Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders

Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders Osama Mohamed, Omar Najm, Shefin F. Shaji This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5723404/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The global growth in infrastructure projects exacerbates the need for ordinary Portland cement (OPC) or other similarly effective binder. The construction industry in general and the production of OPC in particular are responsible for significant contributions to CO 2 emissions into the atmosphere. Ground granulated blast slag (GGBS) and fly ash are industrial byproducts that can be recycled and reused as sustainable alternative binders to OPC to produce concrete. This article evaluated the effect of NaOH activator concertation on the development of 28-day compressive strength of mortar that uses combinations of GGBS and fly ash as binders and activated using Na 2 SiO 3 and NaOH. The Na 2 SiO 3 content was kept constant while NaOH concentration varied from 6 mol/L to 12 mol/L. Three groups of samples were cured in different environments including: 1) immersion in water, 2) ambient conditions, or 3) 7 days of curing under water then 21 days in ambient conditions. Mortar cured under water produced higher compressive strength when GGBS content exceeds 50% of the total binder content, compared to ambient curing. However, when GGBS content was 50% or less of the total binder, the strength of mortar cured under water was comparable to or lower than those cured in ambient conditions. An optimum NaOH concentration of 10 mol/L produced the highest 28-day compressive in mortar with 75% or 100% GGBS binder. Further increase in NaOH concentration resulted in lower compressive strength than mortar produced with 10 mol/L activator concentration. Efflorescence and strength degradation were manifested in ambient-cured mortar samples with slag binder that was activated using relatively low NaOH concentration. Increasing NaOH concentration beyond 6M decreased or eliminated efflorescence and strength degradation in ambient-cured mortar. sustainable concrete compressive strength efflorescence workability activator concentration slag fly ash sodium hydroxide sodium silicate Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction The demand for ordinary Portland cement (OPC) is increasing globally due to the rapid urbanization in many parts of the world. The construction industry in general and the production of OPC in particular are associated with significant emissions of CO 2 . In addition, steel production and other industries produce significant waste that must be recycled. Slag and fly ash are common industrial byproducts that needs to be recycled. This article explores the recycling of ground granulated blast furnace slag (GGBS) and fly ash by reusing these byproducts as binders to replace OPC. In such case, two goals are achieved, recycling of fly ash and GGBS, and eliminating the need for OPC as binder. Unlike OPC, GGBS and fly ash have limited binding properties unless activated in an alkaline medium. Activators like sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) are commonly used together and sometimes separately. Using large amounts of NaOH is also not environmentally friendly, therefore, its content must be controlled [ 1 ]. In addition, the relative amounts of GGBS and fly ash within the total binder influences fresh properties, mechanical strength, and transport characteristics of concrete [ 2 ]. When GGBS constitutes the larger proportion of GGBS + fly ash binder, the binding gel is mostly C-A-S-H, while N-A-S-H represents the bulk of the binding gel when fly ash is the largest constituent of the binder. The volume of permeable voids (VPV) in concrete was found to be the largest when the binder is alkali-activated fly ash. After 18 days or 180 days of curing, the VPV of concrete decreased as fly ash is incrementally replaced with GGBS [ 3 ]. In mortar with alkali-activated GGBS and fly ash binder activated using 12 mol/L NaOH solution, the total volume of pore voids decreased after 28 days of curing compared to 7 days, regardless of the relative contents of GGBS and fly ash [ 4 ]. This indicates alkali-activated GGBS/fly ash binders are capable of developing polymerization products and inducing healthy gain in compressive strength of mortar and concrete. Wardhono [ 5 ] evaluated compressive strength development of mortar with alkali-activated GGBS and fly ash binders up to 28 days. The study reported that the mix with 50%GGB + 50% fly ash binder developed higher compressive strength at each testing age up to 28 days, compared to mixes with higher content of GGBS relative to fly ash. Increasing the concentration of the activator increases solution alkalinity, therefore, enhances the breakdown and dissolution of the aluminosilicate and calcium-based precursors such as GGBS and fly ash. Dissolution occurs after mixing, followed by precipitation at early stages and beginning of hardening, and the reaction continues by solid state mechanisms at later phases [ 6 ]. One measure of the activator concentration is the percentage of Na 2 O to the total mass of the binder. In mortar where the binder is alkali-activated slag and fly ash (AASF), or solely fly ash, increasing the Na 2 O concentration increases the formation of the N-A-S-H gel, the primary product associated with dissolution of fly ash [ 7 ]. Excessively high activator concentration, such as high Na 2 O or NaOH molarity, beyond an optimum value, results in a decrease in mechanical strength [ 8 ]. However, the optimum NaOH concentration also depends on the activator modulus (Ms = SiO 2 /Na 2 O) as well as the curing age. Taghvayi[ 9 ] reported that after 90 days of curing Na 2 O of 6.5% as the optimum concentration for compressive strength when the activator modulus was 0.85, while 5.5% was the optimum concentration when the activator modulus was 1.05. The existence of an optimum NaOH concentration was also observed in mortar with alkali-activated fly ash without GGBS. Kotwal[ 10 ] reported that excessive OH − ions, associated with NaOH concentration beyond an optimum value, increases dissolution but not polycondensation leading to precipitation of binder and loss of strength. It is worth noting that in alkali-activated GGBS fly ash binders, the optimum activator concentration is also related to the silicate modulus and the target concrete properties. Aydin and Badran[ 11 ] reported an optimum silicate modulus of 0.8 and Na 2 O of 6% resulted in minimum water absorption and volume of permeable voids in alkali-activated slag, which influence both strength and durability of mortar. Curing conditions have significant effect on mechanical properties and durability of concrete and mortar with alkali-activated binders [ 12 ]. The effect of curing regime on mechanical properties and durability is also related to the type of activator used. Activator type and concentration also affect the way curing regime influences mechanical properties. Mortar with GGBS binder activated using NaOH solutions exhibited highest compressive strength when cured in ambient conditions while mortar activated using a combination of NaOH and Na 2 SiO 3 and cured under the same conditions showed the lowest compressive strength [ 13 ]. Ambient curing or drying of mortar with alkali-activated GGBS binder at the early ages after demolding is detrimental to mechanical properties [ 13 ]. Liu [ 14 ] reported that the efflorescence in pastes prepared using alkali-activated slag (AAS) were largely sodium carbonate while calcium carbonate, produced by natural carbonation was rarely found where efflorescence was present. The activator used was either NaOH or Na 2 SiO 3 in dosages of 3%, 5%, or 7%. Efflorescence has more adverse effect on degradation and compressive strength of the AAS paste when the activator as Na 2 SiO 3 than NaOH. The use of 5% and 10% calcium stearate (CS) as partial replacement of fly ash eliminated efflorescence in mortar with AAF binder [ 15 ]. CS acts as a water repellent on the walls of the pore system and introduces voids, which create a light weight geopolymer. Cellulose nanocrystals (CNC) reportedly increase the production of C-A-S-H in mortar with 50% GGBS and 50% fly ash activators. Some studies reported an increase in the production of C-A-S-H. The increase in C-A-S-H as CNC content was increased from 0% to 0 0.3% by mass of binder was associated with a corresponding increase in flexural and compressive strength [ 16 ]. C-A-S-H is generally characterized by a soother and more compact structure [ 12 ]. A potential downside to using sustainable concrete with alkali-activated recycled binders is the appearance of efflorescence on the surface of the structural elements, which may undermine aesthetics, compromise structural integrity, and/or undercut the longevity and durability of the structural systems. Efflorescence was reported in mortar samples with alkali-activated binders that were partially immersed in water with the remaining sample portion under ambient conditions [ 17 ]. Due to evaporation of water for sample surface cured under conditions, capillary force drive the activator solution carrying alkaline metals, such as Na, toward the free surface where liquid evaporates leaving the salts deposited on the sample surface. CO 2 diffusion from the atmosphere reacts with precipitated salts forming carbonate salts. Alkali-activated mortar with relatively higher slag content (Ca/(Si + Al)) experienced lower efflorescence compared to samples with lower slag content. The higher slag content compared to fly ash results in producing of the compact C-A-S-H rather the less compact N-A-S-H. An optimum content of slag relative to fly ash provides sufficient calcium to produce more of the compact C-S-H, while at the same time decreases leaching of sodium irons [ 18 ]. Gong[ 19 ] reported that efflorescence and carbonation were present on all materialssamples with AASF binders. However, the type of calcium carbonate (CaCO 3 ) depended on activator concentration and application method (liquid or solid). Vaterite was identified with solid activators while aragonite was one of the carbonates identified with liquid activators. Saludung[ 20 ] reported that slag pastes exhibited the fastest development of efflorescence when activated using 14 mol/L NaOH solution. However, partial replacement of 5–15% of the slag with silica fume decreased efflorescence by limiting migration of alkalis. Allahverdi[ 21 ] reported that pastes with alkali-activated GGBS binders experienced increased efflorescence as Na 2 O increases from 1–6%. However, the samples experienced slight to moderate efflorescence when the activator concentration (Na 2 O) ranged from 1–3%. When the activator concentration is high, the excess unreacted Na 2 O leaches out to react with atmospheric CO 2 leading to formation of sodium carbonate. Sodium polyacrylate, a superabsorbent polymer (SAP), reportedly eliminates efflorescence due to its ability to decrease water permeability and establish a compact network that obstructs sodium ions limiting their ability to migrate to the surface of samples [ 22 ]. Similarly, adding Ca(OH) 2 in the range of 3–9% by weight of GGBS decreases efflorescence by decreasing porosity and water absorption rates [ 23 ]. The reduction in efflorescence was attributed to possible binding of weakly bound Na + due to the production of additional C-A-S-H. Adding 9% silica fume (SF) and 2% nano-silica (NS) to mortar developed using a ternary GGBS + fly ash + steel slag binder eliminated efflorescence entirely [ 24 ]. The dissolvable silica in SF and NS reacted with and consumed the free alkalis creating additional C(N)-A-S-H and C-S-H, thereby, filling larger pores and reducing their size from 26 nm to 10 nm. In fly ash-based specimens, there is less efflorescence and occurs at a slower rate when the activator is NaOH compared to sodium silicate [ 25 ]. Replacing 20% of fly ash with GGBS slows the rate of efflorescence in slag-based samples but doesn’t mitigate the overall efflorescence potential or amount. Heat-treatment of fly ash-based mortar during early curing age not only decreases efflorescence of geopolymers but also enhances mechanical properties. This article presents the findings of a study that evaluated the effect of activator concentration and binder composition on strength development and efflorescence of sustainable mortar with alkali-activated fly ash and GGBS binders. In addition, the effect of curing environment on strength development and efflorescence was evaluated by placing samples for 28 days under: 1) water curing, 2) ambient-curing, or 3) water curing for 7 day, then ambient curing for 21 days. Research indicates that water curing is more effective in enhancing mechanical properties of concrete and mortar with alkali-activated binders [ 26 ]. Ambient curing causes efflorescence to progress faster[ 20 ] than water or heat curing. Water curing followed by ambient curing mimics the realistic environment of structural systems. 2. Materials and Methods Mortar mixes were prepared using three combinations of the recycled GGBS and fly ash: 1) 100%GGBS and no fly ash (S100F0), 2) 75% GGBS and 25% fly ash (S75F25, 3) 50% GGBS and 50% fly ash (S50F50), and 4) 25% GGBS and 75% fly ash (S25F75). The combined weight of GGBS and fly ash binders is 436 kg/m 3 . X-ray diffraction (XRD) of the GGBS and fly are shown in Fig. 1 with broader amorphous hump between 20 0 and 40 0 and peak showing Gehlenite (Ca 2 Al(AlSiO 7 ). XRD plot of fly ash shows typical crystalline phases with strong peaks representing Quartz (SiO₂), Mullite (Al₆Si₂O₁₃), and Hematite (Fe₂O₃). Table 1 shows the chemical composition of GGBS and fly ash used in the study, obtained using X-ray florescence (XRF) spectrometry. The properties of fly ash used in the study are consistent with ASTM C618 class F and N ((SiO2 + Al2O3 + Fe2O3) > 70%)). Table 1 Chemical properties of GBS and fly ash CaO SiO 2 Al 2 O 3 SO 3 Fe 2 O 3 TiO 2 K 2 O MnO SrO ZrO 2 CuO Cr 2 O 3 Y 2 O 3 GBBS (%) 59.41 25.75 8.3 2.81 1.5 1.049 0.61 0.564 0.126 0.065 0.031 0.026 0.016 Fly Ash (%) 4.294 58.158 24.351 0.068 8.517 2.565 1.534 0.094 0.062 0.085 0.037 0.047 0.015 GGBS and fly ash were activated using a combination of sodium hydroxide and sodium silicate solutions such that the ratio of the activator-to-binder is 0.55. NaOH activator solution was prepared by mixing calculated quantities of anhydrous NaOH with fresh water to achieve the target concentration (mol/L). The sodium hydroxide solution was covered and left to cool down before being mixed with the sodium silicate solution (water glass), superplasticizer, and additional water as needed to achieve the NaOH activator concentration. The silicate modulus (Na 2 SiO 3 /NaOH) was maintained at 1.5 for all mixes. A silicate modulus of 1.5 was found to develop favorable mechanical properties for mortar with various combinations of GGBS and fly ash [ 8 ], and for mortar with class C fly ash binder[ 27 ]. The sodium silicate was provided by a local supplied in solution form (water glass) consisting of 28.8 wt% SiO 2 , 9.8 wt% Na 2 O, and 61.4 wt% H 2 O. The dry mix consisted of GGBS, fly ash, and sand. The materials needed for the dry and wet mixtures were weighed separately. Three primary sets of mixes were prepared and placed in different curing environments until test day. One set was cured under water for 28 days, the second set was cured in ambient conditions for 28 days, and a third set was cured for 7 days under water followed by 21 days in ambient conditions. For each of the curing methods, samples were prepared using 100% GGBS and no fly ash (S100F0), 75%GGBS + 25% fly ash (S75F25), and 50%GGBS + 50% fly ash (S50F50), and 25% GGBS + 75%fly ash (S25F75) binders. For each of the four binder combinations, samples were prepared such that the precursor was activated using 6 mol/L, 8 mol/L, 10 mol/L, or 12 mol/L NaOH solutions. The wet and dry mixes were combined in a container and mixed thoroughly to ensure a uniform mix. Varying NaOH concentration is intended to observe its effect on strength development and efflorescence of mortar with alkali-activated GGBS and fly ash binders. NaOH is responsible for early strength development at ambient temperature of mortar and concrete with AAS, developing a clearly distinguishable microstructure within 6 hours of casting [ 28 ]. The mix was cast in 50 x 50 x 50 mm 3 and labelled on top as shown in Fig. 2 . Samples (50 mm x 50 mm x 50 mm) were demolded the following day and placed in the appropriate curing environment as shown in Fig. 3. 3. Experimental Procedures The flowability of mixes was evaluated using a mini flow test. After mixing the wet (alkali-activator solution) and dry components (GGBS, fly ash, sand) thoroughly into a homogenous mix, a measured amount of the mix is poured into a cone placed on top of a metal plate. The excess mortar was cleared and the final surface of the mortar in the cylinder was smoothed as shown in Fig. 4(a). The mini slump cylinder slowly lifted upward causing the mortar to spread on the metal plate as shown in Fig. 4(b). The metal plate was tamped 25 times within a 15 second period. The spread of the fresh mortar mix was measured in two perpendicular directions, then the median value was calculated. The 28-day compressive strength of 50 mm x 50 mm x 50 mm mortar samples was determined according to ASTM C109/109a [ 29 ]. 4. Results and Discussion 4.1 Effect of NaOH Concentration on Setting Time and Mortar Flow Table 2 summarizes the flow (cm) of mortar mixes S100F0 (100%GGBS) and S50F50 (50% GGBS + 50% fly ash). For mixes with 100% GGBS binder, increasing the NaOH from 6 mol/L to 10 mol/L had minor effect on mortar flow. However, the mortar flow at NaOH 12 mol/L was substantially lower than 6 mol/L. Higher solution alkalinity accelerates the dissolution of precursors and increases reaction rate leading to formation of skeletal gel structure and decrease in mortar flow. Table 2 Precursor composition, NaOH setting time, and flow. Trial Number Binder Composition* NaOH Concentration (mol/L) Mortar flow (cm) T 1 S100F0 6 18.5 T 3 S100F0 8 18.4 T 4 S100F0 10 18.3 T 5 S100F0 12 14 T 10 S75F25 6 19.8 T 9 S75F25 8 19.5 T 11 S75F25 10 18.5 T 12 S75F25 12 17.3 T13 S50F50 6 18.7 T15 S50F50 8 18.5 T16 S50F50 10 18.1 T17 S50F50 12 * S100F0 = 100%GGBS + 0% fly ash; S75F25 = 75%GGBS + 25% fly ash; S50F50 = 50%GGBBS + 50% fly ash 4.2 Visual Inspection of Mortar with Alkali-Activated Binders 4.2.1 Change of Samples Color to Green and Bluish Green After removing molds, samples placed in the air or in water were visually inspected. A change of sample color to bluish-green or green was observed on the bottom side of the cubes as shown in Fig. 5(a) and becomes intense and extensive in samples with higher NaOH concentration. For samples cured in ambient conditions, the pigmentation disappears within a day after casting, as shown in Fig. 5(b). The bluish-green discoloration in AAS was attributed to the presence of the trisulfur radical anion ( \(\:{s}_{3}^{-}\) ) while green pigmentation is attributed to disulfur radical anion ( \(\:{s}_{2}^{-}\) ) [ 30 ]. In the reducing environment of water curing, sulfur species can be reduced forming polysulfides or species like disulfur and trisulfur anions. Iron compounds, such as ferrous iron ( \(\:{Fe}^{2+}\) )), Vivianite (Fe₃(PO₄)₂·8H₂O) which are green or bluish green were suggested and possible reasons for the pigmentation. Table 1 shows that both GGBS and fly ash used in the study contain ferric oxide ( \(\:{Fe}_{2}{O}_{3}\) ) and sulfur trioxide ( \(\:{SO}_{3}\) ). Curing underwater maintains a low-oxygen environment and stabilizes the reduced forms. Similarly, high NaOH concentrations provide sufficient alkalinity to further stabilize the reduced sulfur or iron specifies and preserves the color. Therefore, the samples cured under water (Fig. 5(b)) maintained the blue-green and green colors on the surfaces until compression test day, while the colors disappeared within a after demoulding for samples cured in air. It is also possible that exposure to air oxidized the greenish \(\:{Fe}^{2+}\) to \(\:{Fe}^{2+}\) . 4.2.2. Efflorescence in mortar with high slag content and low activator concentration. Mortars with GGBS binder which are activated using NaOH are prone to development of efflorescence induced by the existence of the free Na + originating primarily from the dissolved sodium-based alkaline activators. Free Na + migrates upwards, reacts with atmospheric CO 2 and ultimately precipitate in the form of white-colored Na 2 CO 3 .nH 2 O [ 23 , 31 ]. In this study, significant efflorescence was observed on mortar samples made with GGBS as sole binder (S100F0), activated using 6 mol/L NaOH and cured in air at 22 + 2 0 C, as demonstrated in Fig. 6 . Efflorescence was abundant on the top surface and on the sides of the cube which were also decayed. The compressive strength of S100F0 activated using 6 mol/L NaOH was 15.59 MPa, making it the lowest strength of all tested samples, as detailed later in article. It is likely that effloresce was accompanied with decalcification and dealumination leading to degradation of the C(N)-A-S-H gel, which contributed to decreased compressive strength of ambient cured slag-based mortar samples compared to those cured under water. Furthermore, the deposited salts forming the efflorescence may be a combination of carbonate salts as well leached alkalis due to evaporation of water from the activator solution. The low NaOH concentration in samples with 6 mol/L NaOH concentration resulted a higher porosity, which facilitates leaching of ions to the surface of material where carbonate salts form and deposit on the surface. Figure 7 shows that mortar samples with NaOH concentrations higher than 6 mol/L have also shown some efflorescence. However, with NaOH greater than 6 mol/L, efflorescence was limited or non-existent depending on the NaOH concentration and curing environment. No efflorescence was noted on the surface of mortar with 100% slag binder activated using 12 mole/L NaOH solution. Increased NaOH concentration expedited the polymerization process, leading to the formation of a denser C-A-S-H matrix characteristic of AAS mortar, within the sample core as well as the outer surface exposed to air. The compact C-A-S-H slowed down the transport of natural CO 2 through the pore as well as the migration of salts from within the samples through the pore-structure to the surface. This is validated by the increased compressive strength of air-cured samples when NaOH concentration is increased, as discussed later in this article. Therefore, it is important to note that while high slag content reportedly supports decrease of efflorescence [ 18 ], Fig. 7 shows that it this doesn’t apply to apply to relatively low NaOH concentration (6 mol/L). The susceptibility of mortar with AAS or more than 50% GGBS was also reported by Yao [ 32 ]. However, in the present study, although GGBS-based mortar was vulnerable to efflorescence, that effect was dependent on the activator concentration. Similar to mortar with 100%GGBS activator, ambient-cured mortar with 50%GGBS + 50% fly ash binder exhibited highest efflorescence when the activator concentration was 6 mol/L as shown in Fig. 8(a). It is likely that 6 mol/L didn’t provide sufficient alkalinity to drive geopolymerization reaction, which resulted in a porous network allowing unreacted alkalis to migrate through the pores under capillary forces. Efflorescence on sample surfaces decreased markedly as NaOH activator concentrator increased to 8 mol/L (Fig. 8(b) then 10 mol/L (Fig. 8(c)). It is also noted by comparing Fig. 7 and Fig. 8 that higher deposits were observed in samples with 100% GGBS are lower compared to 50% GGBS. 4.3 Effect of GGBS/Fly Ash Contents and NaOH Concentration on 28-day Compressive strength The compressive strength was tested on 50 mm x 50 mm x 50 mm mortar samples with alkali-activated binder consisting of GGBS or combination of GGBS and fly ash as discussed earlier. The mortar specimens were produced using alkaline solutions with concentrations ranging between 6M (6 mol/L) and 12M (12 mol/L). Three distinct curing conditions were employed: (1) curing in water, (2) curing in air, and (3) an initial 7-day water curing, followed by 21 days of air curing. The specific details and corresponding compressive strengths are provided in Table 3 . Table 3 Compressive strength of mortar with AASF binders after 28-days of curing in water, air, or water + air. Sample No Binder composition NaOH Concentration (mol/L) Air Cured (MPa) Water Cured (MPa) 7 days Water Cured & 21 days Air cured (MPa) T1 S100F0 6 15.59 24.81 10.04 T3 S100F0 8 36.3 61.7 39.45 T4 S100F0 10 50.23 67.5 43.1 T5 S100F0 12 55 62.05 45.1 T10 S75F25 6 26.52 24.11 18.83 T9 S75F25 8 43.67 58.03 36.98 T11 S75F25 10 41.52 59.62 37.75 T12 S75F25 12 47.60 55.80 42.55 T13 S50F50 6 16.99 13.42 9.51 T15 S50F50 8 29.55 36.60 24.25 T16 S50F50 10 40.80 40.63 26.30 T17 S50F50 12 38.97 38.03 33.87 T18 S25F75 6 7.02 4.25 4.25 T20 S25F75 8 15.87 14.93 11.48 T21 S25F75 10 25.44 18.78 14.43 T22 S25F75 12 32.18 25.49 22.50 Increasing the concentration of the NaOH solution increases the solution alkalinity and dissolution rate of the precursor. Therefore, increasing activator concentration increases the development of polymerization products and subsequently, the compressive strength, as shown in Fig. 9 . Curing under water ensures retention of activation reactants within the sample ensuring polymerization reaction continues at the surface as well as the core of the mortar sample. As discussed in the prior section, the reaction on the surface of water-cured slag-based (S100F0) samples was manifested with pigmentation that lasted until compression test day, unlike ambient cured samples where the color disappeared within a day after casting. As a result, figure xxx shows that the 28-day compressive strength of slag-based mortar cured under water is consistently higher than that produced under the other two curing regimes. This is consistent with the findings of Collins and Sanjayan [ 33 ]. In addition, 12 mol/L NaOH concentration corresponds to a decrease in strength of water-cured mortar compared to 10 mol/L concentration, possibly due to excessive dissolution of silica. Studies have consistently demonstrated the existence of an optimal dosage of the alkaline activator that maximizes the compressive strength of concrete/mortar with alkali-activated binders [ 28 , 34 , 35 ]. The decrease in strength beyond an optimum activator concentration may be attributed to the faster dissolution and precipitation of silica depositing on particle surfaces and impeding further polymerization. On the other hand, ambient cured samples didn’t experience excessive dissolution even at 12 mol/L NaOH concentration and continued to gain strength. The optimum NaOH concentration of ambient cured slag-based mortar is higher than water-cured samples. Nonetheless, between NaOH concentration of 6 mole/L and 12 mole/L, water-cured samples exhibit higher 28-day compressive strength. When 25% of the GGBS was replaced with flash to form S75F25 mortars, the peak compressive strength NaOH concentration of 10 mol/L decreased from 67.5 MPa to 61.75 MPa. This is because the compactness of the gel increases with higher GGBS content [ 36 , 37 ]. Furthermore, in S75F25 the 10 mol/L optimum NaOH concentration that achieved the highest strength development persisted, as show in Fig. 10 . Despite some shortcomings, such as relatively short setting time, S75F25 was shown to have lower sorptivity compared to S100F0 and S50F50 for a wide range of NaOH concentrations, indicating superior water transport properties and durability [ 26 ]. When the alkali-activated binder consisted of 75% slag and 25% fly ash, ambient-cured mortar continued to develop strength beyond NaOH concentration of 12M. Water-cured mortar continued to develop high strength than ambient cured mortar. The compressive strength of S75F25 mortar cured under water is consistently higher than ambient cured mortar with the same binder. The pattern is consistent in mortar with S100F0 as well. The availability of moisture helps with both hydration and activation of slag, while ambient curing may lead to loss of moisture leading incomplete activation. It is also possible that loss of moisture at ambient conditions may lead to microcracking, especially in mortar with high content of slag [ 38 ]. At ambient conditions, slag dissolution takes longer, and reaction proceeds from the inner core where moisture is available and makes its way to external surface of the sample. Increasing slag content to 50% of the total binder (S50F50) decreases the 28-day compressive strength- compared to higher slag contents (S100F0 and S75F25, regardless of the curing condition. This is attributed to the increased content of the typically more compact C-A-S-H as GGBS is increased and consistent with published literature. It was reported that C-A-S-H gel remained the dominant geopolymerization product when GGBS was partially replaced with various fly ash contents [ 39 ]. Figure 11 shows that the highest compressive strength in S50F50 was around 40 MPa, corresponding to 10M NaOH concentration, lower than the 60 MPa in S75F25 samples, shown in Fig. 10 . This is because more time is needed to dissolve the slow-reacting fly ash whose content is higher in S50F50. The lower content of slag in S50F50 compared to F75F0 and S100F0 signifies lower amount of C-S-H is formed due to lower content of slag, and hence, relatively lower strength. The relatively lower slag content in S50F50 also means the influence of water in strength developed is decreased as it tends to influence slag, more than fly ash. Therefore, Fig. 11 shows comparable strength development for water cured and ambient cured mortar when the binder is 50% slag. For water and ambient-cured mortar, the optimum NaOH concentration yielding highest strength in S50F50 is 10 mol/L, similar S75F25 and S100F0 as discussed earlier. Increasing NaOH concentration to 12 mol/L is not accompanied by further increase in 28-day strength, but rather a relative decrease compared to the value at 10 mol/L. Several reasons were reported in the literature for the relative decrease in strength with high activator concentrations [ 40 ]. Gebregziabiher[ 28 ] reported the development of a high-density barrier in mortar at elevated activator concentrations that limit diffusion of reactants and curbs strength development at later age. Puertas[ 41 ] reported that all GGBS has completed reacted after 28 days of curing when the binder consists of 50%GGBS and 50% fly ash and the NaOH concentration is 10 mol/L. The geopolymerization product was described as a C-S-H gel with high amounts of Al in the structure. Prior studies have also shown that strength development at 27 0 C is characterized by GGBS activation when its content ranged from 5–50% of the total binder content, unlike curing at 60 0 C which is characterized by the interaction of GGBS and fly ash [ 42 ]. When the content of fly ash is increased to 75% of the total binder, the strength of S25F75 mortar decreases below S50F50 regardless of curing environment or activator concentration within the 6 mol/L to 12 mol/L range evaluated. Included GGBS, even as little as 25% enhances decreases overall porosity and increases tortuosity of the AASF system, leading to a more durable system compared to pure AAF [ 43 ]. As shown on Fig. 12 , the optimum concentration of NaOH is not reached and mortar strength continues to increase with NaOH concentration. This due to the dominance of the slow reacting fly ash and the inability to the small amount of slag to contribute sufficiently to the 28-day strength. The maximum 28-day strength developed by ambient-cured sample was 32.18 MPa at NaOH concentration of 12 mol/L. This result is promising as no heat treatment was applied to samples with 75% fly ash and curing was done under ambient laboratory conditions. Similarly, the 25.44 MPa achieved at 10 mol/L S25F75 is also promising, indicating a balance between limited heat treatment and low NaOH concentration may achieve the desired mechanical strength. It must be noted that high fly ash content promotes the development of more N-A-S-H, which is typically more porous with reduced transport resistance [ 44 ]. Therefore, comparing Fig. 9 , 10 , 11 , and 12 , it can be stated that at any NaOH activator concentration, increasing GGBS content increases the 28-day compressive strength of water-cured mortar [ 8 ], which was consistent with published literature and attributed to the increase of the more compact C-A-S-H [ 45 ]. Therefore, it can be concluded AAS favors water curing to develop mechanical strength than ambient curing. As shown in Fig. 9 , water-cured mortar samples with 100%GGBS binder (S100F0) developed the highes 28-day strength compared to S75F25, S50F50, and S25F75, regardless of NaOH concentration. Samantasinghar [ 34 ] reported the same pattern for mortar with NaOH concentration of 8 mol/L. Deb [ 46 ] reported that mortar with higher GGBS content developed higher compressive strength up to the age of 180 days. The existence of an optimum NaOH molarity for compressive strength of mortar when binder dominated by GGBS binder (S100F0 and S75F25) is clear. The strength increases with molarity, then decreases with further increment of NaOH concentration, as discussed earlier. Figure 13(a) shows that when NaOH concentration is the lowest in the study (6 mol/L), the compressive strength decreased as the curing environment changed from ambient (A) to Water (W), then 7 days under followed by 21 days in ambient conditions (WA), regardless of the relative binder content. The exception to the pattern in Fig. 13(a) is mortar with 100% GGBS binder, where W curing had the highest strength compared to A and WA curing. This is because ambient curing was associated with significant efflorescence under low NaOH concentration as discussed earlier in this article. Mortar samples that contain fly ash favor ambient curing rather than water curing, regardless of NaOH concentration. Figure 13 (a) for 6 mol/L NaOH concentration shows that for samples with fly ash (S25F75, S50F50, S75F25), ambient cured mortar developed higher 28-day compressive compared to water-cured samples or those cured in water than in air. Since S100F0 didn’t contain any fly ash, it favors water curing as mentioned earlier, which produced higher strength than ambient curing. The relationship between ambient curing and fly ash content is influenced by NaOH concentration. Figure 13(b) shows that for NaOH concentration of 8 mol/L, as fly ash content is increased, the relative strength of ambient-cured mortar to water cured mortar also increases. For instance, the strength of ambient-cured S25F75 samples is greater than those cured in water. When NaOH concentration is increased to 10 mol/L the strength of both ambient cured S25F75 and S50F50 exceeded that of water-cured counterparts. The pattern continues as NaOH concentration is further increased to 12 mol/L. Figures 13(a) to (d) show that S100F0 developed higher compressive strength under ambient curing compared to the other binder combinations when NaOH concentration was 10 mol/L or 12 mol/L. However, for NaOH concentration of 6 mol/L, S100F0 developed lower strength than S75F25 and S50F50, and less than S75F25 when NaOH concentration is 8 mol/L. This is attributed to the higher efflorescence inflicting ambient-cured GGBS-dominated samples, especially S100F0 developed using NaOH concentrations of 6 mol/L and 8 mol/L. Figures 13(a) to (d) show that the optimum NaOH concentration where mortar develops the highest compressive strength is 10 mol/L. This is the case for mortars with S100F0, F75F25, and S50F50. Samantasinghar and Singh[ 34 ] reported an optimum NaOH concentration of 8 mol/L, which was noted for GGBS content from 0 to 100%. NaOH concentration beyond the optimum value causes excessive dissolution and leaching of silica from the precursor leading to congealing of particle surfaces and subsequent delay of the geopolymerization process. No optimum NaOH activator concentration was observed in mortar with low GGBS content (S25F75) as higher alkalinity is necessary to dissolve the slow reacting fly ash which exists in larger quantity. Rattanasak[ 47 ] also reported an increased leaching of Si 4+ and Al 3+ after 10 minutes of mixing in fly ash-based pastes when NaOH concentration increased from 5 mol/L to 10 mol/L, which supports the increase in strength shown in Figs. 13(a) to (d) for the mixes with 25% GGBS + 75% fly ash binders. In the present study, Fig. 13 supports that leaching and strength development at the age of 28 days continues to increase as NaOH concentration is increased further to 12 mol/L. Along with activator concentration, the curing environment also influences strength development, especially when NaOH exceeds 12 mol/L [ 8 ]. Increasing NaOH concentration from 10 mol/L to 12 mol/L also increases resistance to chloride ion penetration, carbonation, and acid attack [ 48 ]. The mix with 50%fly ash + 50% GGBS is of particular interest from a professional practice perspective as it develops reasonable compressive strength at ambient conditions for NaOH higher than 6 mol/L. In addition, prior studies have shown acceptable setting time and workability, without retarding admixtures or special mixing methods [ 49 ]. The compressive strength of mortar with 75% fly ash is low even at the highest NaOH concentration of 12 mol/L. Guo [ 27 ] reported a significant enhancement when mortar with AAF binder is heat cured at 75 0 C and the optimum NaOH concentration in such a case is 10 mol/L. Conclusions This study evaluated the effect of NaOH activator concentration on compressive strength development of mortar with alkali-activated ground granulated blast furnace slag (GGBS) and fly ash. The binder, which consists of GGBS, 75% GGBS + 25% fly ash, 50% GGBS + 50% fly ash, or 25%GGBS + 75% fly ash, was activated using a combination of NaOH and Na 2 SiO 3 . The ratio of NaOH/Na 2 SiO 3 was maintained at 1.5 in tests. Samples were cured for 28 days under ambient conditions, under water, or were kept for 7 days under water followed by 21 days in ambient conditions. The following observations were made: When the binder used was alkali-activated GBBS or GGBS and fly ash with a ratio of 3:1, mortar cured under water for 28 days developed higher compressive strength compared to samples cured under ambient conditions, regardless of NaOH concentration. However, when the binder was 50% GGBS + 50% fly ash, mortar cured under ambient conditions or under water developed comparable strength. However, when the GGBS:fly ash ratio was 1:3, mortar cured under ambient conditions developed higher strength. In mortar with high GGBS binder content (100% GGBS and 75% GGBS + 25% fly ash), the 28-day compressive strength increases as NaOH concentration increases from 6 mol/L to 10 mol/L. However, further increase of NaOH concentration to 12 mol/L decreases the 28-day compressive strength of mortar relative to 10 mol/L concentration. Ambient-cured mortar samples with dominant slag binder do not exhibit the 10 mol/L optimum concentration and continue to gain strength with increase in NaOH concentration but remains lower in strength than mortar cured under water. When the amounts of GGBS and fly ash binder are equal (50% GGBS + 50% fly ash), ambient and water-cured mortar samples develop comparable strength. However, when the ratio of GGBS:fly ash becomes 1:3, ambient-cured mortar samples produce higher 28-day strength than water-cured samples. It appears that GGBS favors water curing while fly ash produces higher strength in air-cured mortar samples. Ambient-cured mortar samples with 100%GGBS activated with 6 mol/L NaOH solution developed visible efflorescence, degradation, and decrease in compressive strength compared to water cured samples. This is likely due to due to loss of moisture from the activator solution and precipitation of the activator salts, along with carbonation due to ingress of CO 2 through pores. However, efflorescence was not observed when NaOH was higher than 6 mol/L, likely due to the increased alkalinity of the activator solution which resulted in rapid formation of the more compact C-A-S-H. A more compact pore structure in the outer surface impedes the progression of Na + through the pores to combine with atmospheric CO 2 and form carbonate slats. Efflorescence was not observed in any sample cured under water regardless of NaOH concentration or binder composition. This is attributed to retention of activator of solution as water provided a closed curing medium, which support continued geopolymerization reaction, while blocking access of CO 2 to sample. Declarations Author Contribution O. M. conceptualization, writing the original draft, fund acquisition.O. N. Experimental program, validation, edit draft.S. S. Figures preparation, reference organization. Acknowledgements The authors gratefully acknowledge the financial support provided by the Office of Research and Sponsored Programs (ORSP) at Abu Dhabi University under grant No. 19300899. Data Availability Data is provided within the manuscript. References O.A. Mohamed, O. Najm, E. 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Zuaiter, Setting time, sulfuric acid resistance, and strength development of alkali-activated mortar with slag & fly ash binders, Results in Engineering 21 (2024). https://doi.org/10.1016/j.rineng.2023.101711 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5723404","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":419623818,"identity":"528e4457-43bf-48f4-8339-3a1724c2f9db","order_by":0,"name":"Osama Mohamed","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA90lEQVRIiWNgGAWjYDADAxDBU8HA2ECiljMka+FtI0ILP//hpxt+MGyTM2c/nfjg7Tw72Q0HmB9+YKixwalFckaa2c0ehtvGlj25mw3nbks23nCAzViC4VgabvfcYDC7wcNwO3HDgdxt0rzbmIEMBjMGBrbDuLWcP/7t5h+QlvNvt//mnVMP1ML+jYHh33/cWg7kmN0G23Ijdxszb8NhoBYeMwbGtgN4/JJTdlvG4LaxwY23myXnHDtuPPMwT7FEYl8yTi38/Me33XxTcVvO4Hzuxg9vaqpl+463b/zw4ZsdTi1Q5yFzmIE4gYCGUTAKRsEoGAX4AQBNCVvm972jDQAAAABJRU5ErkJggg==","orcid":"","institution":"Abu Dhabi University","correspondingAuthor":true,"prefix":"","firstName":"Osama","middleName":"","lastName":"Mohamed","suffix":""},{"id":419623819,"identity":"3519377b-da74-47bd-8ed4-6050fe39efb3","order_by":1,"name":"Omar Najm","email":"","orcid":"","institution":"Al Ain University","correspondingAuthor":false,"prefix":"","firstName":"Omar","middleName":"","lastName":"Najm","suffix":""},{"id":419623820,"identity":"4e315bd9-736e-4652-a8a0-7200f5f0b18c","order_by":2,"name":"Shefin F. Shaji","email":"","orcid":"","institution":"Abu Dhabi University","correspondingAuthor":false,"prefix":"","firstName":"Shefin","middleName":"F.","lastName":"Shaji","suffix":""}],"badges":[],"createdAt":"2024-12-27 19:53:03","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5723404/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5723404/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":77305315,"identity":"239b6677-817a-43fc-a796-c14fe1dec3bf","added_by":"auto","created_at":"2025-02-27 08:58:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":70474,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction (XRD) analysis of: (a) GGBS and (b) fly ash.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/ddb93db9f4d96ea06fc216af.png"},{"id":77303418,"identity":"6b66aaa7-a0c2-40f3-aaed-dcb47bb7e777","added_by":"auto","created_at":"2025-02-27 08:51:06","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":112968,"visible":true,"origin":"","legend":"\u003cp\u003eMortar cast in molds and labelled indicated intended curing method W indicates water curing, A indicates Ambient curing, and AW indicates water curing for 7-days followed by ambient curing for 14 days.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/719fa86091d79ecbf16cda10.jpg"},{"id":77307073,"identity":"1f77b5de-0d21-44aa-aa02-c5c7ac235fdf","added_by":"auto","created_at":"2025-02-27 09:14:57","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":353127,"visible":true,"origin":"","legend":"\u003cp\u003e50 x 50 x 50 mm mortar samples cured: (a) in ambient conditions, (b) for 7 days under water + 21 days in ambient conditions, (c) under water.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/7780ea1d52fba2f5243eeb1f.png"},{"id":77305311,"identity":"4d0eee4f-f58d-463d-8bbd-d938f61fda5c","added_by":"auto","created_at":"2025-02-27 08:58:57","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":629747,"visible":true,"origin":"","legend":"\u003cp\u003eMeasurement of mortar flow after mixing a) filling the cylinder with freshly mixed mortar, b) mortar after lifting the cylinder and spreading\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/8d3ee7c73c40b51fa9c7b65d.png"},{"id":77303406,"identity":"4e6480f6-6c43-4097-bc0c-0017a63cfb0d","added_by":"auto","created_at":"2025-02-27 08:50:57","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1067730,"visible":true,"origin":"","legend":"\u003cp\u003eGreen and bluish-Green Pigmentation on mortar samples on: (a) day 0 \u0026nbsp;of air curing; (b) day 1 of air curing\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/f74ebf811025cbf57cb034c8.png"},{"id":77303409,"identity":"f8b33387-2f5d-48b9-aaee-6ca9866fdd41","added_by":"auto","created_at":"2025-02-27 08:50:57","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":645404,"visible":true,"origin":"","legend":"\u003cp\u003eEfflorescence phenomenon observed on all sides of ambient-cured mortar samples with 100% slag binder (S100F0) activated using 6 mol/L NaOH\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/fd705f2608e6fe8c2ecf8828.png"},{"id":77303416,"identity":"d4c8d61a-441c-4683-8316-a1fa0f6d1e89","added_by":"auto","created_at":"2025-02-27 08:51:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1017616,"visible":true,"origin":"","legend":"\u003cp\u003eLimited (a) or no efflorescence (c) observed on ambient-cured mortars samples with 100% slag binder activated using NaOH concentration of: a) 8 mol/L b) 10 mol/L, c) 12 mol/L.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/0dc7d8385ec4c5b7feb4f17d.png"},{"id":77303413,"identity":"b219b993-069e-45e0-b682-4e5c56888fa0","added_by":"auto","created_at":"2025-02-27 08:50:58","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":729096,"visible":true,"origin":"","legend":"\u003cp\u003eEfflorescence is highest on mortar with 50% GGBS+50% fly ash activated using 6 mol/L NaOH and decreases as NaOH concentrations increases to 10 mol/L: (a) 6 mol/L NaOH, b) 8 mol/L NaOH, c) 10 mol/L NaOH.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/8fd55f202dddac8ee6dfcf55.png"},{"id":77305963,"identity":"26214b4c-af34-46d7-a5fe-e6b49da3017b","added_by":"auto","created_at":"2025-02-27 09:06:57","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":93531,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of NaOH activator concentration on strength development of mortar with 100% GGBS (S100F0) cured under water, in ambient conditions, and 7 days under + 21 days in ambient conditions.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/d2f7ca4057fe25cd36a49abf.png"},{"id":77303417,"identity":"bd23fd01-e9c3-4148-978e-c6fbc9719011","added_by":"auto","created_at":"2025-02-27 08:51:02","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":97413,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of NaOH activator concentration on strength development of mortar with 75% GGBS and 25% fly ash (S75F25) cured under water, in ambient conditions, and 7 days under + 21 days in ambient conditions.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/c64e3a98919192233489e9bd.png"},{"id":77303405,"identity":"7d9c09b8-6e92-4353-bb9c-3076240319df","added_by":"auto","created_at":"2025-02-27 08:50:57","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":99138,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of NaOH activator concentration on 28-day compressive strength of mortar with 50% GGBS and 50% fly ash (S50F50) cured under water, in ambient conditions, and 7 days under + 21 days in ambient conditions.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/4f7b46703b9d9eb16981b743.png"},{"id":77303411,"identity":"2e3087e5-69b2-4533-b5f9-b13b96c1c0e6","added_by":"auto","created_at":"2025-02-27 08:50:57","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":93044,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of NaOH activator concentration on strength development of mortar with 25% GGBS and 75% fly ash (S75F25) cured under water, in ambient conditions, and 7 days under + 21 days in ambient conditions.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/c183b987c877c9212a69e23d.png"},{"id":77305314,"identity":"507d9dde-acfb-474d-8cd3-623140a3e7ab","added_by":"auto","created_at":"2025-02-27 08:58:57","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":89761,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of NaOH concentration on 28-day compressive strength for mortar sample cured under water, in ambient conditions, and 7-days under water + 21 days in ambient conditions using NaOH concentration of: a) 6 mol/L, b) 8 mol/L, c) 10 mol/L, and d) 12 mol/L\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/934dbfeac978c6150261a90b.png"},{"id":77307433,"identity":"ef8f9cdd-6c69-41a6-b0e5-48483e358481","added_by":"auto","created_at":"2025-02-27 09:23:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6980444,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5723404/v1/e902422e-f156-4ba5-b964-af2f3ad88cf1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe demand for ordinary Portland cement (OPC) is increasing globally due to the rapid urbanization in many parts of the world. The construction industry in general and the production of OPC in particular are associated with significant emissions of CO\u003csub\u003e2\u003c/sub\u003e. In addition, steel production and other industries produce significant waste that must be recycled. Slag and fly ash are common industrial byproducts that needs to be recycled. This article explores the recycling of ground granulated blast furnace slag (GGBS) and fly ash by reusing these byproducts as binders to replace OPC. In such case, two goals are achieved, recycling of fly ash and GGBS, and eliminating the need for OPC as binder. Unlike OPC, GGBS and fly ash have limited binding properties unless activated in an alkaline medium. Activators like sodium hydroxide (NaOH) and sodium silicate (Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e) are commonly used together and sometimes separately. Using large amounts of NaOH is also not environmentally friendly, therefore, its content must be controlled [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. In addition, the relative amounts of GGBS and fly ash within the total binder influences fresh properties, mechanical strength, and transport characteristics of concrete [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhen GGBS constitutes the larger proportion of GGBS\u0026thinsp;+\u0026thinsp;fly ash binder, the binding gel is mostly C-A-S-H, while N-A-S-H represents the bulk of the binding gel when fly ash is the largest constituent of the binder. The volume of permeable voids (VPV) in concrete was found to be the largest when the binder is alkali-activated fly ash. After 18 days or 180 days of curing, the VPV of concrete decreased as fly ash is incrementally replaced with GGBS [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In mortar with alkali-activated GGBS and fly ash binder activated using 12 mol/L NaOH solution, the total volume of pore voids decreased after 28 days of curing compared to 7 days, regardless of the relative contents of GGBS and fly ash [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. This indicates alkali-activated GGBS/fly ash binders are capable of developing polymerization products and inducing healthy gain in compressive strength of mortar and concrete.\u003c/p\u003e \u003cp\u003eWardhono [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] evaluated compressive strength development of mortar with alkali-activated GGBS and fly ash binders up to 28 days. The study reported that the mix with 50%GGB\u0026thinsp;+\u0026thinsp;50% fly ash binder developed higher compressive strength at each testing age up to 28 days, compared to mixes with higher content of GGBS relative to fly ash.\u003c/p\u003e \u003cp\u003eIncreasing the concentration of the activator increases solution alkalinity, therefore, enhances the breakdown and dissolution of the aluminosilicate and calcium-based precursors such as GGBS and fly ash. Dissolution occurs after mixing, followed by precipitation at early stages and beginning of hardening, and the reaction continues by solid state mechanisms at later phases [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. One measure of the activator concentration is the percentage of Na\u003csub\u003e2\u003c/sub\u003eO to the total mass of the binder. In mortar where the binder is alkali-activated slag and fly ash (AASF), or solely fly ash, increasing the Na\u003csub\u003e2\u003c/sub\u003eO concentration increases the formation of the N-A-S-H gel, the primary product associated with dissolution of fly ash [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Excessively high activator concentration, such as high Na\u003csub\u003e2\u003c/sub\u003eO or NaOH molarity, beyond an optimum value, results in a decrease in mechanical strength [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, the optimum NaOH concentration also depends on the activator modulus (Ms\u0026thinsp;=\u0026thinsp;SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO) as well as the curing age. Taghvayi[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] reported that after 90 days of curing Na\u003csub\u003e2\u003c/sub\u003eO of 6.5% as the optimum concentration for compressive strength when the activator modulus was 0.85, while 5.5% was the optimum concentration when the activator modulus was 1.05. The existence of an optimum NaOH concentration was also observed in mortar with alkali-activated fly ash without GGBS. Kotwal[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] reported that excessive OH\u003csup\u003e\u0026minus;\u003c/sup\u003e ions, associated with NaOH concentration beyond an optimum value, increases dissolution but not polycondensation leading to precipitation of binder and loss of strength. It is worth noting that in alkali-activated GGBS fly ash binders, the optimum activator concentration is also related to the silicate modulus and the target concrete properties. Aydin and Badran[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] reported an optimum silicate modulus of 0.8 and Na\u003csub\u003e2\u003c/sub\u003eO of 6% resulted in minimum water absorption and volume of permeable voids in alkali-activated slag, which influence both strength and durability of mortar.\u003c/p\u003e \u003cp\u003eCuring conditions have significant effect on mechanical properties and durability of concrete and mortar with alkali-activated binders [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The effect of curing regime on mechanical properties and durability is also related to the type of activator used. Activator type and concentration also affect the way curing regime influences mechanical properties. Mortar with GGBS binder activated using NaOH solutions exhibited highest compressive strength when cured in ambient conditions while mortar activated using a combination of NaOH and Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e and cured under the same conditions showed the lowest compressive strength [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Ambient curing or drying of mortar with alkali-activated GGBS binder at the early ages after demolding is detrimental to mechanical properties [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eLiu [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] reported that the efflorescence in pastes prepared using alkali-activated slag (AAS) were largely sodium carbonate while calcium carbonate, produced by natural carbonation was rarely found where efflorescence was present. The activator used was either NaOH or Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e in dosages of 3%, 5%, or 7%. Efflorescence has more adverse effect on degradation and compressive strength of the AAS paste when the activator as Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e than NaOH. The use of 5% and 10% calcium stearate (CS) as partial replacement of fly ash eliminated efflorescence in mortar with AAF binder [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. CS acts as a water repellent on the walls of the pore system and introduces voids, which create a light weight geopolymer.\u003c/p\u003e \u003cp\u003eCellulose nanocrystals (CNC) reportedly increase the production of C-A-S-H in mortar with 50% GGBS and 50% fly ash activators. Some studies reported an increase in the production of C-A-S-H. The increase in C-A-S-H as CNC content was increased from 0% to 0 0.3% by mass of binder was associated with a corresponding increase in flexural and compressive strength [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. C-A-S-H is generally characterized by a soother and more compact structure [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA potential downside to using sustainable concrete with alkali-activated recycled binders is the appearance of efflorescence on the surface of the structural elements, which may undermine aesthetics, compromise structural integrity, and/or undercut the longevity and durability of the structural systems. Efflorescence was reported in mortar samples with alkali-activated binders that were partially immersed in water with the remaining sample portion under ambient conditions [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Due to evaporation of water for sample surface cured under conditions, capillary force drive the activator solution carrying alkaline metals, such as Na, toward the free surface where liquid evaporates leaving the salts deposited on the sample surface. CO\u003csub\u003e2\u003c/sub\u003e diffusion from the atmosphere reacts with precipitated salts forming carbonate salts. Alkali-activated mortar with relatively higher slag content (Ca/(Si\u0026thinsp;+\u0026thinsp;Al)) experienced lower efflorescence compared to samples with lower slag content. The higher slag content compared to fly ash results in producing of the compact C-A-S-H rather the less compact N-A-S-H. An optimum content of slag relative to fly ash provides sufficient calcium to produce more of the compact C-S-H, while at the same time decreases leaching of sodium irons [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGong[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] reported that efflorescence and carbonation were present on all materialssamples with AASF binders. However, the type of calcium carbonate (CaCO\u003csub\u003e3\u003c/sub\u003e) depended on activator concentration and application method (liquid or solid). Vaterite was identified with solid activators while aragonite was one of the carbonates identified with liquid activators.\u003c/p\u003e \u003cp\u003eSaludung[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] reported that slag pastes exhibited the fastest development of efflorescence when activated using 14 mol/L NaOH solution. However, partial replacement of 5\u0026ndash;15% of the slag with silica fume decreased efflorescence by limiting migration of alkalis. Allahverdi[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] reported that pastes with alkali-activated GGBS binders experienced increased efflorescence as Na\u003csub\u003e2\u003c/sub\u003eO increases from 1\u0026ndash;6%. However, the samples experienced slight to moderate efflorescence when the activator concentration (Na\u003csub\u003e2\u003c/sub\u003eO) ranged from 1\u0026ndash;3%. When the activator concentration is high, the excess unreacted Na\u003csub\u003e2\u003c/sub\u003eO leaches out to react with atmospheric CO\u003csub\u003e2\u003c/sub\u003e leading to formation of sodium carbonate.\u003c/p\u003e \u003cp\u003eSodium polyacrylate, a superabsorbent polymer (SAP), reportedly eliminates efflorescence due to its ability to decrease water permeability and establish a compact network that obstructs sodium ions limiting their ability to migrate to the surface of samples [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Similarly, adding Ca(OH)\u003csub\u003e2\u003c/sub\u003e in the range of 3\u0026ndash;9% by weight of GGBS decreases efflorescence by decreasing porosity and water absorption rates [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The reduction in efflorescence was attributed to possible binding of weakly bound Na\u003csup\u003e+\u003c/sup\u003e due to the production of additional C-A-S-H. Adding 9% silica fume (SF) and 2% nano-silica (NS) to mortar developed using a ternary GGBS\u0026thinsp;+\u0026thinsp;fly ash\u0026thinsp;+\u0026thinsp;steel slag binder eliminated efflorescence entirely [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The dissolvable silica in SF and NS reacted with and consumed the free alkalis creating additional C(N)-A-S-H and C-S-H, thereby, filling larger pores and reducing their size from 26 nm to 10 nm.\u003c/p\u003e \u003cp\u003eIn fly ash-based specimens, there is less efflorescence and occurs at a slower rate when the activator is NaOH compared to sodium silicate [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Replacing 20% of fly ash with GGBS slows the rate of efflorescence in slag-based samples but doesn\u0026rsquo;t mitigate the overall efflorescence potential or amount. Heat-treatment of fly ash-based mortar during early curing age not only decreases efflorescence of geopolymers but also enhances mechanical properties.\u003c/p\u003e \u003cp\u003eThis article presents the findings of a study that evaluated the effect of activator concentration and binder composition on strength development and efflorescence of sustainable mortar with alkali-activated fly ash and GGBS binders. In addition, the effect of curing environment on strength development and efflorescence was evaluated by placing samples for 28 days under: 1) water curing, 2) ambient-curing, or 3) water curing for 7 day, then ambient curing for 21 days. Research indicates that water curing is more effective in enhancing mechanical properties of concrete and mortar with alkali-activated binders [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Ambient curing causes efflorescence to progress faster[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] than water or heat curing. Water curing followed by ambient curing mimics the realistic environment of structural systems.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eMortar mixes were prepared using three combinations of the recycled GGBS and fly ash: 1) 100%GGBS and no fly ash (S100F0), 2) 75% GGBS and 25% fly ash (S75F25, 3) 50% GGBS and 50% fly ash (S50F50), and 4) 25% GGBS and 75% fly ash (S25F75). The combined weight of GGBS and fly ash binders is 436 kg/m\u003csup\u003e3\u003c/sup\u003e. X-ray diffraction (XRD) of the GGBS and fly are shown in Fig.\u0026nbsp;1 with broader amorphous hump between 20\u003csup\u003e0\u003c/sup\u003e and 40\u003csup\u003e0\u003c/sup\u003e and peak showing Gehlenite (Ca\u003csub\u003e2\u003c/sub\u003eAl(AlSiO\u003csub\u003e7\u003c/sub\u003e). XRD plot of fly ash shows typical crystalline phases with strong peaks representing Quartz (SiO₂), Mullite (Al₆Si₂O₁₃), and Hematite (Fe₂O₃).\u003c/p\u003e\n\u003cp\u003eTable \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e shows the chemical composition of GGBS and fly ash used in the study, obtained using X-ray florescence (XRF) spectrometry. The properties of fly ash used in the study are consistent with ASTM C618 class F and N ((SiO2\u0026thinsp;+\u0026thinsp;Al2O3\u0026thinsp;+\u0026thinsp;Fe2O3)\u0026thinsp;\u003cem\u003e\u0026gt;\u003c/em\u003e\u0026thinsp;70%)).\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eChemical properties of GBS and fly ash\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCaO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMnO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSrO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eZrO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCuO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eY\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGBBS (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e59.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e25.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.049\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.61\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.564\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.126\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.065\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.031\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.026\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.016\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFly Ash (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.294\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.158\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e24.351\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.068\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.565\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.534\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.094\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.062\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.085\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.037\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.047\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.015\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eGGBS and fly ash were activated using a combination of sodium hydroxide and sodium silicate solutions such that the ratio of the activator-to-binder is 0.55. NaOH activator solution was prepared by mixing calculated quantities of anhydrous NaOH with fresh water to achieve the target concentration (mol/L). The sodium hydroxide solution was covered and left to cool down before being mixed with the sodium silicate solution (water glass), superplasticizer, and additional water as needed to achieve the NaOH activator concentration. The silicate modulus (Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e/NaOH) was maintained at 1.5 for all mixes. A silicate modulus of 1.5 was found to develop favorable mechanical properties for mortar with various combinations of GGBS and fly ash [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e], and for mortar with class C fly ash binder[\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e]. The sodium silicate was provided by a local supplied in solution form (water glass) consisting of 28.8 wt% SiO\u003csub\u003e2\u003c/sub\u003e, 9.8 wt% Na\u003csub\u003e2\u003c/sub\u003eO, and 61.4 wt% H\u003csub\u003e2\u003c/sub\u003eO. The dry mix consisted of GGBS, fly ash, and sand. The materials needed for the dry and wet mixtures were weighed separately. Three primary sets of mixes were prepared and placed in different curing environments until test day. One set was cured under water for 28 days, the second set was cured in ambient conditions for 28 days, and a third set was cured for 7 days under water followed by 21 days in ambient conditions. For each of the curing methods, samples were prepared using 100% GGBS and no fly ash (S100F0), 75%GGBS\u0026thinsp;+\u0026thinsp;25% fly ash (S75F25), and 50%GGBS\u0026thinsp;+\u0026thinsp;50% fly ash (S50F50), and 25% GGBS\u0026thinsp;+\u0026thinsp;75%fly ash (S25F75) binders. For each of the four binder combinations, samples were prepared such that the precursor was activated using 6 mol/L, 8 mol/L, 10 mol/L, or 12 mol/L NaOH solutions. The wet and dry mixes were combined in a container and mixed thoroughly to ensure a uniform mix. Varying NaOH concentration is intended to observe its effect on strength development and efflorescence of mortar with alkali-activated GGBS and fly ash binders. NaOH is responsible for early strength development at ambient temperature of mortar and concrete with AAS, developing a clearly distinguishable microstructure within 6 hours of casting [\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]. The mix was cast in 50 x 50 x 50 mm\u003csup\u003e3\u003c/sup\u003e and labelled on top as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Samples (50 mm x 50 mm x 50 mm) were demolded the following day and placed in the appropriate curing environment as shown in Fig.\u0026nbsp;3.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"3. Experimental Procedures","content":"\u003cp\u003eThe flowability of mixes was evaluated using a mini flow test. After mixing the wet (alkali-activator solution) and dry components (GGBS, fly ash, sand) thoroughly into a homogenous mix, a measured amount of the mix is poured into a cone placed on top of a metal plate. The excess mortar was cleared and the final surface of the mortar in the cylinder was smoothed as shown in Fig. 4(a). The mini slump cylinder slowly lifted upward causing the mortar to spread on the metal plate as shown in Fig. 4(b). The metal plate was tamped 25 times within a 15 second period. The spread of the fresh mortar mix was measured in two perpendicular directions, then the median value was calculated.\u003c/p\u003e\n\u003cp\u003eThe 28-day compressive strength of 50 mm x 50 mm x 50 mm mortar samples was determined according to ASTM C109/109a [\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e"},{"header":"4. Results and Discussion","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 Effect of NaOH Concentration on Setting Time and Mortar Flow\u003c/h2\u003e\n \u003cp\u003eTable\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e summarizes the flow (cm) of mortar mixes S100F0 (100%GGBS) and S50F50 (50% GGBS\u0026thinsp;+\u0026thinsp;50% fly ash). For mixes with 100% GGBS binder, increasing the NaOH from 6 mol/L to 10 mol/L had minor effect on mortar flow. However, the mortar flow at NaOH 12 mol/L was substantially lower than 6 mol/L. Higher solution alkalinity accelerates the dissolution of precursors and increases reaction rate leading to formation of skeletal gel structure and decrease in mortar flow.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePrecursor composition, NaOH setting time, and flow.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTrial Number\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eBinder Composition*\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNaOH Concentration (mol/L)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMortar flow (cm)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e14\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e19.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e19.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT 12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e17.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003ctfoot\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\"\u003e\u003csup\u003e*\u003c/sup\u003eS100F0\u0026thinsp;=\u0026thinsp;100%GGBS\u0026thinsp;+\u0026thinsp;0% fly ash; S75F25\u0026thinsp;=\u0026thinsp;75%GGBS\u0026thinsp;+\u0026thinsp;25% fly ash; S50F50\u0026thinsp;=\u0026thinsp;50%GGBBS\u0026thinsp;+\u0026thinsp;50% fly ash\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tfoot\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 Visual Inspection of Mortar with Alkali-Activated Binders\u003c/h2\u003e\n \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\n \u003ch2\u003e4.2.1 Change of Samples Color to Green and Bluish Green\u003c/h2\u003e\n \u003cp\u003eAfter removing molds, samples placed in the air or in water were visually inspected. A change of sample color to bluish-green or green was observed on the bottom side of the cubes as shown in Fig.\u0026nbsp;5(a) and becomes intense and extensive in samples with higher NaOH concentration. For samples cured in ambient conditions, the pigmentation disappears within a day after casting, as shown in Fig.\u0026nbsp;5(b). The bluish-green discoloration in AAS was attributed to the presence of the trisulfur radical anion (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{s}_{3}^{-}\\)\u003c/span\u003e\u003c/span\u003e) while green pigmentation is attributed to disulfur radical anion (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{s}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e) [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]. In the reducing environment of water curing, sulfur species can be reduced forming polysulfides or species like disulfur and trisulfur anions. Iron compounds, such as ferrous iron (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Fe}^{2+}\\)\u003c/span\u003e\u003c/span\u003e)), Vivianite (Fe₃(PO₄)₂\u0026middot;8H₂O) which are green or bluish green were suggested and possible reasons for the pigmentation. Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e shows that both GGBS and fly ash used in the study contain ferric oxide (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Fe}_{2}{O}_{3}\\)\u003c/span\u003e\u003c/span\u003e) and sulfur trioxide (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{SO}_{3}\\)\u003c/span\u003e\u003c/span\u003e). Curing underwater maintains a low-oxygen environment and stabilizes the reduced forms. Similarly, high NaOH concentrations provide sufficient alkalinity to further stabilize the reduced sulfur or iron specifies and preserves the color. Therefore, the samples cured under water (Fig. 5(b)) maintained the blue-green and green colors on the surfaces until compression test day, while the colors disappeared within a after demoulding for samples cured in air. It is also possible that exposure to air oxidized the greenish \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Fe}^{2+}\\)\u003c/span\u003e\u003c/span\u003e to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Fe}^{2+}\\)\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\n \u003ch2\u003e4.2.2. Efflorescence in mortar with high slag content and low activator concentration.\u003c/h2\u003e\n \u003cp\u003eMortars with GGBS binder which are activated using NaOH are prone to development of efflorescence induced by the existence of the free Na\u003csup\u003e+\u003c/sup\u003e originating primarily from the dissolved sodium-based alkaline activators. Free Na\u003csup\u003e+\u003c/sup\u003e migrates upwards, reacts with atmospheric CO\u003csub\u003e2\u003c/sub\u003e and ultimately precipitate in the form of white-colored Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e.nH\u003csub\u003e2\u003c/sub\u003eO [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eIn this study, significant efflorescence was observed on mortar samples made with GGBS as sole binder (S100F0), activated using 6 mol/L NaOH and cured in air at 22\u0026thinsp;+\u0026thinsp;2 \u003csup\u003e0\u003c/sup\u003eC, as demonstrated in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. Efflorescence was abundant on the top surface and on the sides of the cube which were also decayed. The compressive strength of S100F0 activated using 6 mol/L NaOH was 15.59 MPa, making it the lowest strength of all tested samples, as detailed later in article. It is likely that effloresce was accompanied with decalcification and dealumination leading to degradation of the C(N)-A-S-H gel, which contributed to decreased compressive strength of ambient cured slag-based mortar samples compared to those cured under water. Furthermore, the deposited salts forming the efflorescence may be a combination of carbonate salts as well leached alkalis due to evaporation of water from the activator solution. The low NaOH concentration in samples with 6 mol/L NaOH concentration resulted a higher porosity, which facilitates leaching of ions to the surface of material where carbonate salts form and deposit on the surface.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;7 shows that mortar samples with NaOH concentrations higher than 6 mol/L have also shown some efflorescence. However, with NaOH greater than 6 mol/L, efflorescence was limited or non-existent depending on the NaOH concentration and curing environment. No efflorescence was noted on the surface of mortar with 100% slag binder activated using 12 mole/L NaOH solution. Increased NaOH concentration expedited the polymerization process, leading to the formation of a denser C-A-S-H matrix characteristic of AAS mortar, within the sample core as well as the outer surface exposed to air. The compact C-A-S-H slowed down the transport of natural CO\u003csub\u003e2\u003c/sub\u003e through the pore as well as the migration of salts from within the samples through the pore-structure to the surface. This is validated by the increased compressive strength of air-cured samples when NaOH concentration is increased, as discussed later in this article. Therefore, it is important to note that while high slag content reportedly supports decrease of efflorescence [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e], Fig.\u0026nbsp;7 shows that it this doesn\u0026rsquo;t apply to apply to relatively low NaOH concentration (6 mol/L). The susceptibility of mortar with AAS or more than 50% GGBS was also reported by Yao [\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e]. However, in the present study, although GGBS-based mortar was vulnerable to efflorescence, that effect was dependent on the activator concentration.\u003c/p\u003e\n \u003cp\u003eSimilar to mortar with 100%GGBS activator, ambient-cured mortar with 50%GGBS\u0026thinsp;+\u0026thinsp;50% fly ash binder exhibited highest efflorescence when the activator concentration was 6 mol/L as shown in Fig. 8(a). It is likely that 6 mol/L didn\u0026rsquo;t provide sufficient alkalinity to drive geopolymerization reaction, which resulted in a porous network allowing unreacted alkalis to migrate through the pores under capillary forces. Efflorescence on sample surfaces decreased markedly as NaOH activator concentrator increased to 8 mol/L (Fig. 8(b) then 10 mol/L (Fig. 8(c)). It is also noted by comparing Fig. 7 and Fig. 8 that higher deposits were observed in samples with 100% GGBS are lower compared to 50% GGBS.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e4.3 Effect of GGBS/Fly Ash Contents and NaOH Concentration on 28-day Compressive strength\u003c/h2\u003e\n \u003cp\u003eThe compressive strength was tested on 50 mm x 50 mm x 50 mm mortar samples with alkali-activated binder consisting of GGBS or combination of GGBS and fly ash as discussed earlier. The mortar specimens were produced using alkaline solutions with concentrations ranging between 6M (6 mol/L) and 12M (12 mol/L). Three distinct curing conditions were employed: (1) curing in water, (2) curing in air, and (3) an initial 7-day water curing, followed by 21 days of air curing. The specific details and corresponding compressive strengths are provided in Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCompressive strength of mortar with AASF binders after 28-days of curing in water, air, or water\u0026thinsp;+\u0026thinsp;air.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSample No\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eBinder composition\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNaOH Concentration (mol/L)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAir Cured\u003c/p\u003e\n \u003cp\u003e(MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWater Cured\u003c/p\u003e\n \u003cp\u003e(MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e7 days Water Cured \u0026amp; 21 days Air cured\u003c/p\u003e\n \u003cp\u003e(MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e24.81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.04\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e36.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e61.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e39.45\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e67.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e43.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS100F0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e62.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e45.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e26.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e24.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e18.83\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e43.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e36.98\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e41.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e59.62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e37.75\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS75F25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e47.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e55.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e42.55\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e13.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.51\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29.55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e36.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e24.25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e40.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e40.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e26.30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS50F50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e38.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e38.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e33.87\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS25F75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS25F75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e14.93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e11.48\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS25F75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e18.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e14.43\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eT22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS25F75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e32.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e25.49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e22.50\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eIncreasing the concentration of the NaOH solution increases the solution alkalinity and dissolution rate of the precursor. Therefore, increasing activator concentration increases the development of polymerization products and subsequently, the compressive strength, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e. Curing under water ensures retention of activation reactants within the sample ensuring polymerization reaction continues at the surface as well as the core of the mortar sample. As discussed in the prior section, the reaction on the surface of water-cured slag-based (S100F0) samples was manifested with pigmentation that lasted until compression test day, unlike ambient cured samples where the color disappeared within a day after casting. As a result, figure xxx shows that the 28-day compressive strength of slag-based mortar cured under water is consistently higher than that produced under the other two curing regimes. This is consistent with the findings of Collins and Sanjayan [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. In addition, 12 mol/L NaOH concentration corresponds to a decrease in strength of water-cured mortar compared to 10 mol/L concentration, possibly due to excessive dissolution of silica. Studies have consistently demonstrated the existence of an optimal dosage of the alkaline activator that maximizes the compressive strength of concrete/mortar with alkali-activated binders [\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]. The decrease in strength beyond an optimum activator concentration may be attributed to the faster dissolution and precipitation of silica depositing on particle surfaces and impeding further polymerization. On the other hand, ambient cured samples didn\u0026rsquo;t experience excessive dissolution even at 12 mol/L NaOH concentration and continued to gain strength. The optimum NaOH concentration of ambient cured slag-based mortar is higher than water-cured samples. Nonetheless, between NaOH concentration of 6 mole/L and 12 mole/L, water-cured samples exhibit higher 28-day compressive strength.\u003c/p\u003e\n \u003cp\u003eWhen 25% of the GGBS was replaced with flash to form S75F25 mortars, the peak compressive strength NaOH concentration of 10 mol/L decreased from 67.5 MPa to 61.75 MPa. This is because the compactness of the gel increases with higher GGBS content [\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e]. Furthermore, in S75F25 the 10 mol/L optimum NaOH concentration that achieved the highest strength development persisted, as show in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. Despite some shortcomings, such as relatively short setting time, S75F25 was shown to have lower sorptivity compared to S100F0 and S50F50 for a wide range of NaOH concentrations, indicating superior water transport properties and durability [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eWhen the alkali-activated binder consisted of 75% slag and 25% fly ash, ambient-cured mortar continued to develop strength beyond NaOH concentration of 12M. Water-cured mortar continued to develop high strength than ambient cured mortar. The compressive strength of S75F25 mortar cured under water is consistently higher than ambient cured mortar with the same binder. The pattern is consistent in mortar with S100F0 as well. The availability of moisture helps with both hydration and activation of slag, while ambient curing may lead to loss of moisture leading incomplete activation. It is also possible that loss of moisture at ambient conditions may lead to microcracking, especially in mortar with high content of slag [\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e]. At ambient conditions, slag dissolution takes longer, and reaction proceeds from the inner core where moisture is available and makes its way to external surface of the sample.\u003c/p\u003e\n \u003cp\u003eIncreasing slag content to 50% of the total binder (S50F50) decreases the 28-day compressive strength- compared to higher slag contents (S100F0 and S75F25, regardless of the curing condition. This is attributed to the increased content of the typically more compact C-A-S-H as GGBS is increased and consistent with published literature. It was reported that C-A-S-H gel remained the dominant geopolymerization product when GGBS was partially replaced with various fly ash contents [\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e]. Figure \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e shows that the highest compressive strength in S50F50 was around 40 MPa, corresponding to 10M NaOH concentration, lower than the 60 MPa in S75F25 samples, shown in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. This is because more time is needed to dissolve the slow-reacting fly ash whose content is higher in S50F50. The lower content of slag in S50F50 compared to F75F0 and S100F0 signifies lower amount of C-S-H is formed due to lower content of slag, and hence, relatively lower strength. The relatively lower slag content in S50F50 also means the influence of water in strength developed is decreased as it tends to influence slag, more than fly ash. Therefore, Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e shows comparable strength development for water cured and ambient cured mortar when the binder is 50% slag. For water and ambient-cured mortar, the optimum NaOH concentration yielding highest strength in S50F50 is 10 mol/L, similar S75F25 and S100F0 as discussed earlier. Increasing NaOH concentration to 12 mol/L is not accompanied by further increase in 28-day strength, but rather a relative decrease compared to the value at 10 mol/L. Several reasons were reported in the literature for the relative decrease in strength with high activator concentrations [\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e]. Gebregziabiher[\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e] reported the development of a high-density barrier in mortar at elevated activator concentrations that limit diffusion of reactants and curbs strength development at later age.\u003c/p\u003e\n \u003cp\u003ePuertas[\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e] reported that all GGBS has completed reacted after 28 days of curing when the binder consists of 50%GGBS and 50% fly ash and the NaOH concentration is 10 mol/L. The geopolymerization product was described as a C-S-H gel with high amounts of Al in the structure. Prior studies have also shown that strength development at 27 \u003csup\u003e0\u003c/sup\u003eC is characterized by GGBS activation when its content ranged from 5\u0026ndash;50% of the total binder content, unlike curing at 60 \u003csup\u003e0\u003c/sup\u003eC which is characterized by the interaction of GGBS and fly ash [\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eWhen the content of fly ash is increased to 75% of the total binder, the strength of S25F75 mortar decreases below S50F50 regardless of curing environment or activator concentration within the 6 mol/L to 12 mol/L range evaluated. Included GGBS, even as little as 25% enhances decreases overall porosity and increases tortuosity of the AASF system, leading to a more durable system compared to pure AAF [\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e]. As shown on Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e, the optimum concentration of NaOH is not reached and mortar strength continues to increase with NaOH concentration. This due to the dominance of the slow reacting fly ash and the inability to the small amount of slag to contribute sufficiently to the 28-day strength. The maximum 28-day strength developed by ambient-cured sample was 32.18 MPa at NaOH concentration of 12 mol/L. This result is promising as no heat treatment was applied to samples with 75% fly ash and curing was done under ambient laboratory conditions. Similarly, the 25.44 MPa achieved at 10 mol/L S25F75 is also promising, indicating a balance between limited heat treatment and low NaOH concentration may achieve the desired mechanical strength. It must be noted that high fly ash content promotes the development of more N-A-S-H, which is typically more porous with reduced transport resistance [\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eTherefore, comparing Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e, and \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e, it can be stated that at any NaOH activator concentration, increasing GGBS content increases the 28-day compressive strength of water-cured mortar [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e], which was consistent with published literature and attributed to the increase of the more compact C-A-S-H [\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e]. Therefore, it can be concluded AAS favors water curing to develop mechanical strength than ambient curing. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, water-cured mortar samples with 100%GGBS binder (S100F0) developed the highes 28-day strength compared to S75F25, S50F50, and S25F75, regardless of NaOH concentration. Samantasinghar [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e] reported the same pattern for mortar with NaOH concentration of 8 mol/L. Deb [\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e] reported that mortar with higher GGBS content developed higher compressive strength up to the age of 180 days. The existence of an optimum NaOH molarity for compressive strength of mortar when binder dominated by GGBS binder (S100F0 and S75F25) is clear. The strength increases with molarity, then decreases with further increment of NaOH concentration, as discussed earlier.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;13(a) shows that when NaOH concentration is the lowest in the study (6 mol/L), the compressive strength decreased as the curing environment changed from ambient (A) to Water (W), then 7 days under followed by 21 days in ambient conditions (WA), regardless of the relative binder content. The exception to the pattern in Fig.\u0026nbsp;13(a) is mortar with 100% GGBS binder, where W curing had the highest strength compared to A and WA curing. This is because ambient curing was associated with significant efflorescence under low NaOH concentration as discussed earlier in this article.\u003c/p\u003e\n \u003cp\u003eMortar samples that contain fly ash favor ambient curing rather than water curing, regardless of NaOH concentration. Figure\u0026nbsp;13 (a) for 6 mol/L NaOH concentration shows that for samples with fly ash (S25F75, S50F50, S75F25), ambient cured mortar developed higher 28-day compressive compared to water-cured samples or those cured in water than in air. Since S100F0 didn\u0026rsquo;t contain any fly ash, it favors water curing as mentioned earlier, which produced higher strength than ambient curing.\u003c/p\u003e\n \u003cp\u003eThe relationship between ambient curing and fly ash content is influenced by NaOH concentration. Figure\u0026nbsp;13(b) shows that for NaOH concentration of 8 mol/L, as fly ash content is increased, the relative strength of ambient-cured mortar to water cured mortar also increases. For instance, the strength of ambient-cured S25F75 samples is greater than those cured in water. When NaOH concentration is increased to 10 mol/L the strength of both ambient cured S25F75 and S50F50 exceeded that of water-cured counterparts. The pattern continues as NaOH concentration is further increased to 12 mol/L.\u003c/p\u003e\n \u003cp\u003eFigures\u0026nbsp;13(a) to (d) show that S100F0 developed higher compressive strength under ambient curing compared to the other binder combinations when NaOH concentration was 10 mol/L or 12 mol/L. However, for NaOH concentration of 6 mol/L, S100F0 developed lower strength than S75F25 and S50F50, and less than S75F25 when NaOH concentration is 8 mol/L. This is attributed to the higher efflorescence inflicting ambient-cured GGBS-dominated samples, especially S100F0 developed using NaOH concentrations of 6 mol/L and 8 mol/L.\u003c/p\u003e\n \u003cp\u003eFigures\u0026nbsp;13(a) to (d) show that the optimum NaOH concentration where mortar develops the highest compressive strength is 10 mol/L. This is the case for mortars with S100F0, F75F25, and S50F50. Samantasinghar and Singh[\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e] reported an optimum NaOH concentration of 8 mol/L, which was noted for GGBS content from 0 to 100%. NaOH concentration beyond the optimum value causes excessive dissolution and leaching of silica from the precursor leading to congealing of particle surfaces and subsequent delay of the geopolymerization process. No optimum NaOH activator concentration was observed in mortar with low GGBS content (S25F75) as higher alkalinity is necessary to dissolve the slow reacting fly ash which exists in larger quantity. Rattanasak[\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e] also reported an increased leaching of Si\u003csup\u003e4+\u003c/sup\u003e and Al\u003csup\u003e3+\u003c/sup\u003e after 10 minutes of mixing in fly ash-based pastes when NaOH concentration increased from 5 mol/L to 10 mol/L, which supports the increase in strength shown in Figs. 13(a) to (d) for the mixes with 25% GGBS\u0026thinsp;+\u0026thinsp;75% fly ash binders. In the present study, Fig. 13 supports that leaching and strength development at the age of 28 days continues to increase as NaOH concentration is increased further to 12 mol/L. Along with activator concentration, the curing environment also influences strength development, especially when NaOH exceeds 12 mol/L [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e]. Increasing NaOH concentration from 10 mol/L to 12 mol/L also increases resistance to chloride ion penetration, carbonation, and acid attack [\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eThe mix with 50%fly ash\u0026thinsp;+\u0026thinsp;50% GGBS is of particular interest from a professional practice perspective as it develops reasonable compressive strength at ambient conditions for NaOH higher than 6 mol/L. In addition, prior studies have shown acceptable setting time and workability, without retarding admixtures or special mixing methods [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eThe compressive strength of mortar with 75% fly ash is low even at the highest NaOH concentration of 12 mol/L. Guo [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e] reported a significant enhancement when mortar with AAF binder is heat cured at 75 \u003csup\u003e0\u003c/sup\u003eC and the optimum NaOH concentration in such a case is 10 mol/L.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study evaluated the effect of NaOH activator concentration on compressive strength development of mortar with alkali-activated ground granulated blast furnace slag (GGBS) and fly ash. The binder, which consists of GGBS, 75% GGBS + 25% fly ash, 50% GGBS + 50% fly ash, or 25%GGBS + 75% fly ash, was activated using a combination of NaOH and Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e. The ratio of NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e was maintained at 1.5 in tests. Samples were cured for 28 days under ambient conditions, under water, or were kept for 7 days under water followed by 21 days in ambient conditions. The following observations were made:\u003c/p\u003e\u003cp\u003e \u003c/p\u003e\u003cul\u003e \u003cli\u003e \u003cp\u003eWhen the binder used was alkali-activated GBBS or GGBS and fly ash with a ratio of 3:1, mortar cured under water for 28 days developed higher compressive strength compared to samples cured under ambient conditions, regardless of NaOH concentration. However, when the binder was 50% GGBS + 50% fly ash, mortar cured under ambient conditions or under water developed comparable strength. However, when the GGBS:fly ash ratio was 1:3, mortar cured under ambient conditions developed higher strength.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn mortar with high GGBS binder content (100% GGBS and 75% GGBS + 25% fly ash), the 28-day compressive strength increases as NaOH concentration increases from 6 mol/L to 10 mol/L. However, further increase of NaOH concentration to 12 mol/L decreases the 28-day compressive strength of mortar relative to 10 mol/L concentration. Ambient-cured mortar samples with dominant slag binder do not exhibit the 10 mol/L optimum concentration and continue to gain strength with increase in NaOH concentration but remains lower in strength than mortar cured under water.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eWhen the amounts of GGBS and fly ash binder are equal (50% GGBS + 50% fly ash), ambient and water-cured mortar samples develop comparable strength. However, when the ratio of GGBS:fly ash becomes 1:3, ambient-cured mortar samples produce higher 28-day strength than water-cured samples. It appears that GGBS favors water curing while fly ash produces higher strength in air-cured mortar samples.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAmbient-cured mortar samples with 100%GGBS activated with 6 mol/L NaOH solution developed visible efflorescence, degradation, and decrease in compressive strength compared to water cured samples. This is likely due to due to loss of moisture from the activator solution and precipitation of the activator salts, along with carbonation due to ingress of CO\u003csub\u003e2\u003c/sub\u003e through pores. However, efflorescence was not observed when NaOH was higher than 6 mol/L, likely due to the increased alkalinity of the activator solution which resulted in rapid formation of the more compact C-A-S-H. A more compact pore structure in the outer surface impedes the progression of Na\u003csup\u003e+\u003c/sup\u003e through the pores to combine with atmospheric CO\u003csub\u003e2\u003c/sub\u003e and form carbonate slats.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eEfflorescence was not observed in any sample cured under water regardless of NaOH concentration or binder composition. This is attributed to retention of activator of solution as water provided a closed curing medium, which support continued geopolymerization reaction, while blocking access of CO\u003csub\u003e2\u003c/sub\u003e to sample.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eO. M. conceptualization, writing the original draft, fund acquisition.O. N. Experimental program, validation, edit draft.S. S. Figures preparation, reference organization.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors gratefully acknowledge the financial support provided by the Office of Research and Sponsored Programs (ORSP) at Abu Dhabi University under grant No. 19300899.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData is provided within the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eO.A. Mohamed, O. Najm, E. Ahmed, Alkali-activated slag \u0026amp; fly ash as sustainable alternatives to OPC: Sorptivity and strength development characteristics of mortar, Cleaner Materials 8 (2023) 100188. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.clema.2023.100188\u003c/span\u003e\u003cspan address=\"10.1016/j.clema.2023.100188\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eO.A. Mohamed, R. Al-Khattab, W. 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Zuaiter, Setting time, sulfuric acid resistance, and strength development of alkali-activated mortar with slag \u0026amp; fly ash binders, Results in Engineering 21 (2024). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.rineng.2023.101711\u003c/span\u003e\u003cspan address=\"10.1016/j.rineng.2023.101711\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"sustainable concrete, compressive strength, efflorescence, workability, activator concentration, slag, fly ash, sodium hydroxide, sodium silicate","lastPublishedDoi":"10.21203/rs.3.rs-5723404/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5723404/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe global growth in infrastructure projects exacerbates the need for ordinary Portland cement (OPC) or other similarly effective binder. The construction industry in general and the production of OPC in particular are responsible for significant contributions to CO\u003csub\u003e2\u003c/sub\u003e emissions into the atmosphere. Ground granulated blast slag (GGBS) and fly ash are industrial byproducts that can be recycled and reused as sustainable alternative binders to OPC to produce concrete. This article evaluated the effect of NaOH activator concertation on the development of 28-day compressive strength of mortar that uses combinations of GGBS and fly ash as binders and activated using Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e and NaOH. The Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e content was kept constant while NaOH concentration varied from 6 mol/L to 12 mol/L. Three groups of samples were cured in different environments including: 1) immersion in water, 2) ambient conditions, or 3) 7 days of curing under water then 21 days in ambient conditions. Mortar cured under water produced higher compressive strength when GGBS content exceeds 50% of the total binder content, compared to ambient curing. However, when GGBS content was 50% or less of the total binder, the strength of mortar cured under water was comparable to or lower than those cured in ambient conditions. An optimum NaOH concentration of 10 mol/L produced the highest 28-day compressive in mortar with 75% or 100% GGBS binder. Further increase in NaOH concentration resulted in lower compressive strength than mortar produced with 10 mol/L activator concentration. Efflorescence and strength degradation were manifested in ambient-cured mortar samples with slag binder that was activated using relatively low NaOH concentration. Increasing NaOH concentration beyond 6M decreased or eliminated efflorescence and strength degradation in ambient-cured mortar.\u003c/p\u003e","manuscriptTitle":"Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-27 08:50:52","doi":"10.21203/rs.3.rs-5723404/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5b1c85f1-c81c-4d4e-b789-1b20139e43dc","owner":[],"postedDate":"February 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-02-27T08:50:52+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-27 08:50:52","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5723404","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5723404","identity":"rs-5723404","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}