Resistance to acid degradation, sorptivity, and setting time of geopolymer mortars

Abstract

Experimental evaluations were conducted to determine the water sorptivity, setting time, and resistance to a highly acidic environment, of mortar with alkali-activated ground granulated blast furnace slag (GBS) binder and also of combinations of fly ash and GBS binders. Binders were activated using mixtures of NaOH and Na₂SiO₃ solutions. The molarity of NaOH in the mixtures ranged from 10 mol·L⁻¹ to 16 mol·L⁻¹, and the Na₂SiO₃/NaOH ratio was varied from 1.5 to 2.5. Mortar samples were produced using three binder combinations: 1) GBS as the only binder; 2) blended binder with a slag-to-fly ash ratio of 3:1; and 3) mixed binder with 1:1 ratio of slag to fly ash. Mortar samples were mixed and cured at (22 ± 2) °C till the day of the test. The impact of activator solution alkalinity, activator ratio Na₂SiO₃/NaOH, GBS content on the rate of water absorption were evaluated. After 7, 28, and 90 d of immersion in a 10% sulfuric acid solution, the resistance of a geopolymer matrix to degradation was assessed by measuring the change in sample weight. The influence of solution alkalinity and relative fly ash content on setting times was investigated. Alkali-activated mortar with a slag-to-fly ash ratio of 3:1 had the least sorptivity compared to the two other binder combinations, at each curing age, and for mortars made with each of the NaOH alkaline activator concentrations. Mortar sorptivity decreased with age and sodium hydroxide concentrations, suggesting the production of geopolymerization products. No reduction in weight of sample occurred after immersion in the strong acid H₂SO₄ solution for three months, regardless of binder combination. This was due to the synthesis of hydration and geopolymerization products in the presence of curing water, which outweighed the degradation of the geopolymer matrix caused by sulfuric acid.

Graphical abstract

Keywords

alkali-activated materials / fly ash / sorptivity / durability of concrete / sodium hydroxide / sodium silicates / reduction in CO ₂ emissions / sulfuric acid

1 Introduction

A crucial design/evaluation criterion for structural utilizing ordinary Portland cement (OPC) or other more sustainable binders is their capacity to withstand a harsh environment without a significant reduction in strength either at an early age or over time. Studies have shown that extended exposure to a severe climate that induces a continuous alkali-aggregate reaction or cyclic freezing and thawing can result in a significant loss in strength and the inability of structural elements to support applied loads [1]. The permeability of water through concrete is mostly governed by its pore structure and capillary absorption. Water can transfer corrosive chlorides through interconnected concrete pores to reach and degrade steel reinforcing [2]. Transport of chlorides through concrete is dependent on pore structure type, pore system connectivity, and the presence/severity of microcracks. Fly ash and slag have been researched and used extensively as partial replacements of OPC, and have been shown to improve the fresh characteristics, strength, and durability of concrete [3]. Therefore, in the present study, fly ash and ground granulated blast furnace slag (GBS) were tested as alkali activated mortar binders. GBS and fly ash offer sustainable alternatives to OPC and a potential to construct long-lasting infrastructure.

Collins and Sanjayan [4] proved that the sorptivity of concrete containing an alkali-activated GBS binder is dependent on temperature and curing procedure. When samples are maintained unsealed after casting (exposed to normal air) in a laboratory at 230 °C and 50% relative humidity (RH), the sorptivity increases at a low rate with curing age. In contrast, the sorptivity of all samples reduced when they were cured by immersion in water or when they were sealed and stored in a lab at room temperature.

Allahverdi and Škvára [5] described the sulfuric acid attack on mortar with activated slag-fly ash binder as a two-step process. The initial stage comprises an exchange of ions between cations, such as sodium and calcium, and H⁺ and H₃O⁺ in the solution, as well as an acid assault on the Si-O-Al bonds. This process results in ejection of aluminum (dealuminization) from the aluminosilicate gel. In the second stage, calcium combines with acid-emitted sulfates to generate calcium sulfates, which deposit on the corroded layers. In very acidic settings, the deposition of gypsum crystals produces a protective layer that prevents further matrix breakdown, according to the authors’ premise.

Curing temperature and the ratio of alkaline activator Na₂SiO₃/NaOH were shown to influence the rate of water absorption in mortars containing fly ash binder activated with sodium silicates and sodium hydroxide solutions. When samples of fly ash mortar were initially cured for a period of 24 h at a temperature of 100 °C, they absorbed less water during testing than those initially cured at 70 °C. The effect of temperature curing on water absorption remained the same for all measured alkaline activator Na₂SiO₃/NaOH ratios, including 1.0, 1.5, 2.0, 2.5, and 3.0.

Nedeljković et al. [6] concluded that the resistance to carbonation of mortar increases with the content of GBS, when the binder is a blend of fly ash and GBS activated using an alkaline solution. The pattern was consistent in each of the curing methods employed in their study. When the alkali activated binder consisted of 100% GBS, the pores of the sample were so small in size that CO₂ could not diffuse through them.

Sorptivity of fly ash-based alkali-activated mortar decreased proportionally as fly ash is replaced with 0 to 20% GBS [7]. Along with the decrease in sorptivity, the compressive strength increased. After mortar curing for 28 and 90 d, the sorptivity of the geopolymer blends containing 20% GBS was consistently much lower than that of conventional concrete. The ratio of liquid to binder was 0.55, while the ratio of Na₂SiO₃ to NaOH was 2.5. XRD research revealed that mixtures containing 20% GBS had a more refined microstructure, smaller pores, and fewer cracks than those containing 100% fly ash.

Blended GBS and fly ash binders have been found to provide advantages in terms of concrete compressive strength and durability, compared to conventional OPC-based concrete. Key GBS/fly ash combinations with promising fresh and hardened concrete qualities include binders with equal amounts of GBS and fly ash and binders with GBS-to-fly ash ratio of approximately 3 to 1 [8]. Although GBS enhances early strength development and acid resistance of mortar/concrete, the addition of fly ash could alleviate some of the disadvantages of utilizing GBS, such as rapid setting and high shrinkage [8]. Using alkali activated blended GBS-fly ash binders improved mortar compressive strength for water-cured samples at 28 and 90 d compared to samples that use 100% GBS as the binder [9,10].

Exposure to acids was identified as a source of strength and mass reduction in OPC-based concrete. When sulfuric acid with low pH comes into contact with concrete, it decreases the pH of the concrete, which is initially alkaline with relatively high pH. If the pH of concrete falls below hydrates stability limit, calcium is lost and the hydrates convert into an amorphous hydrogel. The reaction products of an acid attack include calcium salts of the acid and other hydrogels. When concrete is exposed to acid, the naturally soluble calcium hydroxide decomposes rapidly. This is accompanied by the accumulation of considerable amounts of gypsum, which causes surface scaling and concrete softening.

When concrete is exposed to or submerged in an acid-rich environment for an extended period of time, its compressive strength may be significantly diminished [11,12]. Mass loss due to prolonged acid exposure increases as binder content rises [13]. Ren et al. [14] showed, based on a theoretical investigation, that the long-term degradation of GBS/fly ash concrete owing to sulfuric acid exposure could be 52 to 60 percent lower than that of OPC-based concrete.

The dense structure of C-A-S-H produced by geopolymerization of slag could impede penetration of sulfate ions through mortar. Zhang et al. [15] reported that sulfuric acid had no effect on mortar that employs alkali-activated GBS as a binder, and that development of the intrinsically soluble gypsum took place only on the surface of samples. Similarly, in mixtures of fly ash and GBS, an increase in GBS content reduced the negative effects of sulfuric acid. This could be attributed to the finer pore structure and denser matrix of GBS hydration and geopolymerization products.

Immersion of mortar samples in 5% sulfuric or acetic acid, when the binder is activated fly ash, led to a significant decrease in compressive strength and a reduction in mass [16]. Activating the fly ash precursor was done using a combination of NaOH and KOH solutions and sodium silicate. The acidic media led to depolymerization of aluminosilicates and production of zeolites, as detected by X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) images. Gu et al. [17] observed a significant reduction in strength and mass of mortar samples immersed in sulfuric acid for 492 d, when the binder was fly ash activated using alkaline solution.

According to Lee and Lee [18], the degradation of mortar with activated slag-fly ash blended binder after immersion in 10% H₂SO₄ solution is caused by the penetration of {\mathrm{S}\mathrm{O}}_4^{2-} through mortar samples. The liquid penetrability of concrete and mortar is dependent on the air void system and water sorptivity. The authors of that study contended that the fly ash-based geopolymerization product (N-A-S-H) was less susceptible to acid attack than the GBS-based product (C-A-S-H).

Tortuosity, which is defined as the ratio of actual fluid flow path to a hypothetical straight path between end points of a porous material, determines the amount of water ingress through mortar/concrete and influences its endurance. It has been demonstrated that increasing the GBS concentration in fly ash/GBS binders to more than 50% increases the tortuosity and decreases the porosity of mortar [16]. The higher space-filling capabilities of GBS are attributable to C-A-S-H, which exceeds N-A-S-H derived from fly ash.

Calcium supplied by the precursor in alkali-activated GBS/fly ash mortars, even in minute quantities, significantly accelerates the setting and hardening of geopolymerization products [19]. It was believed that quick precipitation of C-S-H offers nucleation sites, which subsequently accelerates the development of geopolymerization products throughout the mixture, resulting in the setting of mortar.

C-N-A-S-H was found as the binding gel structure when the alkali activated binder is a combination of slag and fly ash, provided that slag comprises at least 50 percent of the total binding material. Ismail et al. [20] stated that when fly ash content surpasses GBS, the primary binding phase is N-C-A-S-H with a higher degree of cross-linking than in C-N-S-H gel.

Degirmenci [21] determined the resistance of alkali activated fly ash and slag, and natural zeolites to sulfuric acid by measuring the weight change after 24 weeks of immersion in sulfuric acid. Compared to fly ash and zeolites, mortar samples containing alkali activated GBS showed the least weight loss and the highest residual strength following 24 weeks of exposure to 10% sulfuric acid. In terms of acid resistance, mixtures of GBS and fly ash are advantageous since they produce concrete/mortar that does not require curing at elevated temperatures, but they perform halfway between the good performance of GBS and the comparatively lower performance of fly ash. It was also observed that, following exposure to sulfuric acid, the residual strength of mortar with activated slag as binder improved with an increase in the alkaline activator Na₂SiO₃/NaOH.

This study’s objective is to determine the effects of relative slag and fly ash contents in the alkali activated binder mix, the molarity of NaOH alkaline activator (a measure of solution alkalinity), and the alkaline activator ratio (Na₂SiO₃/NaOH), on mortar sorptivity and resistance to degradation of density due to sulfuric acid.

2 Methodology

The goal of the experimental program is to evaluate alkali-activated mortar samples’ resistance to sulfuric acid degradation, sorptivity, and initial/final setting periods, taking into consideration the influence of relative GBS/fly ash and activator solution alkalinity.

Sorptivity of mortar with activated GBS-fly ash binders was assessed using the experimental approach originally proposed by Hall [22], and adopted in modified form by ASTM C1585 [23]. The purpose of the test was to quantify the water absorption of OPC-based concrete. Nonetheless, it has been demonstrated that the connectedness of the pore system, which determines the transport characteristics of OPC-based mortars and mortars that use alkali activated binders, is comparable [24,25]. The test determines the change in mass of mortar sample due to water absorption through one surface during the immersion time, when the unsealed surface is 1 to 2 mm below water level. The rate of absorption, which is generally considered as an indicator of the pore system’s continuity in geopolymer mortar samples, is calculated using sample weights measured at set time intervals. The details of the experimental program and methodology are published elsewhere [8,26,27].

2.1 Material characteristics and blend proportions

Using X-ray fluorescence (XRF) spectrometry, the chemical composition of GBS and fly ash employed as binders in this work was determined, and the results are presented in Tab.1. The percentages of CaO, SiO₂ + Al₂O₃ + Fe₂O₃, and SO₃ are within the ASTM C267 [28] specifications for class F fly ash.

GBS and fly ash precursors were activated using a mixture of NaOH and Na₂SiO₃ alkaline solutions. To attain the requisite molarity, the sodium hydroxide solution was prepared by dissolving sodium hydroxide flakes in distilled water. Mortar samples were divided into four groups according to their NaOH molarity: 10, 12, 14, and 16 mol·L⁻¹. These four molarities of NaOH were utilized to examine the influence of solution alkalinity on setting time, sorptivity, and resistance to mass decrease owing to sulfuric acid attack. In order to evaluate the impact of activator ratio on sorptivity and sulfuric acid resistance, three variants of each NaOH mixture were formulated with Na₂SiO₃/NaOH ratios of 1.5, 2.0, and 2.5. Therefore, twelve variants of the alkaline activator solution were produced, each with a different ratio of NaOH to activator. To explore the impact of the relative amount of slag or fly ash on mortar properties, three binder combination groups were created: 50% slag + 50% fly ash (G50F50), 75% slag + 25% fly ash (G75F25), and 100% slag (G100). Mortar mixtures were formulated so that each binder (G50F50, G75F25, and G100) was activated by each of the twelve alkaline activators, resulting in a total of 36 mixtures shown in Tab.2.

The ratio of sand-to-binder of 2.75 and the ratio of activator solution-to-binder of 0.55 were maintained for all of the 36 mixtures. Several experimental mixtures were evaluated until the optimal polycarboxylic ether superplasticizer dosage of 2.5% by weight of the binder was determined. MasterGlenium SKY 504 is the trade name of the superplasticizer provided by BASF Corporation. The superplasticizer was applied directly to the activator solution during sample preparation.

2.2 Method of experimentation-sorptivity

The 100 mm × 50 mm mortar specimens were placed in molds for 24 h after mixing and casting. The samples were then removed from the molds and stored (exposed to air) at room temperature in the laboratory until test day, which occurred at 7, 28, and 90 d after preparation. On the day of the test, each sample was oven-dried to remove moisture and then sealed thoroughly on all sides with the exception of the underside. Samples were weighed on a 0.01 g scale. After recording the dry weight, the unsealed base of the sample was placed in the water tank with 1 to 2 mm of the specimen submerged, as depicted in Fig.1. The specimens were then withdrawn from the water tank and weighed at predetermined intervals, beginning with one minute, before being returned to the water tank and immersed again. After 5, 10, 20, and 30 min, measured from the time of first immersion, the process of removing, weighing, and replacing the sample in the water tank was repeated. The procedure was then repeated every hour, beginning with the time of the initial immersion.

At any mass measurement time, the absorption, I, of the test specimen is a function of the change in mass, the exposed surface area of the specimen, and the density of water (0.001 g/mm³). Absorption is given by Eq. (1) [23].

(1)I=\frac{m_t}{0.001a},

wherem_t is the change in mass (grams) of mortar sample at a predetermined time t following immersion in water;a is the specimen’s exposed area (mm²).

When concrete absorption, I, is plotted against the square root of the period at which the change in mass is computed, a bilinear relationship represented by Eq. (2) has often been observed [23]. The validity of Eq. (2) is confirmed in Ref. [29]. The relationship between water absorption and the square-root of measurement time is typically bilinear. The first linear segment of the bilinear relationship corresponds to the initial 6 h of absorption measurements.

(2)I=S_{\mathrm{i}}\sqrt{t}+b,

whereS_i is initial absorption rate (mm· {\mathrm{s}}^{-\frac{1}{2}}).

The magnitude of the initial absorption rate, S_i, was calculated and used as a measure of the sorptivity of mortar samples prepared using the control parameters, such as the relative content of ground granulated blast furnace GBS/fly ash and the molarity of sodium hydroxide solution, specified in this study.

2.3 Experimental procedure for sulfuric acid resistance

In line with ASTM C267 [28], the resistance of alkali-activated mortar to sulfuric acid assault was tested by estimating the change in mass of samples submerged in sulfuric acid. A decrease in mass of mortar sample after submersion in sulfuric acid, if any, is indicative of matrix damage caused by dealumination and/or leaching. After casting the 20 mm × 20 mm × 20 mm samples of mortar, they were left in their molds for 24 h before being removed and their initial weights recorded. Until the day of the test, mortar specimens were submerged in 10% sulfuric acid (H₂SO₄). After 7, 28 or 90 d, samples were withdrawn from the solution and completely dried. The surface-dried samples were weighed using a 0.01 kg scale, and the weight change before and after immersion was noted. The number of cubes cast was sufficient for the average of three samples to be given. The percentage change between each sample’s weight before and after submersion was estimated using Eq. (3):

(3)weigh tchange \left(\mathrm{\%}\right)=[\left(W_{\mathrm{s}\mathrm{u}\mathrm{b}}-W_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}\right)/W_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}]\times 100,

where W_init = weight (g) of specimen prior to immersion in acidic solution; W_sub = weight (g) of specimen after immersion in sulfuric acid solution until test date.

2.4 First and last setting times

The setup time was obtained using a standard Vicat needle and ASTM C191. This study focuses on analyzing the impact of relative amounts of GBS/fly ash and activator molarity on initial and final setting times.

3 Results and discussion

3.1 Effect of solution alkalinity on sorptivity of mortar

Fig.2 displays the initial rate of water absorption, S_i ( \mathrm{m}\mathrm{m}\cdot {\mathrm{s}}^{-\frac{1}{2}}), for mortar samples made using slag as the sole binder. Fig.2(a)–2(d) represent initial water absorption for mortar with NaOH molarities of 10, 12, 14, and 16 mol·L⁻¹, respectively. The figures depict the change in sorptivity with curing age for each of the 1.5, 2.0, and 2.5 alkaline activator ratios. These figures also demonstrate that the sorptivity of G100 mortars generally decreases when the Na₂SiO₃/NaOH ratio increases. Furthermore, at any of the four NaOH molarities, sorptivity declined with curing age for each activator ratio, regardless of the NaOH molarity (10, 12, 14, and 16 mol·L⁻¹). The reduction in sorptivity shows that GBS hydration and geopolymerization products, such as C-A-S-H, significantly reduced mortar pore system connectivity. In conjunction with the effect of silicates provided by the dissolved GBS, a rise in the Na₂SiO₃/NaOH ratio enhanced the supply of silicates from the activator solution, hence promoting the formation of pore-filling calcium-silicate-hydrate and calcium-aluminate-silicate-hydrate.

Fig.3(a)–3(d) demonstrate the effect of the alkaline activator ratio, Na₂SiO₃/NaOH, on the sorptivity of G50F50 mortar prepared using NaOH molarity of 10, 12, 14, and 16 mol·L⁻¹, respectively. Regardless of the molarity of sodium hydroxide solution or Na₂SiO₃/NaOH ratio, it is evident that alkali-activator mortar sorptivity decreases from one curing age to the other, due to the development of geopolymerization products.

Fig.3 shows that at each of the three curing ages, mortar samples prepared using alkaline solution with activator ratio of 2.0 had the least sorptivity compared to Na₂SiO₃/NaOH of 1.5 and 2.5. The pattern applies to G50F50 samples made with NaOH molarities of 10, 14, and 16 mol·L⁻¹. This ratio appears to give the optimal supply of free sodium (Na) for the production of polymerization products such as N-A-S-H, since 50% of the total binder consists of fly ash precursor. On the other hand, hydration of the remaining 50% GBS produces the denser and more compact pore structure that is characteristic of calcium aluminosilicate hydrate (C-A-S-H) and leads to a considerable decrease in sorptivity. The silica given by the Na₂SiO₃ alkaline activator solution complements the Si⁴⁺ and Al³⁺ leaching from the precursor to create sodium-aluminate-silicate hydrate, together with Na²⁺ from the alkaline activator. However, excess supply of silica beyond an optimal value produces a congealing action that may hinder further ion leaching and geopolymerization product formation [30]. Rattanasak and Chindaprasirt [31] also recognized and documented the existence of an optimum Na₂SiO₃/NaOH ratio, which was represented in a decrease in compressive strength when the alkaline activator ratio was larger than or less than the optimum value. Fig.3(b) demonstrates that G50F50 mortar with binder activated using 12 mol·L⁻¹ NaOH solution was the exception. This mortar sample exhibited a lower sorptivity when the alkaline activator ratio was 1.5 as compared to 2.0, which may be associated with solution instability. Further studies are needed to understand sorptivity of G50F50 mortar prepared using 12 mol·L⁻¹ NaOH activator solution.

Fig.4 illustrates the change in sorptivity with curing age for G50F50, G75F25, and G100 mortar samples that were activated using NaOH solutions with molarities from 10 to 16 mol·L⁻¹ in increments of 2 mol·L⁻¹. G75F25 exhibited the lowest sorptivity at all three curing ages (7, 28 and 90 d). G75F25 also had the lowest overall sorptivity in mortar samples made with 10, 12, or 14 mol·L⁻¹ NaOH molarities.

When the fly ash level is high, solution alkalinity promotes the dissolution of Al³⁺ and Si⁴⁺ ions from the aluminosilicate binder due to the catalytic effect of OH^–1 in the alkaline solution [30]. When the concentration of NaOH is increased, more Na²⁺ cations are available to interact with Si⁴⁺ and Al³⁺ to produce sodium aluminate silicate hydrate. Therefore, Fig.4 shows that G50F50 mortars exhibited the least sorptivity after 7, 28, and 90 d of curing, compared with those of G100 and G75F25, when NaOH molarity was 16 mol·L⁻¹. This was due to the effective filling of the pore system as fly ash dissolved in the extremely alkaline solution and formed sodium-silicate-hydrate gel. The dependence of fly ash solubility on NaOH concentration has been reported in the literature [31].

From the standpoint of the molarity of NaOH alkaline activator, Fig.4(a)–Fig.4(d) demonstrate that for each molarity (10 to 16 mol·L⁻¹), sorptivity reduced when sample curing age increased from 7 to 90 d. This pattern holds true for all three binder configurations (G50F50, G75F25, and G100). This decrease in sorptivity with increasing age of curing is indicative of the accumulation of hydration and polymerization products. In mortars made using 10, 12, and 14 mol·L⁻¹, the sorptivity of G50F50 was greater than that of G75F25, as depicted in Fig.4(a)–Fig.4(d). This may be a result of the naturally slow dissolution of fly ash, which slows the formation and growth of polymerization products and subsequently delays the optimization of pore structure connectivity. Nonetheless, despite the existence of 25% fly ash, the sorptivity of G75F25 was lower than that of G100, as depicted in Fig.4(a)–Fig.4(c). This phenomenon occurred consistently at every curing age and each of the four molarities. Since sorptivity was directly related to pore system connectivity, it is likely that cross-linking was more effective when fly ash content was less than GBS, which was the case in the present study. It is also likely that the slag-to-fly ash ratio of 3 to 1 was the most effective in reducing sorptivity NaOH molarities between 10 and 14 mol·L⁻¹. It has also been reported that the slag-to-fly ash ratio of approximately 3:1 leads to higher compressive strength at the ages of 28 and 90 d, in contrast with other slag-to-fly ash ratios reported in various studies [8,9,10,32].

3.2 Effect of mortar immersion in H₂SO₄ solution on the stability of mortar mass

The percentage change in weight of specimens immersed in a 10% H₂SO₄ solution for 7, 28, and 90 d is depicted in Fig.5. Consistently with the findings of Marjanović [32], increasing the ratio of alkaline activator from 1.5 to 2.5 increased the mortar sample weight. G100, G75F25, and G50F50 all followed this pattern. Similarly, sample weight rose for each alkaline activator ratio (1.5, 2.0, or 2.5) in each of the three binders (G50F50, G75F25, and G100) from one curing age to the next. Consequently, immersion in sulfuric acid did not result in a loss in mass; rather, water in the liquid provided a closed curing environment and resulted in a mass gain due to the synthesis of hydration and geopolymerization products. The lone exception was G100, where the change in mass after 28 d of submersion appears to be a test outliner owing to solution instability, but is recorded here to illustrate this potential test problem.

Mortar samples cured in a sulfuric acid solution, as depicted in Fig.6(a), were visually evaluated for obvious damage, as depicted in Fig.6(b). There was no change in color scaling or softening in any of the examined samples.

3.3 Influence of sodium hydroxide concentration on setting times

From the time it is mixed until it is placed, concrete must retain its flowability. Similarly, it is crucial for concrete to attain appropriate strength within a reasonable timeframe so that formwork may be removed and construction can continue as planned. Therefore, the initial and ultimate setting times are essential characteristics for determining the appropriateness of alkali-activated concrete. This article discusses the influence of NaOH molarity and relative GBS/fly ash contents in binder.

Fig.7 illustrates the setting times of G50F50, G75F25, and G100 mortar samples prepared using the four-sodium hydroxide molarities. Setting times decreased consistently with increasing sodium hydroxide molarity. The initial setting time decreased from 20.33 min in mixes with 10 mol·L⁻¹ NaOH concentration, to 11.5 min in mixes with 16 mol·L⁻¹ NaOH concentration. The ultimate setting time for G100 mortars was 43.67 min when NaOH molarity was 10 mol·L⁻¹ and 26.17 min when NaOH molarity was 16 mol·L⁻¹, indicating that increasing solution alkalinity decreases the setting time of alkali activated GBS mortar. When the solution alkalinity of G100 mixtures was increased by raising the NaOH concentration, the rate of GBS dissolution rose, which accelerated the calcium supply and formation of calcium silicates gel. Setting times can be reduced when calcium silicate gel is formed rapidly. As indicated in Tab.1, GBS precursor is rich in calcium in the form of CaO. In addition, the rapid dissolution of aluminates and silicates in the precursor, along with presence of calcium, has been found to change the framework and accelerate the formation of reaction products [33]. Calcium dissolution from mortars containing alkali-activated GBS at low alkalinity results in the formation of C-S-H and C-A-S-H, which promotes early strength development [19] and rapid setting. This is consistent with the trend in setting time shown in Fig.7(a) and 7(b).

Mortars containing an alkali-activated binder with a slag-to-fly ash ratio of 3:1 have superior 7-d and 28-d strengths [9,32], particularly at a NaOH molarity of 10 mol·L⁻¹. Consequently, the G75F25 mortars assessed in this study are intriguing. Although the initial and ultimate setting periods of G75F25 are longer than those of G100, additives are still necessary before this mixture may be considered for practical applications.

Fig.7(a) and 7(b) clearly demonstrate that, for each NaOH molarity, the greater the GBS concentration, the shorter the initial and final setting periods, confirming the Ref. [34]. With retarders or other additions such as microsilica, the quick setting time induced by a high GBS content can be alleviated [35].

As illustrated in Fig.7(a) and 7(b), the setting time of G75F25 and G50F50 for NaOH molarities of 10, 12, and 14 mol·L⁻¹ rose significantly when fly ash content was raised from 25% to 50% of the total binder. This is because it takes longer to dissolve and polymerize significant quantities of fly ash in the binder when the NaOH molarity is relatively low (10, 12, and 14 mol·L⁻¹). As demonstrated in Fig.7(a) and 7(b), start and final setting periods of G75F25 and G50F50 were shorter at the highest NaOH molarity (16 mol·L⁻¹) than at the lowest three molarities (10, 12, and 14 mol·L⁻¹), as greater solution alkalinities accelerate and promote fly ash-based geopolymerization product. Initial setting times for G50F50 mixtures ranged between 30 and 45 min, and final setting times ranged between 75.5 and 108.3 min.

Notably, the setting times of mortars with slag content greater than 50%, depicted in Fig.7, were shorter than those of mortar made from activated fly ash as sole binder, or with OPC [10]. The relatively short setting time of mortars containing greater than or equal to 50% GBS is caused by the rapid dissolution of GBS and release of calcium into the system, which accelerates the formation of calcium-based silicate gel. It has also been observed [36] that the initial setting time decreased according to the starting ratio of SiO₂/Al₂O₃. Higher SiO₂/Al₂O₃ ratios are typical of mixtures with a higher GBS content, with G100 mortars exhibiting the shortest setting times and G50F50 mortars the longest. In this study, the SiO₂/Al₂O₃ ratios of G100 mixtures varied between 4.33 and 4.55. Due to the presence of 25% fly ash, the range for G75F25 blends was reduced to 3.56–3.77. Due to the increase in fly ash concentration to 50% of the total binder, the range for G50F50 mixtures reduced to 3.17–3.27.

Examining Fig.7(b), it is found that raising the molarity of NaOH in G100 mortars decreased the ultimate setting time. This pattern was driven by the rapid calcium release caused by the dissolution of GBS as the alkalinity of the solution rose. The increased alkalinity has been found not to be accompanied by an increase in strength [37], as calcium generates more precipitates than calcium silicate hydrate in such instances. As illustrated in Fig.7, substituting 25% of the slag with fly ash (G75F25) improved the final setting time compared to G100 (b). As NaOH molarity grew from 10 to 16 mol·L⁻¹, however, the pattern flipped and the ultimate setting time increased. In G75F25, the addition of fly ash lengthened the setting time because the higher solution alkalinity appeared to slow the setting time and inhibit the formation of GBS-based silicate gel. In contrast to G100 and G75F25 mortars, when the fly ash content was increased to 50% in G50F50 mortars, the final setting time increased significantly; however, the tendency then reversed, and increasing the NaOH molarity decreased the final setting time. The decrease in final setting time with increasing molarity of G50F50 mortar was consistent with Lee and Lee’s [38] findings, but for a lower molarity range and a 4:1 fly ash-to-GBS ratio. Fig.7(b) demonstrates that at a molarity of 10 mol·L⁻¹ NaOH, G100 and G75F25 have a final setting time that was almost identical, but substantially shorter than G50F50. At high NaOH molarity (16 mol·L⁻¹), G50F50 and G75F25 mortars had a final setting time that was almost same, but significantly longer than G100 mortar.

4 Conclusions

This study evaluated the sorptivity, resistance to sulfuric acid, and setting time of alkali-activated mortar samples containing either slag as binder or a combination of fly ash and slag. The ratio of GBS-to-fly ash in total binder, the alkaline activator ratio (Na₂SiO₃/NaOH), and the activator solution alkalinity were considered. The analyzed alkaline activator ratios were 1.5, 2.0, and 2.5. Multiple mixtures of 10, 12, 14, and 16 mol·L⁻¹ NaOH concentrations were formulated and tested to determine the influence of solution alkalinity. To determine the impact of an acidic environment on alkali-activated mortars, samples were cured in a 10% H₂SO₄ solution for 7, 28, and 90 d, and the weight change was determined. The principal conclusions of the study are:

1) Sorptivity of mortar samples declined with curing age, from 7 to 28 d, and from 28 to 90 d, for the slag content of 50% or higher that is evaluated in this study. The pattern was consistent for each of the four solution alkalinities studied, and regardless of the alkaline activator ratio. This shows a continual filling of pore system and a reduction in pore connectivity as a result of the formation of geopolymerization products and efficient cross-linking.

2) When the solution alkalinity is relatively low to medium (NaOH = 10, 12, or 14 mol·L⁻¹), G75F25 mortars exhibited the lowest sorptivity, in contrast with G100 and G50F50. The pattern was consistent after 7, 28, and 90 d of curing. This reveals that a slag-to-fly ash ratio of 3:1 generates the most effective reduction in pore system connectivity and the strongest cross-linking at 10 to 14 mol·L⁻¹ NaOH molarities. When solution alkalinity is high (NaOH molarity = 16 mol·L⁻¹), G50F50 mortars exhibit the lowest sorptivity at each age of curing compared to G100 and G75F25. In this instance, the sorptivity of G50F50 is dominated by fly ash dissolution and geopolymerization, which are improved in extremely alkaline solutions.

3) Mortar samples containing activated slag as the sole binder demonstrated decreasing sorptivity as the sodium silicate/sodium hydroxide ratio increased. Mortar samples made with each of the four solution alkalinities exhibited the same pattern. This may be due to the silica content in sodium silicate solution, which promotes the formation of calcium-silicate hydrate. However, G75F25 and G50F50 mortars, which contain 25% and 50% fly ash respectively, did not exhibit this pattern. This is because the supply of silica from the dissolution of fly ash is slower than from slag, and more dependent on solution alkalinity.

4) G50F50 mortars demonstrated the lowest sorptivity when the alkaline activator Na₂SiO₃/NaOH ratio was 2.0. The pattern remained consistent after 7, 28, and 90 d of curing. The pattern was also consistent in mortar prepared using 10, 14, and 16 mol·L⁻¹ NaOH solutions.

5) Mortar samples containing slag or mixtures of slag and fly ash binders activated using alkaline solutions, exhibited neither apparent damage nor mass loss after being immersed in sulfuric acid for 90 d. Mass of mortar samples rose from one curing age to the next, indicating continual geopolymerization product development.

6) When GBS content was substantial, as it was in G100 and G75F25 mortars, raising the sodium hydroxide solution concentration increased both the start and final setting times. This is a result of calcium’s fast release at high solution alkalinities and the creation of calcium-based products and precipitates. In contrast, the setting time of mortar samples increases with increasing fly ash content due to the slower dissolution and delayed formation of fly ash-based geopolymerization products at any solution alkalinity.

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