External Sulfate Attack of Ambient-Cured One-Part Alkali-Activated Self-Consolidating Concrete

Abstract

The mechanism of sulfate attack on alkali-activated materials, particularly the alkali-activated self-consolidating concrete (AASCC), is complex and contradictory. This could be due to the wide range of precursor and activator materials used in the production of AASCC mixtures, which has called into question the reliability and validity of existing evaluation procedures and practices. This paper presents a systematic research effort on AASCC mixtures, based on granulated blast-furnace slag, prone to various sulfate attack scenarios that are thought necessary to establish a proposed criterion. The conducted experimental design demonstrated that single-, binary-, and ternary-precursor AASCC samples, activated with 1:1 Na₂CO₃ and MetaNa₂SiO₃, partially submerged in sodium, magnesium, and mixed sulfate solutions could experience a dual sulfate attack scheme. Sulfate attack can occur in the immersed section in sulfate solutions, while physical sulfate attack can occur in the portion above the solution level. The influence of physical sulfate attack on the concrete’s characteristics was not significant given that the damage was confined to the outer surface. However, the damage was primarily monitored by the AASCC different systems’ pore structure, which resulted in the leaching of ions from samples to solutions. It was found that maintaining the pH in the sulfate solutions increased the rate of damage of AASCC mixtures. Furthermore, binary, and ternary precursor blends partially replacing slag with SF, or FA resulted in decreased porosity, surface scaling, and AASCC deterioration caused by an expansion in the volume of very small diameter pores. Finally, in all AASCC systems, gypsum and ettringite were the primary degradation products of sulfate attack.

1. Introduction

External sulfate attack (ESA), caused by the invasion of sulfuric ions in soils, underground, marine, or industrial wastewater, is a substantial means of deterioration of concrete in-service. Even though sulfate typically damages the cement-paste matrix, its adversity depends on the types of binder, nature, and concentrations of sulfate solutions, concrete quality, and surrounding conditions. Generally, cement paste pores are filled with a highly basic solution (i.e., pH > 12.5). Consequently, any medium with a lower pH value would be an aggressive condition for the cementitious matrix. Concrete exposed to ESA suffers from expansion, cracking, strength loss, and eventually, disintegration.

Different test methods are utilized to examine the ESA mechanisms and effects, e.g., ASTM C1012 “Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution” [1]. This commonly used standard was criticized due to its limitation (orientation-expansion effect). In 1997, Taylor reclaimed that laboratory research focuses on expansion and specimen cracking, but field experiences denoted a higher loss of adhesion and strength as a prevalent sign of deterioration [2]. Additionally, most of the existent standardized durability testing methods have especially been founded to test the long-term durability performance of Portland cement systems. The lack of standardized testing and evaluation criteria for alkali-activated materials (AAMs) symbolizes one of the main barriers facing its spread as no reliable long-term durability data are available. Various testing methods are thus vital to better predict the AAM’s different damage mechanisms while being exposed to sulfate attacks (i.e., partial immersion exposure tests).

Thus, this research was dedicated to investigating the chemical and physio-chemical degradation mechanisms (physical or chemical) of alkali-activated self-consolidating concrete (AASCCs) partially exposed to aggressive aqueous solutions. This is crucial as the transport mechanism in partially exposed concrete structures does not appear to cause the same distress as in entirely exposed concrete components. Diffusion is the main transport mechanism that triggers a higher deterioration rate under saturated conditions. It is also known to be slow compared to other mechanisms, as an aqueous ionic transport mechanism, and is not apparent until a thermal or concentration gradient is generated. Hence, the “sulfate-related deterioration rate defies the expectations of the relevant models” [3].

2. Research Significance

The mechanism of sulfate attack on AAM, particularly AASCC, is complex and contradictory. This could be due to the wide range of precursor and activator materials used in the production of AASCC mixtures. The physical form of sulfate attack is not evident when testing a fully saturated specimen, the standard method of exposure recommended by the ASTM C1012 [1]. In addition, there is no standard test available for the partially immersed concrete in sulfate solutions. This has called into question the reliability and validity of existing evaluation procedures and practices [4,5,6]. This paper presents a systematic research effort on AASCC mixtures prone to various sulfate attack scenarios that are thought necessary to establish a proposed criterion. AASCCs were partially exposed to 10% single and mixed sulfate solutions in a controlled pH environment as part of accelerated degradation tests, a step to simulate harsh field conditions. This would result in a better understanding of the sulfate attack mechanism as well as the development of more reliable assessment tools for predicting AASCC performance and service life.

3. Experimental Program

3.1. Materials and Mixture Proportion

Ground granulated blast furnace slag (GGBFS) was employed as the main precursor to produce single, binary, and ternary AASCC. Fly ash Class-F (FA) and silica fume (SF) were added with various proportions replacing GGBFS. The physical and chemical properties of the precursor materials, along with the X-ray diffraction patterns, were previously published in a previous work [7]. Two dry-powder activators, MetaNa₂SiO₃ and Na₂CO₃ with a 1:1 ratio, were mutually utilized to activate the AASCC mixtures. The used Na₂SiO₃ had the density of 1.09 g/cm³ and a molar ratio of 1.0. The Na₂CO₃ powder had ≥99.5% purity and the density of 2.53 g/cm³. Natural siliceous sand with a fineness modulus of 2.5, a specific gravity value of 2.68, and water absorption of 1.5% was used. Furthermore, coarse aggregate with a maximum nominal size of 19 mm, a specific gravity value of 2.71, and water absorption of 0.4% was used. Their volumetric contents were set to 60% and 40%, respectively. The water-to-binder (w/b) ratio was fixed at 0.40 by the mass of the binder. It is important to note that no superplasticizers were used in the production of AASCC mixtures. AASCC mixtures were divided into three groups: single-, binary-, and ternary-precursor groups. The only difference amongst mixtures in the same group is the activator content (i.e., 16, 20, and 25%). Mixtures with appropriate performance, i.e., the ability to balance the restricted fresh characteristics, strength gain, and expected durability performance, were chosen for further long-term investigation. The precise proportions of the mixtures were disclosed in a previous research paper [7].

Five AASCC mixtures were prepared with different precursor blends and compositions but the same activator dosage (25%), as indicated in

Table 1. Summary of mixture’s proportions used for the designed AASCC mixtures (in kg/m³).

Mixture ID	Slag	FA	SF	Sand	CA
S-25	525	0	0	646	830
B1-25	472.5	0	52.5	652	825
B2-25	367.5	157.5	0	637	840
T1-25	315	173	37	625	840
T2-25	289	210	26	624	840

. The S-25 designation refers to a single precursor 100% GGBFS AASCC combination activated with 25% dosage. Both binary AASCC groups used 90% GGBFS and 10% SF (assigned code B1) or 70% GGBFS and 30% FA (assigned code B2). The AASCC ternary mixtures contained 60% GGBFS, 33% FA, and 7% SF (code T1) or 50% GGBFS, 45% FA, and 5% SF (code T2).

Along with ASTM C1012 [1], samples were continuously exposed to aggressive solutions of sodium sulfate (Na₂SO₄) and magnesium sulfate (MgSO₄). The selection of the different cations (Na⁺ and Mg²⁺) accompanying ${\mathrm{SO}}_4^{2-}$ shows not only several attacking and destruction mechanisms but also various solubilities. Although sodium is not an aggressive component, it has a significant impact on the solubilization of cement species and the pH of the pore solution. The effect of magnesium sulfate is far more substantial than either sodium or calcium sulfate. Magnesium and sodium solubilities were 35.7 g and 28.1 g in 100 g water at 25 °C, respectively [8]. These values are significantly higher than that of calcium (i.e., 0.205 g in 100 g water at 25 °C). The sulfate salts used in assessing the sulfate resistance of AASCC mixtures were Na₂SO₄ and MgSO₄ of purity > 99%. Each solution was prepared by dissolving either 50 g or 100 g of the solutes in 1 L tap water to obtain (5% and 10%) of the sulfate solution.

3.2. Testing Program

Several mixtures were tested to evaluate their fresh and mechanical properties, according to EFNARC requirements [9]. Mixtures satisfying the limits were considered for additional investigation targeting a balance of the fresh characteristics, strength gain, and expected durability performance. A synthesis of AASCC, mixtures with optimum performance were regarded for further assessment in this paper [10,11]. After the mixing procedure, the fresh properties were evaluated, and numerous specimens were casted to study the hardened properties. Cylindrical specimens with dimensions of 100 Ø 200 mm were cast without any vibration. Instantly after casting, samples were covered to prevent the evaporation of water and left intact at room temperature (23 ± 2 °C). Past 24 ± 4 h, the samples were demolded and cured in an ambient curing condition until the age of testing.

3.2.1. Single Damage-Factor

In partly exposed sulfate structures, the lower portion may be saturated with seawater, but the upper remains at ambient temperature and humidity conditions. The deterioration commonly occurs when the sulfate is drawn into the concrete, permeated upwards, and then evaporated upon reaching the surface. Accordingly, various types of degradation, i.e., leaching, microcracking, paste and permeability characteristic alteration, efflorescence, and surface scaling, may appear. The degradation can be accelerated through the transfer of larger amounts of sulfate throughout the matrix (Exposure I scenario of the current work). For instance, Boyd and Mindess [12] partially exposed concrete cylinders to a 5% Na₂SO₄ solution and concluded that while scaling implies significant damage, the internal damage caused by sulfate attacks may be more severe. Up to now, this physical form of sulfate attack is not evident when testing a fully saturated sample. Additionally, there is no available standard for the partially immersed concrete in sulfate solutions.

The concrete deterioration scheme proposed for single damage factor tests was divided into six groups (Figure 1). Sulfate solutions were replenished every 4 weeks with fresh ones. The importance of monitoring the pH of the sulfate solutions, although not specified in ASTM C1012 [1], is associated with the real field conditions in which concrete would be exposed to a continual supply of sulfate ions [13,14]. Mixtures were evaluated based on their visual appearance, changes in mass, cross-section variation, and pH values of the sulfate solutions for 183 d at the end of each 30 d.

A step to simulate harsh field conditions, AASCCs were partially exposed to 10% Na₂SO₄ and 10% MgSO₄ in addition to a mixed solution of 10% Na₂SO₄ +10% MgSO₄ in laboratory conditions. The attack intensity is well known to be mainly dependent on the pH of aggressive solutions. Attempting to control the pH is a simulation of field conditions in which concrete is exposed to an environment that usually contains mobile sulfate. Therefore, the effect of monitoring the pH of the solution on the leaching mechanisms of the AASCC mixtures was assessed. Alternatively, the resistance of AASCC mixtures to 5% Na₂SO₄ and 5% MgSO₄ was evaluated for comparative purposes. The initial pH value for sulfate solutions was kept in the range of 6.0–8.0, as recommended by the ASTM C1012 [1]. The exposure to the sulfate solutions lasted for 6 months (26 weeks) under ambient laboratory conditions. For specimens in controlled pH environments, the pH was maintained at the range of 6.0–8.0 by titration with dilute sulfuric acid solutions at constant time intervals. To guarantee a homogenous distribution of the solutions, regular stirring was accompanied by adding a pH regulator (i.e., sulfuric acid solutions).

3.2.2. Multiple Damage-Factor

Field concretes undergo wetting and drying cycles due to fluctuations in water levels caused by flooding, rainwater runoff, tides, and the cyclic migration of sulfate-containing groundwater through capillarity. Such activities induce the crystallization of sulfate salts under the concrete surface layer leading to cracking, scaling, and delamination. Existing test approaches cannot capture these complicated deterioration mechanisms (i.e., continuous immersion exposure). Besides, standard immersion tests have frequently presented different performances relative to the observed ones in real-field cases. A mandatory shift to performance-based specifications demands the elaboration of a performance-based evaluation that better depicts field conditions.

The question of whether wetting–drying cycles and the underlying mechanisms of this form of damage are physical or chemical is still controversial among researchers. For instance, the length change of concrete cylinders exposed to Na₂SO₄ solutions by 2.1% in a 20-year long-term research program was investigated [15]. Samples were exposed to wetting–drying cycles to accelerate the test. In each cycle, the samples were immersed in sulfate solutions for 16 h at room temperature, then air-dried for 8 h at 54 °C. A year after, it was reported that eight years of continuous immersion exposure triggered comparable damage to concrete samples under wetting–drying. De Almeida [16] proposed immersing samples in 16% Na₂SO₄ solutions for 2 h (wetting) followed by drying at 105 °C for 10 to 15 h. It was concluded that specimens failed under physical rather than chemical action. Likewise, mortar cylinders were partially immersed in 10% sodium and 10% magnesium sulfate solutions and exposed to 32–95% constant or cyclic relative humidity (RH) for up to 151 h [17]. This extensive study reported a thick efflorescence layer on the surface of partially immersed specimens in 10% Na₂SO₄ solution only, particularly with cyclic RH. The resistance of mortars to 5% sulfate solution accompanied with wetting–drying cycles were investigated [18]. Each cycle consisted of the exposure at room temperature for 6 d, followed by drying for 1 d at 100 °C. The latter exposure was observed to be more aggressive compared to continuous immersion exposure and has contributed to the complete disintegration of the samples within 17 weeks. Also, Haynes et al. [19] experimented on partially immersed concrete cylinders in (5% Na₂SO₄) and subjected them to several temperatures and RH. The conclusion was that specimens disintegrated after the exposure to cycles between 20 °C with 82% RH and 40 °C with 31% RH in two-week intervals.

In-depth research is still required on sulfate attack assessments, including wetting–drying cycles. Several questions remain unanswered, e.g., the real mechanisms of deterioration after these cycles and the degree and time of drying that can simulate the real field conditions. Variations in these parameters remain a challenge for researchers and standardization agencies. Besides, like partial immersion exposure tests, there is no available standard for concrete exposed to drying and wetting cycles. Further, the properties of AAM in various exposure conditions simulating field-like conditions need to be tackled. This section would provide an adequate understanding of the mechanisms of deterioration of AASCC mixes in the evaporation zones, resembling partially buried concrete elements, to develop reliable data on their durability. For multiple damage factor, mixes were evaluated while the exposure to different sulfate attack environments was considered with other concurrent damage mechanisms (i.e., cyclic environmental conditions).

After the curing process of AASCC mixtures, concrete cylinders were kept in ambient laboratory conditions (i.e., 23 ± 2 °C, RH of 50%) for 48 h to remove the excess moisture from surfaces. This technique would guarantee sufficiently dry samples for a uniform basis of analogy. After that, their preliminary physio-mechanical characteristics were recorded before the exposure to sulfate occurs. Specimens were partially immersed in highly concentrated solutions of (10%) sodium sulfate and (10%) magnesium sulfate solutions to evaluate the resistance AASCCs to combined effects of ESA with other ongoing deteriorating mechanisms (i.e., cyclic environmental conditions). Figure 2 illustrates the technique used to resemble the performance of concretes exposed to ESA [20]. The exposed part of the specimens, up to ⅓ of the cylinder height, was fully immersed and sealed to minimize the evaporation of solutions and ensure that they were uptake by the cylinder. This section investigated the capability of holistic testing techniques in capturing the AASCC deterioration mechanisms within a reasonable testing duration. Therefore, sulfate solutions with a very high concentration were used to accelerate the deterioration process.

The effect of interacting deterioration parameters, partial immersion in sulfate solutions under cyclic environmental conditions could depict the seasonal variations of atmospheric temperature (T) and RH found in many geographic locations, i.e., south of the USA. These unfavorable exposure conditions, an exposed portion of specimens to cycles of varied ambient temperature and RH, resembled a repeated crystallization deliquescence and hydration–dehydration processes that can cause rapid decay of AASCC mixtures. The repetitive crystal growth of sodium and magnesium sulfate from anhydrous to hydrous states was associated with a significant volume increase due to phase changes. Based on the stability phase diagram of the Na₂SO₄ solution, the direct precipitation of thenardite (above ~32.4 °C) and the rapid crystallization of mirabilite after thenardite dissolution causes a higher deterioration level in porous mixtures than mirabilite crystallization only [21]. Based on the available thermodynamic data and close to room ambient temperature, the crystalline phase of MgSO₄ (epsomite) would be stable. However, under dry conditions, epsomite would tend to dehydrate to form a monohydrate kieserite phase [22].

In this study, cylindrical specimens were subjected to alternating temperature and RH cycles for 183 d incessantly. Each cycle (24 h) includes two consecutive stages; an 8 h hot/dry stage (40 ± 2 °C and 35 ± 5% RH) followed by a 16 h temperate/humid stage (20 ± 2 °C and 90 ± 5% RH). The loss in the sulfate solutions was due to the continuous uptake, mainly during the dehydration (drying) cycles. They were frequently refilled up to one third of the cylinder’s height. Moreover, sulfate solutions were renewed every 30 cycles. A conceptual framework is presented that considers four main visible deteriorating phenomena by ESA, i.e., spalling, delamination, cracking, and loss of cohesion (mass and strength loss). These complex physicochemical processes were monitored at two levels: macroscopic and integrated microscopic.

3.2.3. Mass Monitoring

To study the performance of AASCC mixtures in sulfate environments, specimens were partially immersed in sulfate solutions after a 90-d curing age. The masses of the samples were recorded monthly for 183 d. For mass change measurements, samples were weighed before the exposure to sulfate solutions at age 90 d and were taken as initial masses (M_i). After a specified time interval, samples were removed from the sulfate solutions and were brushed using a nylon brush to remove the attached debris. Under ambient laboratory conditions, samples were left to dry for 30 min before visual inspection, and measurements of masses. The change in weight (MC, %) was calculated as follows:

$$MC (\%)=\frac{M_i-M_j}{M_i}\times 100$$

(1)

where M_i (g) is the average weight of two cylindrical samples; and M_j (g) is the average weight of two cylindrical samples exposed to sulfate solutions.

3.2.4. Cross-Section Variations Monitoring

During the sulfate attack, cross-sectional variations of samples can be monitored by certain geometrical parameters, such as sample volume, longitudinal expansion, and lateral variations of transverse sections (radius). The longitudinal expansion is the most questioned geometrical parameter under the ASTM C1012 [1]. Expansion and swelling of the cementitious matrix under sulfate attacks are frequently related to the formation of ettringite and gypsum due to crystal growth pressure [3]. Gypsum and ettringite formations require a calcium source that can be supplied from the leaching of portlandite and C-S-H. Portlandite leaching and progressive C-S-H decalcification led to a softened cement matrix and reduced strength. The formation of calcium sulfate-containing degradation products is related to Portland cement behavior. Expansion, however, does not capture certain degradation-induced changes, while volume and sample monitoring for the radius enables the evaluation of leaching activities and geometrical variations accordingly. The extent of leaching of elements is affected by the pH level, binder microstructure, permeability, and porosity of the material considered. The behavior of various AASCC mixture systems under sulfate attack is associated with the existence of C-S-H and C-A-S-H hydrotalcite gel; the former phase maybe highly resilient to sulfate attack.

The cylindrical samples’ longitudinal and volume variations were directly evaluated from experimental data where the proposed lateral vs. longitudinal deformation pattern corresponds to the deformation path. This allows various binder materials to be classified according to the magnitude of the deformation in concrete mixtures. The lateral deformation is more sensitive to sulfate attacks than longitudinal expansion, making this approach useful in testing low expansion materials (i.e., AAMs). Equation (2) refers to the relative variation in the cross-sectional radius derived from the relative variation of the elongation x and the relative variation in volume z monitored during the test.

$$y=\sqrt{\frac{1+z}{1+x}}-1$$

(2)

where x is ∆L/Li, y is ∆R/Ri, and z is ∆V/Vi with Vi initial global volume, Li initial sample length, and Ri initial radius of a cross-section.

3.2.5. Microstructural Observations

After the visual inspection of test specimens, their degradation mechanisms were analyzed using X-ray diffraction (XRD), ICP-OES (inductively coupled plasma-optical emission spectrometry), ion chromatography, and differential scanning calorimetry (DSC) at a heating rate of 10 °C/min. X-ray diffraction (XRD) was conducted employing a Bruker D8 Advance diffractometer (CuKα radiation, 1.5406 Å) with an imaging plate detector to collect data in a range of 10–90° 2θ. XRD measurements were performed on powder samples, passing #200 sieve (75 µm), extracted from the superficial surfaces of the exposed samples (0–15 mm) after each exposure interval. For further medium analysis with the ICP and ion chromatography, the solutions’ pH measurements and ion concentrations, i.e., Ca²⁺, Na⁺, ${\mathrm{SO}}_4^{2-}$, and Mg²⁺, was performed after the immersion of AASCC specimens.

4. Results and Discussions

4.1. Exposure I: Controlled pH

Sulfate penetration is often governed by diffusion through the pore structure and is consequently slow in the early stages of exposure. While exposed to a 10% sulfate solution in step 1, gypsum precipitation and ettringite production may reduce and offset the sulfate concentration in the pore solution. The presence of microcracks after a period of exposure raises the sulfate concentration in the pore solution in step 2 as sulfate can pass through the open cracks unhindered. For AASCC cylinders partially exposed to various concentrations of sulfate solutions, it was found that the crystallization pressure does not increase comparably with the supersaturation of the pore solution. Instead, changes in solution concentration due to the dual interactions between the paste and sulfate solutions may affect the crystallization pressure less in the Exposure I (high supersaturation) scenario than in the Exposure III (low supersaturation) environment. Then again, due to kinetic effects, the lower the concentration of sulfate in solutions, the slower the formation of expansive products, such as ettringite, resulting in lower crystallization pressure [23]. The sulfate concentration has a significant effect considering different exposure scenarios. The difference in crystallization pressure in AASCC cylinders have different causes. Using 10% sulfate solutions (Exposure I) and 5% solutions (Exposure III), the sulfate concentrations inside the cylinders were expected to be alike despite the variation in sulfate concentration between the container solution and samples. Still, the higher stress could be due to higher sulfate concentration (Exposure I), promoting cylinder supersaturation.

4.1.1. Visual Appearance

Single-precursor mixtures. The Malhotra et al. [24] rating system was used to assess the visual situation of each concrete cylinder after six months of partial immersion in sulfate solutions. The visual examination of AASCC single-precursor mixtures (S-25) revealed no signs of deterioration at the samples surface in the 10% Na₂SO₄ (

Table 2. AASCC mixtures exposed to 10% sulfate solutions at a controlled pH environment.

Sulfate Solution	Exposure I
	S-25	B1-25	B2-25	T1-25	T2-25
MgSO4
Na2SO4
Mixed

) and 10% Na₂SO₄ + 10% MgSO₄ mixed solutions in a controlled pH environment. An exception was noticed when the concrete cylinders were exposed to a 10% MgSO₄ solution. The exposed areas were cracked, and surface delamination was observed.

Binary-precursor mixtures. The samples of binary-1 mixes using 10% SF replacing slag and subjected to 10% sulfate solutions showed that magnesium sulfate greatly affected the concretes properties. The AASCC binary-1 mixture showed an expansion and cracking after exposure to 10% MgSO₄. The exposed area of the concrete mixture also suffered a massive loss (Table 2). Samples in mixed solutions exhibited a significant surface spalling above the exposed area to sulfate solution. While in Na₂SO₄ solution (Table 2), cracking along the length of the sample was more distinct. For binary-2 mixtures using 30% FA replacing slag, no visible signs of deterioration were detected after exposure to different sulfate solutions after 6 months. However, cracks were noticeable in the surface of samples exposed to 10% MgSO₄ and 10% mixed sulfate solutions.

Ternary-precursor mixtures. No visible signs emerged of deterioration while exposing the ternary mixtures to different sulfate solutions in a controlled pH environment. However, the ternary-1 set of mixtures yielded a greater surface scaling at the exposed area in the 10% MgSO₄ (Table 2) than in 10% Na₂SO₄ sulfate solution.

4.1.2. Mass and Cross-Section Variations

Single-precursor mixtures. Figure 3a shows the mass variation of single-precursor AASCC cylinders after the partial immersion in 10% MgSO₄, 10% Na₂SO₄, and 10% mixed controlled pH sulfate solutions. While Figure 3b shows the cross-section variation of these specimens after the partial immersion for 182 d. When exposed to a 10% MgSO₄, 10% mixed, and 10% Na₂SO₄ sulfate solutions, the cross-section decreases (0.5%, 1%, and 1.4%) were negligible, while the mass remained to decrease but below 1%, probably due to alkalis and calcium leaching. The absence of significant damage in AASCC single-precursor specimens using 100% slag in the 10% sulfate solutions suggests the scarce formation of expansive products, such as ettringite and gypsum. The presence of aluminum in C-A-S-H and hydrotalcite gels, combined with the absence of portlandite, can result in high resistance to sulfate attack in single-precursor mixtures containing 100% slag [25]. The XRD analysis suggests that the supply of calcium and/or aluminum in single-precursor mixtures is insufficient to allow ettringite formation, as shown in Figure 4. However, traces of gypsum were detected in the samples exposed to 10% MgSO₄ solutions.

Binary-precursor mixtures. The mass and cross-section variations of binary-precursor specimens after 6 months of partial immersion in 10% different sulfate solutions were presented in Figure 5. It can be adhered that the results in binary mixtures vary depending on the precursor type used, either SF in binary-1 or FA in binary-2. In the controlled 10% MgSO₄ solution, the binary-1 AASCC mixture made with 10% SF replacing slag yielded a maximum mass and cross-section loss of ~6.6% and 11.5%, respectively. This expected behavior can be attributed to the two-way ion diffusion between the MgSO₄ sulfate solution and mixtures. The high concentrations of Ca, Mg, and Na in the binary-1 AASCC mixture have been observed in ion chromatography data in

Table 3. Ion chromatography analysis and pH after 182 d in 10% sulfate solutions.

Sulfate Solution	Mixture ID	pH	Element Concentration (mg/L)
			Na	Ca	Mg	SiO3
10% MgSO4	Single	10.1	3515	756	87	26
	Binary-1	9.40	32,485	34,820	581	48
	Binary-2	10.1	6468	604	257	396
	Ternary-1	9.90	6920	836	999	25
	Ternary-2	10.1	3516	13,679	1368	135
10% Na2SO4	Single	13.6	5739	42	247	22
	Binary-1	9.80	98,134	25,322	436	64
	Binary-2	11.3	4070	112	262	320
	Ternary-1	12.8	4439	202	25	16
	Ternary-2	12.8	5190	614	3	81
10% MgSO4 + 10% Na2SO4	Single	10.9	6299	522	145	20
	Binary-1	9.80	88,184	22,405	1266	53
	Binary-2	10.3	5886	460	794	638
	Ternary-1	10.2	5800	1348	1678	25
	Ternary-2	10.3	6783	5846	1109	84

, resembling the sulfate resistance performance of the binary-1 mixture. The binary-2 mixture made with 30% FA, on the other hand, had a mass and cross-section loss that was ~86% lower than the binary-1 mixture. When exposed to 10% Na₂SO₄ and mixed sulfate solutions, the mass loss was limited to 1% in binary-1 and 1.5% in binary-2 mixtures. For both binary sets, the minimal mass and cross-section variation was observed in 10% Na₂SO₄ due to the C-(N)-A-S-H gel changes, leading to the formation of microcracks within the binary network structure. However, the MgSO₄ attack mechanism can cause decalcification of calcium-containing systems such as C-(N)-A-S-H gels with high calcium content. This can lead to the formation of gypsum (CaSO₄·2H₂O) gels that lack cementing properties [4,5,6,26].

In the AASCC binary-1 mixtures with 10% SF substituting slag, XRD analysis revealed various degradation products in different sulfate solutions (Figure 6). Gypsum was the predominant expansive product freebie in MgSO₄ solution samples after 6 months of immersion. It is important to note that gypsum formation can result in softening and a loss of mass and strength over time. However, a trace of ettringite was found in samples subjected to Na₂SO₄ solution due to the interactions between sulfate ions and AASCC paste components. Likewise, ettringite was present in the samples exposed to a 10% MgSO₄ + 10% Na₂SO₄ mixed solution, and a significant amount of thenardite precipitates were identified. The damage above the solution level can be explained through the salt crystallization pressure theory. A supersaturated solution can create salt crystals, which exert pressure on the concrete pore walls, potentially damaging the matrix [27].

Overall, the average pore diameter determines the permeability, which affects the ion exchange rate after exposure to a sulfate environment. Therefore, increasing the densification level of the forming pore structure appears to enhance the performance of concrete exposed to aggressive environments. The positive substitution of slag with SF in binary-1 mixtures reduced their durability when exposed to external sulfate environments. This could be due to an increase in the small-diameter pores percentage in binary-1 mixtures and an increase in capillary suction. As a result, the specimen’s upper surface area for evaporation increased, making the cylinder more vulnerable to physical attacks.

The only main component responsible for sulfate attack detected in the degraded sections above the solution level in all AASCC samples was gypsum (Figure 7). However, there was evidence of physical sulfate attack damage in the form of surface scaling above the solution level, notably in the samples subjected to 10% mixed solution. Thus, chemical sulfate attack resulting from dual interactions between sulfate solutions and samples is more likely to be the dominating mechanism.

Ternary-precursor mixtures. The mass and cross-section variations of the ternary-precursor mixtures were almost identical. Both sets lost ~2.3% of their mass but remained constant until the end of the test (Figure 8). At the same time, ternary-1 specimens resulted in ~2% and 4% cross-section loss in 10% MgSO₄ + 10% Na₂SO₄ and 10% Na₂SO₄ solutions, respectively. The mass loss in ternary blends can be credited to the occurrence of C-A-S-H gels in calcium-rich mixtures due to the inclusion of Al₂O₃ in their structure, which resembles the sulfate attack mechanism in OPC mixtures. Incorporating low or free-calcium precursors, on the other hand, increased the ion exchange reactions between the various sulfate solutions and the ternary-precursor matrix. However, because of the high competition between carbonate and sulfate ions, extensive ettringite formation may be overlooked and kept under control when a sodium carbonate activator is used.

XRD analyses (Figure 9) verified the structural changes in ternary-precursor blends, revealing thenardite in the damaged regions of samples exposed to a 10% Na₂SO₄ solution; gypsum was formed in specimens immersed in a 10% MgSO₄ solution. Overall, the formation of expansive gypsum and ettringite crystals is associated with exposure to external sources of sulfate attack. These crystals can colonize and accumulate in the concrete mixture’s pores. Once filled, these crystals can initiate significant volumetric strain, leading to microcracking and concrete deterioration. Thus, crystallization of sulfate in macroscopic pores and cracks is desired. This is related to the formation of large-size expansive crystals, i.e., ettringite, which cannot exert a high crystallization pressure and thus are unlikely to be the source of the destruction. Since small crystals are prone to dissolution due to their unstable state, they are in equilibrium with higher sulfate concentrations.

4.1.3. Ion Chromatography

Table 3 shows the ion chromatography analysis of different AASCC systems in controlled 10% sulfate solutions after 182 d of exposure. Na⁺, Ca²⁺, and Mg²⁺ concentrations in 10% MgSO₄, 10% Na₂SO₄, and 10% Na₂SO₄ + 10% MgSO₄ sulfate solutions were significantly increased during the testing period. For single-precursor mixtures, concrete cylinders with 100% slag showed high concentration levels of leached Ca when exposed to 10% MgSO₄ (756 mg/L) and 10% MgSO₄ + 10% Na₂SO₄ (522 mg/L) than in 10% Na₂SO₄ (42 mg/L) sulfate solutions. In contrast, the concentrations of Na and Ca in all sulfate solutions were extremely high, especially in binary-1 mixtures with 10% SF replacing slag, indicating that these elements were leached from the mixtures into solutions, which was in accord with the literature [4,28]. The decrease in pH of sulfate solutions ≤ 9.8 resulted in the decalcification of C-(A)-S-H gel, which was revealed by the increased Ca concentration in the sulfate solutions. For example, in MgSO₄ and mixed sulfate solutions, the Ca concentration was 34,820 mg/L and 25,322 mg/L, respectively compared with 22,405 mg/L in Na₂SO₄ solutions.

Exposure of binary-2 silicon-rich mixtures to various sulfate solutions resulted in extensive leaching of Si, Na, and Ca into the solutions. For instance, in 10% MgSO₄ and 10% Na₂SO₄ solutions, the concentration of SiO₃ was ~396 and 320 mg/L, respectively. While in 10% mixed sulfate solution, the concentration of SiO₃ recorded was 638 mg/L after 182 d. This can be caused by the leaching of the unreacted sodium silicate content present in the pore solution or the gel structure. However, due to the continuous monitoring of pH and the depletion of silicon sources present in the AASCC binary FA-slag mixtures, the amount of leached Si decreased over time. The high pH value of the sulfate solutions used to make binary-2 mixtures have been identified as a contributing factor to the Si leaching, which was shown earlier [29,30,31].

The Na⁺, Ca²⁺, and Mg²⁺ concentrations in ternary-precursor mixtures also increased over time, proving a more distinct impact of aggressive sulfate solutions on calcium-containing compounds, such as C-(N)-A-S-H gel in MgSO₄ and hybrid solutions. The test results of both ternary sets showed that higher pH values between 10 and 13 after 182 d of testing due to pore solution alkalis leaching or Ca release into the solutions due to the system’s reaction with different sulfate compounds and ion exchange [4,28,32]. Ternary-2 and ternary-1, e.g., had Ca concentrations of 13,679 and 836 mg/L, 614 and 202 mg/L, and 5846 and 1348 mg/L in 10% MgSO₄, 10% Na₂SO₄, and mixed sulfate solutions, respectively.

4.2. Exposure II: NonControlled pH

4.2.1. Visual Appearance

Single-precursor mixtures.

Table 4. AASCC mixtures exposed to 10% sulfate solutions at uncontrolled pH environment.

Sulfate Solution	Exposure II
	S-25	B1-25	B2-25	T1-25	T2-25
MgSO4
Na2SO4
Mixed

shows the visual appearance of AASCC single-precursor mixtures exposed partially to an uncontrolled pH environment. It revealed no signs of deterioration in 10% MgSO₄ solution, few spalls of concrete at the exposed areas of specimens immersed in 10% Na₂SO₄, cracking and mass loss of concrete over the sulfate solution level of specimens immersed in 10% mixed solution. All specimens were surrounded with whitish precipitates, which indicated that the concrete had been affected.

Binary-precursor mixtures. The uncontrolled pH exposure condition of binary-1 specimens to sulfate solutions for 6 months resulted in a higher level of deterioration, unexpectedly (Table 4). The severe loss of the concrete matrix at the bottom edge and above the exposed area was caused by the exposure to the MgSO₄ solution. In 10% Na₂SO₄ solution, broken portions above the partially exposed areas were noticeable, while white deposits were observed in 10% mixed solutions. For the binary-2 set of mixtures, in an uncontrolled pH sulfate environment, the surface erosion was minimal in all samples above the exposed area; however, it was significantly greater in 10% MgSO₄ (Table 4) and 10% MgSO₄ + 10% Na₂SO₄ sulfate solutions.

Ternary-precursor mixtures. After 6 months of exposure to sulfate in an uncontrolled pH environment, the scaling of ternary-2 (Table 4) concrete surfaces appeared higher than that of ternary-1 mixes, which appeared above the sulfate solution level.

4.2.2. Mass and Cross-Section Variations

Single-precursor mixtures. Figure 10 shows the time-dependent mass and cross-section variations of AASCC 100% slag specimens during the partial immersion in 10% MgSO₄, 10% Na₂SO₄, and 10% MgSO₄ + 10% Na₂SO₄ non-controlled pH sulfate solutions. White deposits were observed for all mixtures when exposed to all sulfate solutions, although mass loss remained constant ≤1%, most likely due to alkalis leaching in a near-neutral pH environment.

On the other hand, the reaction between sulfate and AASCC slag mixtures caused a change in the cross-section of specimens, especially before the renewal of solutions ~100 d. Exposure to 10% MgSO₄ solutions for 30 d produced a 2% change in cross-section, which is about double the increase in the other sulfate solutions, yet the change was minimal till the end of the test. In 10% Na₂SO₄ and mixed sulfate solutions, a progressive increase in cross-section loss was noticed until the end of the test, indicating the little possible development of expansive ettringite or gypsum.

Binary-precursor mixtures. The mass change of binary-precursor samples exposed to 10% MgSO₄, 10% Na₂SO₄, and 10% mixed solutions in an uncontrolled pH environment is illustrated in Figure 11. The mass loss noted for binary-1 samples subjected to MgSO₄ solution was 0.4% after 30 d of exposure, declining until the sulfate solution was renewed, yielding a mass gain of nearly 0.1% after 90 d. Samples continued to lose mass when the pH dropped between 6–8 at the end of the exposure period (2.1%) during the solution renewal. A different trend is observed for binary-2 samples in MgSO₄ solutions, where a marginal mass gain (0.1%) was noticed after 30 d, followed by a gradual mass loss reaching ~1%. However, neither the binary set of samples exposed to Na₂SO₄ and mixed sulfate solutions were significantly affected, with mass losses of (0.8% and 0.7%) for binary-1 and (1.1% and 0.9%) for binary-2, respectively. Figure 11b shows the cross-section changes that occurred due to the degradation and surface scaling of the AASCC two binary-precursor sets. The highest cross-section change was observed in the MgSO₄ solution for the binary-1 specimens (4.4%) made with 10% SF replacing slag, followed by cylinders exposed to Na₂SO₄ and mixed sulfate solutions with up to 2.4%. On the other hand, the binary-2 AASCC portion immersed into all sulfate solutions was mostly intact for all cylinders. Above the solution level, exposure to 10% mixed, 10% Na₂SO₄, and 10% MgSO₄ sulfate solutions for 182 d resulted in 2%, 1.7%, and 1.4% cross-section variation, respectively.

Ternary-precursor mixtures. After 182 d of exposure to sulfate solutions, the exposed surface of the specimens had no deposits and mostly showed no visible signs of deterioration in both sets of ternary mixtures. However, there were some mass changes in the specimens of <4% at the end of the testing period (Figure 12a). The ternary-1 samples prepared using a higher slag content lost a mass of 3.2% in the mixed sulfate, 2.5% in the Na₂SO₄, and 2.2% in the MgSO₄ sulfate solutions after 182 d of immersion. The extent of damage to ternary-1 specimens due to partial uncontrolled sulfate attack was not greatly associated with the cross-section changes. Ternary-2 cylinders gained 0.1% in the magnesium sulfate solution after 90 d of exposure and lost 0.5% at the end of the test. However, ternary-2 mixtures exposed to mixed and sodium sulfate solutions lost 1.8% and 0.5% at an uncontrolled pH environment. In the solutions of Na₂SO₄ + MgSO₄ and Na₂SO₄, ternary-2 samples had the most cross-section changes: 1.1% and 1.3% loss, while around 1.5% gain in the MgSO₄ solution (Figure 12b).

4.3. Exposure III: 5% Sulfate Solution—Controlled pH

4.3.1. Visual Appearance

Single-precursor mixtures. No signs of deterioration were detected after the exposure of slag-AASCC mixtures to 5% MgSO₄, 5% Na₂SO₄, and 5% mixed sulfate solutions, as shown in

Table 5. AASCC mixtures exposed to 5% sulfate solutions at controlled pH environment.

Sulfate Solution	Exposure III
	S-25	B1-25	B2-25	T1-25	T2-25
MgSO4
Na2SO4
Mixed

. Overall, it can be noticed that all single-precursor samples stayed visually intact with visual integrity, except the ones subjected to 5% MgSO₄ solution.

Binary-precursor mixtures. For binary-1 mixtures using 10% SF replacing slag after 6 months of partial immersion in the 5% different sulfate solutions. The deteriorating conditions were most pronounced in 5% MgSO₄ (Table 5) and 5% Na₂SO₄ sulfate solutions. Cracks were detected along the length of the samples in all sulfate solutions. The soft whitish material above the exposed area also showed signs of damage. However, all binary-2 specimens were visually minimally intact, with some superficial cracks visible above the sulfate’s solutions level.

Ternary-precursor mixtures. For both ternary-1 and ternary-2 AASCC mixtures using slag, FA, and SF, the immersed portions in the 5% sulfate solutions were mainly intact. Surface cracks and damage in the concrete’s ternary-1and ternary-2 were confined to the concrete segments above the sulfate solutions level, the maximum exposure level.

4.3.2. Mass and Cross-Section Variations

Single-precursor mixtures. The mass change of samples made from 100% slag binder after 182 d of partial immersion in 5% of various sulfate solutions is illustrated in Figure 13. Exposing specimens to 5% MgSO₄ solution resulted in a 1% loss of mass, which is higher by 11%, and 67% than when exposed to 5% Na₂SO₄ and mixed sulfate solutions. However, the cross-section variation of the same set of samples immersed in 5% MgSO₄ solution was approximately 25% lower than that of the specimens exposed to 5% Na₂SO₄ and mixed sulfate solutions. The absence of significant deterioration in single-precursor slag specimens in 5% Na₂SO₄ and mixed solution could prove that expansive ettringite or gypsum formation is relatively low. On the other hand, specimen damage in MgSO₄ solutions could be related to the decalcification and transformation of C-(N)-A-S-H gels.

Binary-precursor mixtures. Figure 14a shows the mass and cross-section change of the binary-precursor cylinders in different 5% sulfate solutions. In the sodium sulfate solution, the binary-1 samples formulated with 10% SF replacing slag performed better than the binary-2 and had around 1.3% mass increase compared with 0.2% mass loss over the time of the experiment. Again, a similar trend was observed in the 5% mixed sulfate environment, with binary-1 gaining 0.2% mass after 182 d of exposure compared to losing 0.7% mass in binary-2. In contrast, in the binary-1 and binary-2 samples exposed to 5% MgSO₄ solutions, masses fluctuated and steadily increased from the initial value, i.e., 0.9% and 0.8%, respectively. The corresponding cross-section variations after 182 d of partial exposure are shown in Figure 14b. Some fluctuation of the cross-section was detected in all binary-precursor samples, especially in the magnesium and mixed sulfate solution. For example, the variations of the binary-precursor specimens finally equilibrated with a 5% and 1.2% decrease compared to the initial value in the binary-1 and binary-2 specimens, respectively. However, in the sodium sulfate solutions, 2.8% cross-section decline was measured in the binary-1 samples compared with ~1.7% in binary-2 samples. Almost similarly in the solution of sodium sulfate + magnesium sulfate, where 2.2% and 1.5% reduction was found in the binary-1 and binary-2 specimens, respectively.

Overall, in binary mixes exposed to 5% solution, the damage was limited to the microstructural matrix in addition to few traces of damage above the embedded portion of cylinders, while the portion immersed in the solution was recovered in intact condition. The physical sulfate attack and leaching of ions were the proposed primary theories of damage. In contrast, a significant and steady paste transformation was observed after exposure to a 5% MgSO₄ solution, but little damage was observed in a 5% Na₂SO₄ solution. These results corroborate the findings of Ismail et al. [4], who discovered that when binary mixtures are exposed to MgSO₄ solution, the C-A (N)-S-H undergoes decalcification and silicate polymerization, but it remained essentially intact after exposure to Na₂SO₄.

Ternary-precursor mixtures. For ternary-precursor mixtures exposed to different 5% sulfate solutions, the mass and cross-section variations are shown in Figure 15. After 182 d of immersion, ternary-1 cylinders with a higher Ca content showed slightly more damage than ternary-2 cylinders. For example, after immersing ternary-1 samples in MgSO₄, Na₂SO₄, and mixed solutions, mass losses of 3.3%, 2.5%, and 2.7% were reported, respectively. In contrast, the mass variation of all ternary-2 samples decreased as the CaO amount decreased, resulting in 0.4%, 2.1%, and 2.3% mass loss in MgSO₄, Na₂SO₄, and mixed solutions, respectively. Contrary to expectations and mass change results of ternary-1 samples, cross-section variation in Na₂SO₄ and mixed sulfate solutions for ternary-2 mixtures was high at ~2.1% and 2.3%, respectively, then in MgSO₄ solution at ~0.4%.

4.3.3. Ion Chromatography

Table 6. Ion chromatography analysis and pH after 182 d in 5% sulfate solutions.

Sulfate Solution	Mixture ID	pH	Element Concentration in Different Sulfate Solutions (mg/L)
			Na	Ca	Mg	SiO3
5% MgSO4	Single	12.1	4205	62	313	24
	Binary-1	9.80	51,829	1103	120	72
	Binary-2	10.5	1900	62	26	4598
	Ternary-1	9.90	1426	876	1441	101
	Ternary-2	11.1	18,826	10,123	1271	127
5% Na2SO4	Single	13.6	5081	38	193	23
	Binary-1	10.5	124,089	1795	67	72
	Binary-2	12.9	6170	50	356	552
	Ternary-1	12.8	4730	876	1832	49
	Ternary-2	12.7	7769	4596	1111	81
5% MgSO4 + 5% Na2SO4	Single	13.0	5617	423	105	28
	Binary-1	13.0	72,134	7719	371	41
	Binary-2	10.9	5325	238	375	25
	Ternary-1	10.1	1950	588	933	16
	Ternary-2	10.5	8924	8322	4446	58

shows the variation in pH and element concentration of solutions containing the three different series of AASCC samples, single-, binary-, and ternary-precursor samples and subjected to 5% MgSO₄, 5% Na₂SO₄, and 5% MgSO₄ + 5% Na₂SO₄ sulfate solutions. The pH of the solutions increased tremendously during the exposure to different sulfate solutions to maintain electroneutrality due to the migration and counter diffusion of alkali ions from the specimens to solutions. Prior to the immersion of specimens, the pH of all sulfate solutions was in range of 6–8, and after 182 d of exposure, the pH increased considerably. The deterioration levels are due to the concentrations of the elements leached up to 182 d, after which the damage of the cylinders in 50 g/L solutions takes off, indicating the onset of significant cracking as found by Ye et al. [33].

Despite the pH rise caused by interactions between sulfate solutions and AASCC various matrices, all elements were highly soluble at low sulfate concentrations, such as 5% solutions, as opposed to their status at higher sulfate concentrations as 10% solutions. In single-precursor systems, the extensive damage of the AASCC matrix was observed during immersion in 5% MgSO₄ + 5% Na₂SO₄ solutions where the leached Ca concentration was 423 mg/L compared to 7719 mg/L in binary-1 and 238 mg/L in binary-2 systems. In contrast, extensive deterioration and Ca leaching were observed when low-calcium systems, i.e., ternary-precursor systems, especially in ternary-1 samples (10,123 mg/L) in MgSO₄ sulfate solutions.

4.4. Exposure IV: Drying and Wetting Cycles

After 30 d of continuous exposure to daily wetting and drying cycles with drying for 8 h to a temperature of 40 ± 2 °C and RH of 35 ± 5%, followed by a wetting cycle for 16 h to a temperature of 20 ± 2 °C and RH of 90 ± 5%, efflorescence appeared on the concrete cylinders’ drying surface above the solution level. In addition, salt precipitates were found on the immersed portion of the samples in sulfate solutions. According to previous studies [19,20,21,27,34,35,36], this exposure condition is favorable for the formation and precipitation of hydrous mirabilite from anhydrous thenardite and epsomite from kieserite, which can cause damage. In the Na₂SO₄-H₂O system, thenardite (Na₂SO₄) transformation into mirabilite (Na₂SO₄·10H₂O) occurs at 20 °C and below 76% RH. This process generates an increase in the volume of 314% [37]. Meanwhile, three stable crystalline phases exist in the MgSO₄-H₂O system: epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), and kieserite (MgSO₄·H₂O). When RH reaches > 80%, kieserite transforms into hexahydrite and epsomite, with the latter produced by hydration of hexahydrite [38]. The exposure lasted for six months (183 cycles of wetting and drying), and all samples were examined to diagnose the rate of damage. Higher surface scaling of concrete surfaces appeared on some cylinders above the sulfate solution level due to the crystallization of salts. Other samples, in contrast, revealed that the portion of concrete immersed in the solution was relatively ruined as the leaching process’s kinetics increased with temperature [39]. It should be noted that the reduced permeability can be attributed to the denser structure of aluminosilicate gels, which reduced the manifestation of microcracking due to weathering effect. The schematic diagram (Figure 2) illustrates the relevant sulfate efflorescence phenomena.

4.4.1. Visual Appearance

Single-precursor mixtures. After six months of exposure, i.e., 183 cycles of wetting and drying, the specimens exhibited signs of damage. Surface scaling appeared on the surfaces of single-precursor cylinders exposed to 10% MgSO₄ and 10% MgSO₄ + 10% Na₂SO₄ mixed solutions (

Table 7. AASCC mixtures exposed to 10% sulfate solutions under drying and wetting cycles.

Sulfate Solution	Exposure IV
	S-25	B1-25	B2-25	T1-25	T2-25
MgSO4
Na2SO4
Mixed

). The concrete mass loss and surface spalling were most pronounced in the 10% MgSO₄ solution. However, exposed cylinders to 10% Na₂SO₄ solution showed no signs of degradation. It is worthy to mention that the cyclic conversion of sulfate crystals can generate stresses in concrete pores. This phenomenon can lead to surface scaling and substantial deterioration levels [35].

Binary-precursor mixtures. For binary-1 mixtures, the mass loss due to the exposure to various sulfate solutions was caused in MgSO₄ and hybrid solutions. Instead, less degradation was noted with the Na₂SO₄ solution (Table 7) with less surface scaling and concrete damage. A similar tendency was seen in binary-2 mixtures but to a lesser extent. The binary-2 AASCC cylinders exhibit surface erosion and the manifestation of aggregates after the exposure to 10% MgSO₄ and 10% MgSO₄ + 10% Na₂SO₄ sulfate solutions under drying and wetting cycles on top of the solution level.

Ternary-precursor mixtures. Ternary-1 and ternary-2 samples displayed some alterations in appearance, especially in the solution of 10% MgSO₄ under drying and wetting cycles. In the 10% Na₂SO₄ and 10% mixed sulfate solutions, samples were covered by a 1-mm-thick white cover, and a horizontal cracking in the ternary-2 specimens was found in the latter solution (Table 7). Ternary-1 samples deteriorated less than ternary-2 samples that reported the most significant mass and cross-section variations. Significant deterioration tends to occur when magnesium cation is present in the sulfate solution. For instance, ternary-2 samples in 10% MgSO₄ and 10% MgSO₄ +10% Na₂SO₄ lost ~5.4% and 5% of their weights, respectively.

4.4.2. Mass and Cross-Section Variations

Single-precursor mixtures. Figure 16 reports the time-dependent mass and cross-section changes made from 100% slag after 182 d of exposure to sulfate solutions partially and under 183 drying and wetting cycles. In all sulfate solutions, atypical surface scaling of the immersed portion of concrete cylinders was observed (Table 7), in contrast to the expected damage in part above the level of the solution. After 60 d in MgSO₄ solution, single-precursor cylinders lost ~8% of their mass, which dropped sharply to 2.4% at the end of the exposure time. Single-precursor samples exposed to 10% Na₂SO₄ solution lost 33% less mass than those exposed to 10% MgSO₄ solution, although the cross-section varied by roughly 2%. On the other hand, cylinders immersed in mixed sulfate solutions demonstrated an overall mass gain (0.8%) at 182 d, indicating that porosity decreased with immersion time. It should be noted that all single AASCC cylinders were intact above the solution level, indicating that the high sulfate concentration combined with drying and wetting cycles did not accelerate the damage mechanism.

Binary-precursor mixtures. Figure 17 shows the mass and cross-section results of binary-precursor specimens immersed in various sulfate solutions and subjected to 183 drying–wetting cycles. For all binary samples exposed to different sulfate solutions, the mass loss and physical degradation arose due to spalling of the concrete surface above the solution level. This could be due to the nucleation and growth of sulfate crystals in the supersaturated solution, i.e., thenardite, which exerts stresses on the binary-precursor AASCC, causing damage and mass loss above the solution level and drying and wetting cycles. For example, the mass variations of binary-1 mixtures immersed in MgSO₄ solutions resulted in roughly 2% mass loss and cross-section variations compared with 1.4% mass loss and 2.2% cross-section variations in mixed sulfate solutions after 182 d. After 6 months of drying and wetting cycles, thenardite and ettringite were detected in binary-1 samples exposed to 10% Na₂SO₄ solution (Figure 18), even though a significant amount of gypsum was found in samples subjected to 10% MgSO₄ solution. This suggests that the drying and wetting cycles had a considerable effect on the rate of AASCC degradation.

Similarly, surface scaling (Table 7) began on the drying surfaces of the binary-2 cylinders immersed in both MgSO₄ and mixed sulfate solutions, i.e., 1.2% and 0.7% mass loss, respectively. Noteworthy is the fact that the crystalline phase of MgSO₄ (epsomite) would be stable close to room ambient temperature based on the thermodynamic data, but in dry conditions, epsomite would tend to dehydrate and produce the monohydrate kieserite phase; this process would develop a high crystallization pressure. However, after 60 d, binary-1 samples exposed to Na₂SO₄ showed a 1% mass gain, which reversed to a 1% mass loss after the testing time is over. This can be attributed to the direct precipitation of thenardite (above ~32.4 °C) and the quick crystallization of mirabilite following thenardite dissolution, which causes a higher degradation level in porous mixtures mirabilite crystallization alone [40].

Ternary-precursor mixtures. Figure 19a,b illustrates the ternary-precursor mixture mass fluctuations and their related specimen’s cross-section variations after six months of physical sulfate exposure. After one month of exposure, i.e., 30 cycles of drying and wetting, both ternary-precursor cylinders lost around 5% of their mass, while the exposure to all sulfate solutions was most probably due to the crystallization pressure on pores walls. These values continued to increase till the end of the testing period. All ternary-1 samples with a higher CaO content had similar cross-section variations ranging from 1.4 to 1.9% when exposed to all sulfate solutions. However, cross-section results of low-calcium ternary-2 samples revealed higher variability in MgSO₄ (2%), Na₂SO₄ + MgSO₄ (1.2%), and Na₂SO₄ (0.3%) sulfate solutions. The dissolution and crystallization of soluble salts in porous materials like concrete due to the variations in temperature and RH can cause phase changes between thenardite (Na₂SO₄) and mirabilite (Na₂SO₄·10H₂O) as well as kieserite (MgSO₄) and epsomite (MgSO₄·7H₂O).

4.5. Thermogravimetry (TGA/DTG)

The TGA/DTG profiles for all AASCC mixtures are shown in Figure 20, with mass loss ranging from 18 to 24% during the period of observation. The differential thermograms (DTG) of various mixtures exhibited peaks at various temperatures. Differential thermograms (DTG) of single-, binary-, and ternary-precursor mixtures showed distinct peaks at different temperatures, designated as A–C. The first two peaks (A and B) were observed at temperatures ranging from 70 °C to 120 °C for all AASCC mixtures exposed to various sulfate solutions due to the elimination of free water in the pores and the dehydration of the C-A (N)-S-H, ettringite, or gypsum gel products, these phases were detected by XRD in all specimens. Weak endothermic peaks in the range of 375–485 °C were detected in all samples designated to the dehydration of hydrotalcite at the spot labeled with C [41]. The decomposition of carbonate minerals, notable calcite, was explained with the weight loss observed in all samples within a temperature range of 675–735 °C (spot D). The existence of various structural forms of carbonates, particularly weakly crystalline or amorphous forms, in the matrix may be indicated by the elevated decomposition temperature, which ranges between 820–930 °C (spot E).

4.6. Proposed Deterioration Mechanisms

The suggested mechanism for magnesium sulfate attack, sodium sulfate, or a mixture of both sulfate solutions is shown in Figure 21. Step 1 entails exposing AASCC specimens to different sulfate solutions: (a) Na₂SO₄ and (b) MgSO₄ with a pH range of 6–8. After one month of exposure to sulfate solutions, the pH of the solution changes to 11–12. Therefore, sulfate solutions were renewed, and the pH of the solution was kept low during Step 2 to maintain a consistent rate of attack; thus, the attack was expected to proceed. In Step 3, expansive gypsum and ettringite were anticipated to form in the surface regions of the AASCC cylinders near the sulfate solutions. However, the core beneath tries to resist the sulfate attack or decalcify the strength-giving products, resulting in a complete breakdown and disintegration of the specimen.

The reaction mechanisms between the AASCC matrix and different sulfate solutions differ depending on the type of sulfate and hydration products in AASCC, as shown in Figure 21. In Na₂SO₄ solution, ettringite formation, alkali leaching, slight decalcification, and dealumination from C-A-S-H phases (Figure 21a) are assumed to occur in AASCC cylinders, as noticed from the analytical results. Nevertheless, it is believed that the presence of carbonate ions from the sodium carbonate activator inhibits ettringite formation because of the presence and competition mechanisms of carbonate ions and sulfate ions.

Experience with magnesium sulfate has frequently revealed that regular concrete can suffer from the formation of a layer of brucite Mg(OH)₂ in the pore solution due to reactions between Mg²⁺ and OH⁻ ions. This can disrupt further attacks since brucite precipitation is known to be rapid at pH ~10.5 [42]. A direct decalcification of C-S-H may occur due to prolonged pH monitoring, which may later react with Mg²⁺ and ${\mathrm{SO}}_4^{2-}$ to form M-S-H. The potential degradation mechanisms of AASCC cylinders subjected to MgSO₄ sulfate solutions are shown in Figure 21b. The anticipated attack by magnesium sulfate prompted the development of a “gypsum-layer”. Gypsum precipitates may form because of the reaction between available Ca²⁺ in the pore solution and ${\mathrm{SO}}_4^{2-}$, as well as carbonate-bearing phases because of the presence of carbonates in the pore solution.

Cross-section variations of AASCC specimens in all sulfate solutions were followed by a two-stage procedure. During Stage 1, the primary stage, the cross-section of the specimens gradually increased up to 60 d of exposure. A very modest increase followed this gradual change in the cross-section results (Stage 2), where the level of change was nearly constant. Instead, cross-section variations fluctuated at a continuously increasing rate until the end of the testing phase, considering the multi-damaging effect of a monitored pH environment, high sulfate concentration, and 183 cycles of drying and wetting.

5. Conclusions

Considering the synergistic effect of various parameters in various sulfate solutions, this paper provided a thorough understanding of the resistance of different AASCC mixture designs, activated with 1:1 two dry-powder activators, i.e., anhydrous sodium metasilicate (Na₂SiO₃) and sodium carbonate (Na₂CO₃). The findings of this investigation allow for the following inferences:

• The consequences of continuous immersion in 10% sodium, 10% magnesium, and 10% mixed sulfate solutions indicate that, except for binary-1 mixtures with SF replacing slag, all the tested AASCC mixtures were resilient to sulfate for up to 6 months. This could be due to these materials’ impermeability when different precursor blend ratios and compositions are used.
• In all AASCC cylinders, the main product of degradation in 10% MgSO₄ and 10% MgSO₄ + 10% Na₂SO₄ sulfate solutions is gypsum, whereas ettringite was produced in 10% Na₂SO₄ sulfate solutions.
• The ion chromatography analysis of the sulfate solutions revealed that, during sample exposure, there was severe leaching of Na, Ca, and Si from the structure of AASCC blends, particularly in MgSO₄ and mixed solutions.
• The exposure of singles, binary-2, and ternary-precursor mixtures resulted in pastes rich in C-(N)-A-S-H gels, other carbonate products, and mesopores rather than micropores, which delayed or inhibited the crystal growth of expansive products.
• To increase the hostility of the exposure environment, high sulfate concentrations, pH control, fractional immersion, and wetting–drying cycles were used; however, the synergistic action of all parameters increased the sulfate crystallization.
• Compared to the 10% continuous partial immersion (Exposure I), the continuous partial immersion with an uncontrolled pH condition (Exposure II), the 5% continuous partial immersion (Exposure III), and the 10% continuous partial immersion with wetting–drying cycles (Exposure IV). Exposure IV and I had nearly identical damage mechanisms, with the latter being the most severe.