Analysis of the Mechanical Properties, Durability, and Micro-Mechanisms of Alkali-Activated Fly Ash Mortar

Abstract

The search for sustainable and economical alternative materials has become a top priority in response to the increasing scarcity of natural river sand resources; as a result, a new alkali-activated granulated blast-furnace slag (GGBS)/fly ash (FA) composite cement material innovatively using Tuokexun Desert sand as aggregate has emerged as a good strategy. In this study, GGBS/FA was used in place of cement; the effects of the water glass modulus, alkali equivalent, and FA content on the material’s properties were systematically studied, and the hydration reaction mechanism and durability characteristics were revealed. The material was found to form a stable calcium aluminosilicate hydrate (C-(A)-S-H) gel structure under a specific ratio, which not only displayed excellent mechanical properties (a compressive strength of up to 83.2 MPa), but also showed outstanding resistance to high temperatures (>600 °C) and acid–alkali erosion. Microscopic analysis showed that the phase transition behaviour of C-(A)-S-H was a key factor affecting the material properties under high-temperature and acid–alkali environments. This study provides a new method for the preparation of high-performance building materials using local materials in desert areas, which is of great significance for promoting the construction of sustainable infrastructure in arid areas.

1. Introduction

When producing construction mortar, engineering sand (such as river and artificial machine-made sands) is the most commonly used fine aggregate raw material [1]; however, river sand is increasingly mined and utilized following the continuous expansion of infrastructure and real estate construction, resulting in resource depletion, rising prices, and serious river ecological and environmental problems [2]. In this context, the search for sustainable, easily accessible, and comparable fine aggregates becomes urgent. Ming Zhang explores the potential of using desert sand (DS) as an alternative to SS in producing ultra-high-performance alkali-activated concrete (UHPAAC) [3]; AkhtarMohammad Nadeem Akhtar uses desert sand to replace 50% river sand to make desert sand concrete [4]; Syed Minhaj Saleem Kazmi’s study develops high-strength desert sand concrete (DSC) using 100% DS through compression casting. Nine concrete mixes were prepared with varying DS replacement levels (0, 50, 100%) and design strengths (30, 50, 70 MPa) [5]. Xinjiang is a vast desert with extremely rich desert sand reserves, but its large-scale utilization in construction projects is still relatively limited. Effectively introducing the desert sand resources [6,7] in this region to the construction mortar preparation system could greatly reduce the transportation and procurement costs of raw materials, alleviate the pressure of river sand resource shortages, and also provide new methods of local desert management and resource utilization, benefiting regional engineering construction in terms of both economy and practicality [8]. The fine particles in desert sand can fill the pores between the aggregates, optimize particle gradation, and improve compactness, thereby enhancing the mechanical strength of the mortar [9]. The chemical composition of desert sand mainly consists of SiO₂, Al₂O₃, CaO, etc., and these microscopic components usually contain certain active substances that can enhance the volcanic ash reaction effect in the system, promote the secondary hydration reaction, improve the structure of the interfacial transition zone, and further enhance the long-term durability of the mortar [10,11]. In this study, Xinjiang Tuokexun Desert sand is selected as the research object, with its application potential as a fine aggregate in construction mortar being systematically discussed.

Alkali-activated cementitious material (AACM) is a type of hydraulic cementitious material [12] containing natural minerals like silicates and aluminates, industrial solid waste, and other artificially synthesized materials [13]. AACM is considered a green, low-carbon cementitious material and has a three-dimensional silicon–aluminum tetrahedral structure [14]. Current practice in sustainable cement production predominantly utilizes industrial by-products rich in aluminosilicates—particularly FA and ground granulated GGBS—as precursor materials, typically activated using alkaline solutions such as sodium silicate (water glass) and sodium hydroxide [15]. This cementitious material displays excellent mechanical properties; is resistant to acid and alkali corrosion [15], freezing and thawing, high temperatures, and carbonation when compared to ordinary silicate cement [16]; and involves a simple preparation process. Additionally, due to the lack of high-temperature firing and the resourceful utilization of industrial solid waste [17], this method consumes little energy [18] and is thus a relatively green and environmentally safe alternative to OPC [19].

The performance of AACM is highly dependent on both the proportion and dosage of the composite alkali activator used. P. Kryvenko’s results [20] indicate the importance of the Al₂O₃ content in the cement and aggregate. Together with components that are able to actively interact with alkalis in the presence of reactive SiO₂, the processes taking place during the alkali–aggregate reaction can present either destructive or constructive effects. Jiao et al. [21] investigated the effects of using alkali equivalents of 6%, 8%, and 10% to prepare alkali-activated mortar on its performance; the results indicated that the alkali equivalent significantly influenced the mortar’s performance. The optimal compressive strength was achieved at an alkali equivalent of 8%, while further increases in alkali content resulted in a strength reduction. Dong et al. [22] investigated the impact of precursor composition, alkali activator dosage, and liquid–solid ratio on the workability, mechanical strength, and hydration process of alkali-activated GGBS/FA cements, finding that higher GGBS dosages not only promoted faster alkali activation but also improved the compressive strength of the resulting material. Yang et al. [23] analyzed the microstructure of AACM using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), concluding that the increase in FA reduced the formation of C-(A)-S-H and the strength of the material and inhibited hydrated calcium silicate crystallization.

FA and GGBS composites can potentially be used to make AACM [24]. Adding a certain amount of slag to the alkali-activated FA system [13] can increase the hydration rate, promote the formation of more gel, and form a denser microstructure [25,26]; introduce more Ca²⁺, promote polymerization reactions, and improve the density and mechanical properties of hydration products [27]; and enhance the compressive strength, which decreases as the water–cement ratio increases (Lee et al. [28]). Van Jaarsveld and Lee [29] employed FA with varying CaO levels as a raw material during AACM manufacturing, discovering that the amorphous CaO content of FA impacts the compressive characteristics of AACM and that the release of Ca²⁺ into alkaline environments is accelerated. Lilong Wang designed a composite cementitious material with GGBS as the precursor [30]. Harini Konduru used silica ash and GGBS waste as precursor materials in the alkali-activated polycondensation reaction to determine the feasibility of producing a one-component alkali-activated ground polymer for use in cast-in-place construction [31].

Based on previous analyses, this study systematically investigated the influence mechanisms of key parameters, such as water glass modulus, alkali equivalent, and FA content, on the mechanical properties of desert sand-based AACM. The fly ash–slag composite system is used as the precursor, and new environmentally friendly building materials are prepared using Xinjiang Tokexun Desert Sand as the fine aggregate and a Na₂SiO₃-NaOH composite solution as the activator. By conducting a macroscopic performance test under standard curing conditions, we clarified the influence of different mix ratios on the mechanical properties of AACM mortar and evaluated its durability under harsh environments, including extreme temperatures and strong acid and alkali conditions. The reaction mechanism of AACM gelling materials was investigated using microscopic techniques such as X-ray diffraction (XRD), Fourier infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The research results deepen our understanding of the reaction mechanism of alkali-excited materials and provide new ideas for the development of high-performance green building materials suitable for harsh environments.

2. Raw Materials and Experimental Methods

2.1. Raw Materials

The SEM (Hitachi S4800, Tokyo, Japan) images of the raw materials are given in Figure 1. FA and GGBS are characterized by spherical particles of various sizes and irregular angular-shaped particles, respectively, while TD is primarily characterized as an irregular ellipsoid with good roundness, chronic wind erosion, and a loose surface. Figure 2 illustrates the mineral phases of FA, GGBS, and TD. GGBS displays a broad peak at 2θ between 20 and 40°, indicating the existence of amorphous SiO₂ and significant chemical reactivity, while FA is mostly made up of SiO₂, mullite, and iron trioxide with high crystallinity. SiO₂ is the most common mineral in TDs, alongside a small quantity of sodium feldspar.

Table 1. The chemical composition of FA, GGBS, and TD.

Materials	Chemical Composition (%)
	SiO2	Al2O3	CaO	Fe2O3	MgO	SO3	Na2O	K2O	TiO2
FA	40.31	24.38	8.02	10.27	5.75	5.23	2.6	1.28	0.72
GGBS	34.50	17.70	34.00	1.03	6.01	0.04	/	/	/
TD	73.10	14.63	2.69	2.13	1.37	/	2.70	2.96	/

displays the chemical compositions of FA, GGBS, and TD as determined by means of X-ray fluorescence (XRF (Bruker S2 PUMA, Billerica, MA, USA)). FA, GGBS, and TA primarily contain SiO₂, Al₂O₃, Fe₂O₃, and CaO, SiO₂, CaO, Al₂O₃, and MgO, and a high concentration of silica-aluminum substances as well as a small amount of alkali metal oxides, respectively. Additionally, GGBS shows good reactivity, with activity and water hardness coefficients of 0.51 and 1.67, respectively.

2.2. Alkali Activator

The experimental design is specifically for the selected parameter range (water glass modulus, alkali equivalent, and FA content).

The chemical composition of water glass can be expressed as Na₂O·nSiO_2, where n is the modulus of water glass, as depicted by the molar ratio of SiO₂ and Na₂O. In this experiment, three different water glass moduli of 1.2, 1.4, and 1.6 were used to ensure a better excitation effect. To reduce the existing water glass modulus from 3.12 to 1.2, 1.4, and 1.6, NaOH must be added. The calculation process is as follows:

Supposed: if the required Na mass in the experiment is x, then the required Na₂O mass is 1.348x;

The number of substances in the final solution SiO₂/Na₂O = 1.2, where the SiO₂ is provided by the raw water glass solution;

The mass of Na₂O in the original solution is 1.348/3.12 = 0.432x;

Then, NaOH needs to provide Na₂O mass of 1.348x − 0.432x = 0.916x;

From: 2NaOH → Na₂O = H₂O;

Result: M_(NaOH) = 0.916x × 80/62 = 1.182x;

M_{(Water Glass)} = 0.432x/8.7% = 4.966x.

According to the amount of cementitious material, the amount of water glass and the addition of NaOH flakes can be calculated.

Following preliminary experiments and the relevant literature, the mechanical properties of GGBS and FA content were 80% and 20%, respectively, and were optimal after curing to the corresponding age.

The modulus of the water glass employed in this test was M = n(SiO₂)/n(Na₂O), where M, the ratio of the amount of SiO₂ to Na₂O, was found to be 3.12. The Na₂O and SiO₂ contents were 8.2% and 26.4%, respectively. NaOH was used in a flake form, with a purity of 98%, and the water used was laboratory tap water. Different amounts of NaOH were added to water glass to prepare water glass solutions with different moduli. The alkali activators were configured the day before the experiment, with rapid stirring to dissipate the heat produced by the dissolution of NaOH and Na₂SO₄.

2.3. Preparation of the Sample

Based on earlier research, the impacts of different water glass moduli (1.2, 1.4, and 1.6) and alkali equivalents (4%, 6%, and 8%) on the mechanical properties of FA composite cementitious materials were investigated. The specific raw material ratios for the tests and the experimental samples are presented in

Table 2. Desert sand-based alkali-activated GGBS and FA mixture ratio design.

Number	Water-to-Glue Ratio	TD (g)	GGBS (g)	FA (g)	Water Glass Modulus (n)	Alkali Equivalent (%)	Alkali Activator (g)	Water (g)
1	0.5	1350	360	90	1.2	4	81.74	182
2	0.5	1350	360	90	1.2	6	123.46	160.34
3	0.5	1350	360	90	1.2	8	164.21	138.78
4	0.5	1350	360	90	1.4	4	81.74	182
5	0.5	1350	360	90	1.4	6	123.46	160.34
6	0.5	1350	360	90	1.4	8	164.21	138.78
7	0.5	1350	360	90	1.6	4	81.74	182
8	0.5	1350	360	90	1.6	6	123.46	160.34
9	0.5	1350	360	90	1.2	8	164.21	138.78

Note: The amount of alkali exciter is calculated according to the alkali equivalent; the water referred to is tap water; the amount of water needs to be subtracted from the water consumption of the water glass; and cement is completely replaced by GBSS and FA.

and Figure 3, respectively. The effects of varied FA doping (0%, 20%, 30%, and 40%) on material properties were investigated using the optimal water glass modulus and alkali equivalent. All water–cementitious ratios were fixed at 0.5, with the specific raw material ratios for the tests presented in

Table 3. Desert sand-based alkali-activated FA mixture ratio design.

Number	Water-to-Glue Ratio	TD (g)	GGBS (%)	FA (%)	Waterglass Modulus (n)	Alkali Equivalent (%)	Alkali Activator (g)	Water (g)
1	0.5	1350	450	0	1.2	6	123.46	160.34
2	0.5	1350	360	90	1.2	6	123.46	160.34
3	0.5	1350	315	135	1.2	6	123.46	160.34
4	0.5	1350	270	180	1.2	6	123.46	160.34

Note: The amount of alkali exciter is calculated according to the alkali equivalent; the water referred to is tap water; the amount of water needs to be subtracted from the water consumption of the water glass; and GGBS and cement are completely replaced by GBSS and FA.

. Three parallel samples were set up for all experimental groups, and the data are presented in the form of “mean ± standard deviation”.

For sample preparation, first add FA and slag powder into the planetary cement mortar mixer and stir at low speed for 60 s to ensure thorough mixing; then, add the preconfigured alkaline activator solution and mix at low speed for 30 s; and finally, add desert sand and stir quickly for 240 s before stopping. The mixed slurry is poured into a 40 mm × 40 mm × 160 mm mould, vibrated for 60 s to eliminate air bubbles, and then placed in a conventional curing environment for 24 h. The specimens are demoulded, tagged, and then cured in a normal curing box until the appropriate age. The temperature and relative humidity of the normal curing box are 20 ± 2 °C and ≥95%, respectively.

2.4. Performance and Characterization Tests

2.4.1. Mechanical Performance Testing and Durability Tests

Mechanical Performance Tests

The flexural and compressive strength of the test specimen are tested according to the method specified in GB/T 17671-2021 [32] “Test method of cement mortar strength (ISO method)” using the fully automatic compression and bending machine used in said experiment.

(1)

Flexural strength test:

The specimen to be tested is set horizontally on the supporting column, and the lower pressure plate slowly rises until close to the specimen surface; the controller then zeroes the load data and presses the begin button, after which the test press squeezes the specimen, breaking it into two pieces; the flexural test is finally terminated, and the screen shows the extreme value of the stress and the value of the strength. The test span is 100 mm, with a loading speed of 50 N/s ± 10 N/s.

The flexural strength is calculated according to Formula (1):

$${\mathrm{R}}_{\mathrm{f}}={\displaystyle \frac{1.5{\mathrm{F}}_{\mathrm{f}}\mathrm{L}}{{\mathrm{b}}^3}}$$

(1)

where

${\mathrm{R}}_{\mathrm{f}}$—the flexural strength (MPa);

Ff—the load applied during fracture (N);

L—the distance between supporting cylinders (mm);

b—the side length of a square cross-section of a prism (mm).

(2)

Compressive strength test:

When three specimens were examined for flexural strength, half were chosen for compressive strength testing, during which the loading speed was 2400 N/s ± 200 N/s.

The compressive strength is calculated according to Formula (2):

$$R_c\mathit{=}{\displaystyle \frac{F_c}{A}}$$

(2)

where

$R_c$—compressive strength (MPa);

Fc—maximum load at failure (N);

A—compressed area (mm²).

Durability Test Design

Select the sample prepared with the optimal ratio for durability testing.

(1)

High-temperature resistance performance test

The specimens prepared with the optimal mix ratio were placed in a standard curing room for 28 d before being removed and placed in muffle furnaces at 200 °C, 400 °C, 600 °C, and 800 °C for high-temperature resistance testing, where the heating rate of the muffle furnace was 15 °C/min. After holding for 2 h, the muffle furnace was turned off and cooled to room temperature before the specimen was taken out to test the mass change rate and flexural and compressive strength.

(2)

Acid and alkali resistance test

The specimens prepared with the optimal mix ratio were placed in a standard curing room for 28 d before being taken out and immersed in H₂SO₄ and NaOH solutions with concentrations of 0.2 mol/L, 0.5 mol/L, 0.8 mol/L, and 1.0 mol/L for 7 d and 28 d, respectively. After reaching the corresponding age, their mass was tested, the mass change rate was calculated, and the flexural strength and compressive strength were tested.

(3)

Quality change testing

The volume of the specimen is 40 mm × 40 mm × 160 mm, and Formula (3) is used to calculate the mass change rate of the alkali-activated FA composite cementitious material.

$$\mathrm{Wm} = {\displaystyle \frac{{\mathrm{M}}_1-{\mathrm{M}}_0}{{\mathrm{M}}_0}} \times \mathrm{100\%}$$

(3)

where Wm, M₁, and M₀ indicate the change rate (%), initial sample mass (g), and sample mass after testing (g).

2.4.2. Microstructure Characterization

Following compressive testing at various ages, the core material of the specimens was immersed in anhydrous ethanol for 24 h to arrest hydration. After drying at 60 °C for 24 h, the samples were then characterized using XRD (Bruker D8 Advance, Billerica, MA, USA), FTIR (Great 10, Tianjin, China), and SEM. XRD patterns were collected on a Bruker D8 Advance diffractometer with a 2θ range, a step size, and a counting time of 10–80°, 0.02°, and 2 s per step, respectively. The microstructure of hydration products was examined using a scanning electron microscope. Prior to imaging, the specimens were sputter-coated with a gold layer and analyzed at an accelerating voltage and a current of 15 kV and 10 mA, respectively. FTIR spectroscopy was conducted on a Great 10 spectrometer using the KBr pellet method (sample/KBr mass ratio = 1:200). Spectra were acquired over 64 scans in the range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹.

3. Results and Discussion

3.1. Mechanical Properties

3.1.1. Water Glass Modulus and Alkali Equivalent

It can be seen from Figure 4 that, when the water glass modulus increases from 1.2 to 1.6, the compressive strength and flexural strength of the alkali-activated GGBS/FA composite cementitious material decrease. When the system alkali equivalent increases from 4% to 8%, the compressive strength of the sample increases, while the flexural strength first increases and then decreases (Figure 4a). A system alkali equivalent of 4% indicates that GGBS and FA are not fully involved in the hydration reaction due to the system’s low OH^- content and that the gel material generated is insufficient to fill the gaps of and bond the desert sand particles, resulting in poor mechanical properties (Figure 4c). At an 8% alkali equivalent, the 28 d compressive strength is 83.2 MPa, 71.8 MPa, and 60.8 MPa with water glass moduli of 1.2, 1.4, and 1.6, respectively, highlighting that the optimum compressive strength of 83.2 MPa is reached with a water glass modulus of 1.2 and an alkali equivalent of 8%. At an optimal ratio of a 4% alkali equivalent and M = 1.2, the 28 d compressive strength was 204.4% that of the 3 d compressive strength. In Figure 4a, increasing the alkali equivalent from 4% to 8% increased the compressive strength at 3 d, 7 d, and 28 d by 626.8%, 303.9%, and 175.5%, respectively, with a similar trend being observed in Figure 4b,c, indicating that the alkali equivalent had a significant influence on early strength, most notably increasing it (Figure 4a). When the alkali equivalent increases from 4% to 6%, flexural strength gradually increases by 173.3%, 110.8%, and 68.6% at 3 d, 7 d, and 28 d of age, respectively; however, this tends to decrease when the alkali equivalent increases from 6% to 8%. In Figure 4b,c, the splitting strength also exhibits a tendency to increase first before decreasing. The optimal flexural strength of 8.6 MPa was achieved at M = 1.2 and a 6% alkali equivalent.

At the same water glass modulus, as the alkali equivalent increases, the compressive strength of alkali-activated GGBS/FA composite cementitious materials continues to increase significantly. At the same alkali equivalent, as the water glass modulus increases, the compressive strength of the sample decreases [26]. The influence of alkali equivalent on the mechanical properties of alkali-activated GGBS/FA composite cementitious materials is much greater than that of the water glass modulus. As the alkali concentration in the solution increases, the Ca-O, Al-O, and Si-O bonds in GGBS and FA break [27]. As the pH rises, the alkali activation of GGBS/FA increases and Ca²⁺ and Al³⁺ are dissociated and polymerized, producing C-A-H, C-S-H, C-A-S-H, and other complicated gelling products [28]. Over time, the gel then thickens and the degree of polymerization improves, following which the structure becomes denser and the specimen’s strength increases [29].

3.1.2. FA Content

Figure 5 shows that, as FA doping increased, the composite materials’ 3 d and 7 d compressive strengths, 28 d compressive strength, and split strength all showed trends of increasing and then decreasing to varying degrees. This is because amorphous FA activation is difficult, and increasing FA doping in GGBS leads to decreased gel generation. Early activation of the gel can not fully fill and bond the voids, meaning the compressive strength and splitting strength are decreased. When the time is increased to 28 d and the addition of FA is above 20%, the gel product generated by the synergistic reaction of GGBS and FA increases, showing the highest strength among the compared specimens.

Specifically, the compressive strength of the optimal group declined by 10.4% and 6.2% compared to that of 0% FA doping for 3 and 7 d, respectively, while increasing by 13.7% for 28 d. The optimum group’s compressive strength increased by 33.4%, 30.4%, and 27.2% compared to 40% FA doping after 3 d, 7 d, and 28 d, respectively. Meanwhile, the ideal group’s flexural strength increased by 17.8%, 4.2%, and 16.2% and 35.9%, 25.4%, and 22.9% compared to 0% and 40% FA doping for 3 d, 7 d, and 28 d, respectively.

3.2. Microstructure Characterization

3.2.1. XRD Analysis

It can be observed from Figure 6 that the main phases of hydration products include quartz (SiO₂), C-(A)-S-H, Anorthite (CaAl₂Si₂O₈), etc. There are a large number of SiO₂ phases in the cementitious material under different conditions and ages, which may be the result of FA and GGBS component residues that have not fully participated in the reaction. In XRD plots, 2θ shows a broad, diffuse peak at around 20–40°, resulting from the main hydration product of reactive SiO₂ and Al₂O₃ in the alkali-activated GGBS/FA composite cementitious material. This suggests that an amorphous hydration product, namely, the C-(A)-S-H gel [33,34], was formed in the material and acts as the main source of the material’s later strength. Figure 6a shows the diffraction peak at 2θ = 28° for a water glass modulus of 1.2, corresponding to calcium feldspar’s product composition. The amorphous gel phase is the largest at modulus 1.4, which is consistent with the optimum mechanical qualities. In Figure 6b, the hydration products show obvious changes at different alkali equivalents; however, at a 4% alkali equivalent, the SiO₂ peak is sharp, with fewer hydration products, such as calcium feldspar and C-(A)-S-H, being generated, largely as a result of the system’s low alkali concentration, the insufficient hydration reaction, and the complex crystalline phase reaction [35]. Figure 6c shows a sharp diffraction peak at 2θ = 35° at 3 d, the corresponding product of which is Magnetite, a mineral not involved in the hydration reaction in the original FA. The “hump” area corresponding to the hydration product C-(A)-S-H gel increases with age; Figure 6d shows the appearance of plagioclase at different FA concentrations, with the maximum “hump” area and strength being observed at a FA concentration of 20%, consistent with the analysis of mechanical properties.

In this study, the hydration products worth exploring are the C-(A)-S-H phase and the calcite phase. The “bulge peak” area corresponding to C-(A)-S-H varies with changes in reaction conditions and age. The calcite feldspar phase, whose types are diversified, participates in the hydration reaction and develops into the C-(A)-S-H phase with increasing age, thus acting as the main source of strength in the later stage of alkali-excited GGBS/FA composite cementitious materials.

3.2.2. FTIR Analysis

Figure 7 depicts the effects of the water glass modulus, alkali equivalent, time, and FA dosing on gel FTIR. The characteristic absorption band at around 3447 cm⁻¹ is the hydroxyl group, caused by the stretching vibration of the O-H group in the H₂O carried by C-(A)-S-H in the system [36]; the bending vibration of the C-(A)-S-H gel-bound water in the system is responsible for the peak at 1603 cm⁻¹ [37]; the stretching vibration of the O-C-O bond at 1359 cm⁻¹ suggests the existence of carbonates in the gel, most likely resulting from the reaction of alkalis with CO₂ to form carbonates [38]; and the absorption peak appearing at around 1130 cm⁻¹ is caused by the antisymmetric stretching vibration of Si-O-Si bonds [39]. Figure 7a indicates that the absorption peak at around 980 cm⁻¹ corresponds to the Si-O symmetric stretching vibration of Q2, which is a characteristic peak of C-A-S-H gels [40] and weakens with an increasing water glass modulus. Figure 7b indicates that, when the base equivalence increases, the primary peak at around 980 cm⁻¹ moves to lower wave numbers because this increase promotes the dissociation of Si-O and Al-O bonds and the creation of Al-O-Si bonds, resulting in an increase in C-(A)-S-H [41]. Figure 7c demonstrates that the absorption peak around 980 cm⁻¹ strengthens over time, which is consistent with the findings that XRD produces more gels as time passes. Figure 7d shows that when the FA content is 20%, the absorption peak of the hydration product is greater than the strength, indicating that under this content, the Al-O bond replaces the Si-O bond, the reaction is sufficient, and more hydration products, namely, C-S-H or C-(A)-S-H gel, are generated. The symmetric stretching vibration of Si-O-Si corresponds to around 775 cm⁻¹. The absorption peak appearing at around 465 cm⁻¹ at low wavenumbers is caused by the in-plane bending vibration of the (Si-O-Si) plane. The shift in the absorption band at about 900 cm⁻¹–1200 cm⁻¹ and the absorption peak at about 460 cm⁻¹ both indicate that C-S-H or C(A)-S-H gel appears in the alkali-activated GGBS/FA composite cementitious material.

No significant differences were found in the FTIR spectra of alkali-excited GGBS/FA prepared at different ages, indicating that the complex bonding methods of calcium carbonate salts, bound water, silicon oxygen bonds, and aluminum oxygen bonds coexist in the materials and that the spectral absorption peaks of 400 cm⁻¹ were found to be dense and diverse under different conditions, especially at around 1000 cm⁻¹. Additionally, the Si-O-T bonding methods of hydration products in the system were complex and diverse, with different coordination situations being found during the bonding process.

3.2.3. SEM Analysis

It can be observed from Figure 8a,b,g that the hydration product gel C-(A)-S-H shows a highly polymerized three-dimensional network structure in the system under different moduli of the sample, which is already very compact and fine under a 1.4 modulus. The denser this structure is, the greater the degree of hydration in the system and the better the mechanical properties, as is consistent with the analysis of mechanical properties. Figure 8c demonstrates that, at a 4% alkali equivalent, there are many micropores on the surface but very little gel product is produced, as is consistent with the XRD results; this is as a result of the alkali equivalent being insufficient to activate the GGBS and FA, thus resulting in an inability to produce a high number of hydration products [42]. Figure 8d shows that the system has a good alkaline environment and the hydration reaction proceeds smoothly at a 6% alkali equivalent; however, there is still a small amount of GGBS and FA that is not involved in the hydration reaction, as seen in the XRD results. Figure 8e illustrates that the material hydration process produces a modest amount of C-(A)-S-H [43] at 3 d. While GGBS has a high glassy phase, which is favourable for alkali activation, FA crystalline phases are difficult to activate because they have more glassy phases and thus prolong the reaction; however, no FA globular particles were observed, implying that FA vitreosity has been destroyed or covered by a gel phase [44]. Figure 8g shows that when the alkali equivalent, modulus, and age are 8%, 1.2, and 28 d, respectively, the angular dissolution of GGBS disappears, the morphology changes dramatically, the flocculent gel is more encapsulated and covers the surface of GGBS, the internal structure of the specimen is denser, and the amorphous gel material effectively bonds the desert sand particles, as is consistent with the XRD and mechanical properties results.

3.3. Durability Performance Test

3.3.1. High-Temperature Resistance Performance Test

As shown in Figure 9, as the temperature increases, the mass loss rate of alkali-activated GGBS/FA composite cementitious material increases while the compressive and flexural strength both decrease. At 200 °C, 400 °C, 600 °C, and 800 °C, the mass loss rates of the samples were 8.37%, 9.4%, 9.91%, and 11.9%; the compressive strength decreased by 11.5%, 35.8%, 55%, and 90.9%; and the flexural strength decreased by 52.3%, 75.6%, 90.7%, and 95.3%, respectively. At temperatures below 600 °C, the specimen’s mass loss is minimal due to water loss from free and partially bound water in the gel [45], causing gaps and cracks in the cementitious material and thus reducing its mechanical characteristics. As the calcination temperature rises above 600 °C, the specimen’s mass loss rate sharply increases due to the destruction of the hydration product C-(A)-S-H at high temperatures, causing cracks to expand and extend within the specimen; the compressive and flexural strength also decrease significantly. As the temperature reaches 800 °C, the decomposition of the gelation product C-(A)-S-H is exhausted [46], the specimen’s appearance changes from yellow–grey to yellow–white, cracks, and shrinkage deformation are obvious, and a large amount of powder appears on the surface due to serious shedding. Additionally, the composition of the physical phase shifts from the amorphous phase to the crystalline phase, with the material almost losing its working performance. Overall, the alkali-activated GGBS/FA composite cementitious material exhibits superior thermal stability at elevated temperatures.

3.3.2. H₂SO₄ Resistance Test

Furthermore, as exhibited in Figure 10, the mass loss rate of alkali-activated GGBS/FA composite cementitious materials increased with the H₂SO₄ concentration, while the compressive and flexural strength dropped. After 28 d of immersion in H₂SO₄ at concentrations of 0.2 mol/L, 0.5 mol/L, 0.8 mol/L, and 1.0 mol/L, the compressive and flexural strength declined by 16.6%, 23.9%, 30.4%, and 38.1% and 34.9%, 38.4%, 40.7%, and 47.7%, respectively. The corrosive effect of H₂SO₄ solution on alkali-activated GGBS/FA cementitious material increases with its concentration. The most obvious rate of mass loss after 7 d of immersion in H₂SO₄ solution results from H₂SO₄ rapidly reacting with the surface material, thus causing the surface material to become brittle through decalcification. Once the reaction of the surface layer is complete, H₂SO₄ slowly penetrates into the sample through the contact surface, which is a gradual and incomplete process, thus resulting in slower mass loss. The specimen’s strength loss is also related to the gel material’s internal decalcification reaction, producing SiO₂ and CaSO₄ [47].

3.3.3. NaOH Resistance Test

Figure 11 depicts that the concentration of NaOH increases with the specimen mass and compressive strength; the higher the NaOH concentration, the older the specimen and the faster the rate of mass loss; and the compressive strength and flexural strength decrease as the concentration increases. The rate of change in mass over 28 d in NaOH solutions with concentrations of 0.2 mol/L, 0.5 mol/L, 0.8 mol/L, and 1.0 mol/L was 2.78%, 0.82%, −1.35%, and −2.53%, respectively. The lower concentration of NaOH solution acts as an activation agent in the silica-aluminum raw material, activating the unreacted silica-aluminate and generating C-(A)-S-H to fill the gel material interior and thereby increasing the specimen’s overall compressive strength. At a concentration of 0.5 mol/L, the mass growth rate was higher at 7 d than at 28 d due to the fact that the early stage activation of NaOH continued to activate GGBS and FA to create C-A-S-H gel, causing the mass to grow, whereas in the later stage, NaOH reacts with C-A-S-H to generate the less dense N-A-S-H, thus resulting in mass loss. However, less bonding is necessary in N-A-S-H than in C-(A)-S-H, where it effectively fills the specimen’s voids, increasing compressive strength but decreasing flexural strength. Under a 1.0 mol/L NaOH concentration, the compressive strength of the 28 d specimens insignificantly decreased by around 10.1% as NaOH migration is limited by the specimen block’s void space. Consequently, at lower alkali concentrations, the material is activated, whereas at higher concentrations, the material is partially eroded, increasing and reducing the mass and strength, respectively. The test results indicate that alkali-activated GGBS/FA composite cementitious materials display high alkali resistance.

4. Conclusions

In this study, alkali-activated composite cementitious mortar was made using Tuokexun Desert sand, GGBS, and FA as raw materials and tested for compressive and flexural strength, high-temperature resistance, and acid–base resistance under conditions such as a suitable alkali activator ratio and an appropriate FA mixture. The following main conclusions were drawn:

(1) When the water glass modulus is 1.2, the FA dosage and alkali equivalent are 20% and 8%, respectively; after 28 d of curing, the compressive strength reaches a maximum of 83.2 MPa, while the flexural strength reaches a maximum of 8.6 MPa at a 6% alkali equivalent.

(2) At a water glass modulus of 1.2, the compressive strength of the 8% alkali equivalent sample increased by 72.9% compared to the 4% sample. Conversely, at an alkali equivalent of 8%, the compressive strength of the 1.2 water glass modulus sample increased by 26.9% compared to the 1.6 modulus sample. The effect of the alkali equivalent on alkali-activated GGBS/FA composite cementitious materials is much greater than that of the water glass modulus, playing a key role in the hydration rate of the composite material.

(3) When the water glass modulus was 1.2, the alkali equivalent was 8%, and the addition of FA was 20%, the C-(A)-S-H gel hydration product at 28 d displayed the most C-(A)-S-H, the densest structure, the best strength, and the best resistance to high temperatures and acid–base properties.