Experimental Study on the Physical Properties of Autoclaved Bricks Made from Desert Sand and Their Resistance to Sulfate Attacks

Abstract

In order to optimize the application of desert sand autoclaved bricks in rural construction in Xinjiang, this study focuses on the research and development of MU15-grade desert sand autoclaved bricks. Experimental investigations were conducted to examine the relationship between the water absorption rate of desert sand autoclaved bricks and the duration of water absorption while analyzing the impact of the water absorption rate on the compressive strength of these bricks. Additionally, experimental research was carried out to evaluate the appearance, compressive strength, and pore structure of autoclaved bricks after sulfate erosion. The results indicate the following. (1) With an increasing immersion time, the water absorption rate of desert sand-based autoclaved bricks initially rises and then declines, reaching approximately 14.74% when immersed for 4 h, which is close to the saturation water absorption rate. (2) The compressive strength of desert sand-based autoclaved bricks gradually decreases with an increasing water absorption rate, reaching its lowest point when saturation is attained, with a strength loss rate of approximately 33.18%. (3) Finally, after sulfate erosion, cracks and detachment appear on the surface of desert sand-based autoclaved bricks, and these cracks extend and propagate with the continuous accumulation of eroded products. Simultaneously, this process leads to an increase in the proportion of harmful pores by 0.96%, thereby causing a deterioration in strength. Through data analysis, a decay curve of the compressive strength erosion coefficient of desert sand-based autoclaved bricks with the number of sulfate erosion cycles was established, with good accuracy. This study provides theoretical references and technical support for the performance characteristics of desert sand-based autoclaved bricks and their application in rural construction in Xinjiang.

1. Introduction

In recent years, the scale of infrastructure construction in China has expanded continuously, leading to increased consumption of building materials. However, excessive exploitation of river sand and mineral resources has severely damaged the ecological environment, necessitating the urgent need to find alternative sources of sand to alleviate the supply-demand imbalance [1]. Xinjiang, located in northwest China, has a desert area exceeding 400,000 square kilometers. Following the principle of adapting to local conditions, accelerating the industrial application of desert sand can not only effectively alleviate the sand supply-demand conflict but also has significant environmental benefits. As a key node of the “Belt and Road Initiative” [2], the construction in rural and town areas of Xinjiang predominantly consists of masonry structures. Therefore, rational use of desert sand in the preparation of building materials to serve local engineering projects in Xinjiang is an initiative with notable economic benefits and social value. Moreover, the efficient utilization of desert sand in concrete and mortar indicates that focusing on autoclaved desert sand bricks is an essential area of research [3,4,5].

Autoclaved bricks, a multifunctional and environmentally friendly wall building material, are typically produced by using various industrial waste materials as raw ingredients. Over the years, scholars both domestic and abroad have extensively researched different types of autoclaved bricks. Among them, Chindaprasirt et al. [6] obtained autoclaved fly ash bricks using fly ash and lime and investigated the strength formation mechanism of these bricks. Tayfu et al. [7] utilized lime as a binding material and primarily employed fly ash as a raw material, successfully manufacturing lightweight autoclaved fly ash bricks. Yonghao et al. [8] produced autoclaved ash-sand bricks using copper tailings and river sand, meeting the requirements of Grade MU15 national standards. The strength of these bricks mainly derives from the formation of tobermorite during the autoclaving process. Zhou et al. [9] manufactured autoclaved bricks using electrolytic manganese slag and cement, ensuring compliance with specified heavy metal content regulations and rendering them suitable for the construction industry.

The applicability of autoclaved bricks made from desert sand in the masonry structures of rural villages in Xinjiang is closely related to their durability. Northwestern China, which is rich in desert resources, contains thousands of inland salt lakes and vast saline-alkali lands [10], with high concentrations of sulfates present in the soil and groundwater (Figure 1). Physical and chemical processes result in cracks, spalling, and subsequent degradation of the substrate strength [11,12,13]. For instance, certain hydraulic structures in Xinjiang are exposed to environments with high concentrations of sulfate ions, resulting in corrosive damage shortly after concrete casting [14,15]. The mechanisms of sulfate erosion and matrix degradation vary depending on the ecological environment. Santhanam et al.’s research [16] indicates that the rate of sulfate erosion is influenced by factors such as the types of hydration products, the salt content within the pores, temperature fluctuations, the immersion depth of the specimens, and the wet-dry cycles. While scholars continue to study and explore the erosion process, they also adopt corresponding measures to enhance the matrix’s resistance to sulfate erosion [17,18].

Autoclaved bricks made from desert sand, the subject of this study, are processed from lime, coarse aggregate, desert sand, and water through procedures including mixing, stirring, digestion, shaping, and autoclaving [19]. Previous studies have established that the primary hydration products of autoclaved desert sand bricks are platy tobermorite, needle-like gismondine, and flocculent calcium silicate hydrate (C-S-H) gel. These hydration products, along with unreacted desert sand and coarse aggregates, form a dense network structure, enhancing the strength of the autoclaved bricks. To better apply autoclaved desert sand bricks under the unique climatic conditions of the Xinjiang region, this study focuses on key performance indicators such as water absorption, the compressive erosion resistance coefficient, and the pore structure. By thoroughly investigating these indicators, this research aims to provide technical support for the engineering applications of these bricks in the sulfate-rich areas of Xinjiang.

2. Experimentation

2.1. Material Properties

The desert sand used in this experiment was sourced from the Gurbantünggüt Desert located in Shashanzi Town under the 83rd Regiment of the Xinjiang Production and Construction Corps. It had an average particle size ranging from 100 to 300 μm (Figure 2). The chemical composition of this desert sand is presented in

Table 1. Chemical composition of desert sand.

Composition	SiO2	Al2O3	Na2O	CaO	K2O	Fe2O3	MgO	Other
Content (%)	67.10	17.90	4.94	4.22	3.48	1.35	0.83	0.18

.

The lime used in this experiment was obtained from a lime factory in the 83rd Regiment of the Xinjiang Production and Construction Corps. All properties of the lime met the requirements of the “Hydrated lime for silicate building products” standard JC/T621-2021 [20]. The composition of the lime is detailed in

Table 2. Chemical composition of lime.

Composition	SiO2	Al2O3	Na2O	CaO	K2O	Fe2O3	MgO	SO3
Content (%)	7.22	5.67	0.36	76.79	0.07	0.42	2.60	0.21

.

The coarse aggregate used in this experiment consisted of a mixture of sand and stone or leftover materials from the conveyor belts beneath mining sites. It had an average particle size ranging from 1 to 3 mm, with the primary chemical component being CaCO₃.

Ordinary tap water was used.

2.2. Preparation of Desert Sand Autoclaved Bricks

The mix proportion of MU15-grade desert sand autoclaved bricks used in this experiment was as follows (wt%): desert sand accounted for 70.8%, aggregate accounted for 14.2%, and lime accounted for 15%, with a water-to-material ratio of 15% [21]. The preparation method for desert sand-based autoclaved bricks involves batching, mixing, pressing, curing, and autoclaving. Initially, the raw materials are precisely weighed according to the formulation, followed by a 2 min mixing process. Subsequently, the mixed materials are pressed into molds for shaping and allowed to cure for an initial 10 h period to achieve preliminary hardening. Finally, the cured bricks are subjected to high-temperature and high-pressure steam treatment in an autoclave to ensure final hardening and strength enhancement. Upon completion of the autoclaving process, desert sand-based autoclaved bricks are obtained as depicted in Figure 3, with the standard dimensions for MU15 bricks being 53 mm × 115 mm × 240 mm.

2.3. Experimental Methods

Following the specifications outlined in “Test Methods for Masonry Units” GB/T 2542-2012 [22], absorption rate and compressive strength tests were conducted on the developed MU15-grade desert sand-based autoclaved bricks. The relationship between the absorption rate and absorption time, as well as the compressive strength, was studied in accordance with Equation (1) for the compressive strength and Equation (2) for the absorption rate. Simultaneously, the dried autoclaved bricks were subjected to sulfate erosion testing using a sulfuric acid dry-wet test chamber (HDGS-54). The erosion medium consisted of a 5 wt% sodium sulfate solution. The soaking time was set to 15 h, followed by air drying for 1 h, drying at a temperature of (80 ± 5) °C for 6 h, and air cooling for 1 h, cooling at a temperature of (25 ± 5) °C for 1 h, with each cycle totaling 24 h. Dry-wet sulfate erosion cycles were conducted for 10, 20, 30, 40, 50, and 60 cycles. Upon completing the designated cycle period, in accordance with the standard “Autoclaved Fly Ash Sand Solid Bricks and Solid Blocks” GB/T 11945-2019 [23], visual changes were recorded, the compressive strengths of the specimens were measured, and the compressive erosion resistance coefficient was calculated as per Equation (3):

$$R_p={\displaystyle \frac{P}{L\times B}}$$

(1)

Here, R_p stands for the compressive strength of the sand-compacted bricks from desert sands, measured in MPa, while p stands for the maximum failure load. The dimensions (L and B) of the compressed surface were assessed in millimeters (mm):

$$W={\displaystyle \frac{m-m_0}{m_0}}\times 100\%$$

(2)

In this equation, m₀ and m represent the dry mass and wet mass of the specimens, respectively, with units in kilograms (kg):

$$K={\displaystyle \frac{f_i}{f_0}}$$

(3)

In this equation, K represents the relative coefficient of compressive erosion resistance, f_i denotes the compressive strength (MPa) of the specimen after i days of erosion, and f₀ signifies the initial compressive strength (MPa) of the specimen:

$$S{E}^2={\displaystyle \frac{s^2}{n}}$$

(4)

In this equation, SE² represents the error variance, s² denotes the sample variance, and n is the sample size.

2.4. Instrumentation for Testing

After vacuum drying the crushed samples of DSAB, the microstructure of the hydrated products within the samples was observed using a scanning electron microscope (SEM) (Regulus 8100, made by HITACHI, Tokyo, Japan). The SEM was operated at a scanning voltage of 20 kV for imaging the micromorphology of the samples after gold sputtering.

The mineral composition of the specimen surface was measured using a Rigaku Ultima IV fully automated multifunctional X-ray diffractometer from Japan. The diffraction angles ranged from 10° to 80°, with a scanning speed of 2° per minute.

The internal structure of the DSABs was observed and studied using a high-performance automated mercury intrusion porosimeter (AutoPore LV9510, Micromeritics, Communications Drive, Norcross, GA, USA). Samples were taken from within the bricks, with the dimensions not exceeding 1 cm × 1 cm × 1 cm and a mass ranging from 2 to 3 g. Mercury intrusion porosimetry (MIP) experiments were conducted to investigate the pore structure inside the samples.

3. Experimental Result Analysis

3.1. Experimental Analysis of Water Absorption Rate in Desert Sand-Based Autoclaved Bricks

3.1.1. Analysis of Water Absorption Rate Experiment

The test results for the water absorption rate of desert sand-based autoclaved bricks are presented in

Table 3. Water absorption test result table.

Water Absorption Duration	0 s	30 s	60 s	3 min	6 min	10 min	20 min	40 min	1 h	2 h	4 h	8 h	12 h	24 h
Average water absorption rate (%)	0	1.63	2.57	3.84	4.80	5.79	7.10	8.76	9.89	12.15	14.10	14.38	14.42	14.74

. The relationship curve between the water absorption rate and absorption time is depicted in Figure 4. It can be observed from Figure 4 that the water absorption rate of the desert sand-based autoclaved bricks initially increased rapidly with an increasing absorption time before stabilizing. The saturation water absorption rate was recorded to be 14.74%, indicating a relatively strong water absorption capacity. During the early stage of water absorption (from 0 s to 6 min), the average water absorption rate of the samples was 1.2674 kg/h, accounting for 32.56% of the total water absorption. In the intermediate stage (6–40 min), the average water absorption rate decreased to 0.1840 kg/h, representing 26.87% of the total absorption. In the late stage (240 min), the average water absorption rate further decreased to 0.0013 kg/h, contributing to 4.34% of the total absorption, indicating that the samples were nearly saturated. These results suggest that during the initial stage of water absorption, capillary action dominated, resulting in a rapid increase in water uptake primarily through large pores. Conversely, in the later stage, diffusion became predominant, leading to a gradual reduction in the absorption rate as water ingress occurred mainly through medium and small pores, leading to increased saturation and slowed absorption rates for the autoclaved bricks [24].

3.1.2. Analysis of the Impact of the Water Absorption Rate on the Compressive Strength

In this study, four sets of autoclaved bricks made from desert sand of the same age were established, denoted as groups A, B, C, and D. These groups were subjected to compression strength tests after water absorption periods of 0 min, 10 min, 60 min, and 240 min, respectively. Group A served as the control group and consisted of autoclaved bricks dried prior to testing. The results of the compression strength tests are presented in

Table 4. Results for water absorption and compressive strength.

Group	Average Water Absorption (%)	Average Compressive Strength (MPa)
A (Non-absorbent)	0	17.60
B (Water absorption for 10 min)	5.79	15.79
C (Water absorption for 1 h)	9.89	12.92
D (Water absorption for 4 h)	14.10	11.76

. It is evident from Table 4 that there exists a significant relationship between the compression strength of autoclaved bricks made from desert sand and their water absorption rate. Additionally, the autoclaved bricks exhibited the highest compression strength when completely dry, measuring 17.60 MPa.

The relationship between the water absorption rate and compressive strength of the autoclaved bricks made from desert sand is illustrated in Figure 4. From the graph, it can be observed that the compressive strength of these bricks decreased with an increasing water absorption rate. Notably, the rate of strength degradation was highest during the initial 10–60 min of water absorption, while it slowed down between 1 h and 4 h of absorption. At 10 min of water absorption, the compressive strength of the autoclaved bricks was approximately 90% of the dry state, while after 1 h of absorption, it ranged between 70% and 75% of the dry state. After 4 h of water absorption, when the bricks reached near saturation, the compressive strength measured 11.76 MPa, with a corresponding strength loss rate of 33.18%. Within the scope of this experimental study, there exists a quadratic relationship between the water absorption rate and compressive strength of the autoclaved bricks made from desert sand, with a correlation coefficient of 0.984 indicating a strong fit.

3.1.3. Microscopic Analysis of Water Absorption Rate

Figure 5 and Figure 6 present the SEM and XRD images of the desert sand autoclaved bricks, respectively. The strength of these bricks primarily arises from the dense network structure formed by the interweaving and aggregation of hydration products such as calcium silicate hydrate (C-S-H), tobermorite, and gismondine with desert sand and aggregates, along with the physical interlocking and close contact between aggregate particles [25]. However, during water absorption, prolonged immersion in water causes some smaller soluble substances to dissolve and migrate from the interior of the specimens to the surface. As the water evaporates from the bricks, only large flaky calcium silicate hydrate remains, hindering the development of strength. As depicted in Figure 6a, the crystalline and non-crystalline phases of the hydration products intertwine, filling the voids left after the dehydration of desert sand-based autoclaved bricks and ensuring the stability of the microstructure [26]. Figure 6b illustrates the internal structural changes in the desert sand-based autoclaved bricks. As the water absorption time increased, the calcium compounds reacted with the water, altering the chemical properties within the bricks and promoting the dissolution of other minerals and compounds, resulting in an increase in the total porosity and coarsening of the pore size while simultaneously allowing free water to enter the autoclaved bricks, weakening the bond between aggregates [27]. Consequently, the compressive strength of the desert sand-based autoclaved bricks gradually decreased with an increasing water absorption rate.

3.2. Investigation on Sulfate Erosion Resistance of Desert Sand-Based Autoclaved Bricks

3.2.1. Alterations in Appearance

The process and apparent damage patterns of sulfate erosion for the desert sand-based autoclaved bricks are illustrated in Figure 7. As depicted, the specimens exhibited a smooth and undamaged appearance prior to erosion. After 30 cycles of erosion, there was slight powdering and minor flaking at the edges and corners, as shown in Figure 7c. This phenomenon primarily arose due to the elevated concentration of sulfate ions at the edges and corners caused by three-dimensional diffusion, resulting in a relatively faster generation rate of erosion products. Additionally, subsequent drying caused moisture evaporation, which led to supersaturation of Na₂SO₄ solution at the edges and corners, manifesting as salt efflorescence on the specimen’s outer surface, as depicted in Figure 7d. Internally, crystalline expansion forces led to crack formation, facilitated by the cumulative effects of sulfate salt wet-dry cycles, resulting in continuous crack propagation and eventual detachment of solidified products and fillers from the specimen’s smooth surface. Upon 60 cycles of erosion, ettringite (AFt), gypsum, and other erosion products progressively accumulated within the confined space of the mortar, generating expansion stresses. When the internal stress differences reached a critical threshold, expansion cracks developed. Simultaneously, a loss of moisture from within the autoclaved brick led to coarsening of the pore diameters, increased porosity, and diminished bonding strength. The stability of the internal structures, particularly the interface transition zones, underwent a drastic decline, leading to macroscopic cracking and spalling, as illustrated in Figure 7e,f.

3.2.2. Impact of Sulfate Erosion on Compressive Strength

The compressive resistance coefficient of desert sand autoclaved bricks under sulfate erosion is shown in Figure 8. As indicated by Figure 8, within the range of 10–60 erosion cycles, the compressive resistance coefficient of the desert sand autoclaved bricks initially increased and then decreased, peaking at 30 cycles. This behavior can be attributed to the high porosity of the desert sand autoclaved bricks. In the early stages of sulfate erosion, sulfate ions readily penetrated, leading to structural changes within the bricks, a decrease in density, and a slight reduction in compressive strength. As the erosion test progressed, expansive erosion products such as AFt and gypsum partially filled the pores and cracks, improving the density. This was reflected macroscopically as a slight increase in compressive strength after 30 erosion cycles. These findings contrast with those of Nie Dan [28], whose study indicated a regular decline in the compressive strength of clay bricks with an increasing sulfate erosion time. Notably, the compressive strength of the desert sand autoclaved bricks was slightly higher than that of clay bricks after 30 cycles of erosion.

The error variances for autoclaved bricks subjected to 0–60 cycles of erosion were 0.00086, 0.00067, 0.0018, 0.0030, 0.0031, 0.00056, and 0.00068, respectively. The small error variances suggest a high level of precision and consistency in the data.

When AFt and gypsum absorb water and expand to fill the pores, the resulting expansive stress can cause structural damage. Additionally, during the wet-dry cycles, the evaporation of the sulfate solution created a supersaturated solution, leading to the precipitation of sodium sulfate crystals. This process induced the conversion between sodium sulfate decahydrate and anhydrous sodium sulfate. When the crystallization pressure exceeded the pore wall strength, large internal cracks formed within the bricks, gradually developing into through-cracks. This phenomenon manifested macroscopically as a reduction in the compressive strength of the autoclaved bricks.

The reaction processes of autoclaved bricks made with desert sand subjected to sulfate erosion are shown in Equations (5) and (6). Additionally, gypsum further reacts with C3A to form ettringite (AFt), as illustrated in Equation (7):

$${\mathrm{Ca}\left(\mathrm{OH}\right)}_2{+\mathrm{SO}}_4^{2-}\to {\mathrm{CaSO}}_4+2{\mathrm{OH}}^{-}$$

(5)

$${\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{CH}+2\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\overline{S}{H}_2+2\mathrm{NaOH}$$

(6)

$${\mathrm{C}}_3{\mathrm{A}+3\mathrm{CaSO}}_4·{2\mathrm{H}}_2{\mathrm{O}+26\mathrm{H}}_2\mathrm{O}\to \mathrm{AFt}$$

(7)

The relationship between the compressive durability coefficient K of the desert sand-based autoclaved bricks and the number of cycles n of sulfate wet-dry cycles is shown in Equation (8), with the corresponding curve depicted in Figure 9. The coefficient of determination for the fitted curve was 0.96, indicating a high level of accuracy in the fitting results:

$$K=\left\{\begin{array}{ccc}-0.0370n+1& 0\le n<10& \\ 5.5\times {10}^{-4}{n}^2-0.0075n+0.65& 10\le n<30& , R^2=0.96\\ 2.0793e({\displaystyle \frac{-n}{22.0482}})+0.3914& 30\le n& \end{array}\right.$$

(8)

3.2.3. Microstructure of Sulfate Erosion

Calcium hydroxide (CH) was primarily concentrated at the interface junction, weakening the bond between the solidified product and the filler and making it susceptible to erosion in sulfate-rich environments, leading to crack formation. As depicted in Figure 10a, with the progression of the erosion test, a significant portion of the preexisting calcium hydroxide (CH) crystals were the first to be eroded, while the residual crystal edges exhibited serrated features [29]. After 30 cycles of erosion, distinct gypsum formations were not observed within the specimens. However, around the pores, numerous needle-like and rod-like crystals of AFt precipitated in a hedgehog-like manner on the original solid phase surface, as illustrated in Figure 10b. This phenomenon arose from the initial involvement of potassium SO₄²⁻ in the chemical reactions during early-stage erosion, leading to decreased concentrations. Consequently, gypsum, with relatively higher reaction activation energies, continued to react with compounds such as C₃A, forming AFt with lower reaction activation energies and resulting in a higher abundance of AFt crystals.

Observation of Figure 10c,d reveals that after 60 cycles of erosion, a considerable amount of erosion products, including AFt, gypsum, and sodium sulfate crystals, adhered to the surface of the autoclaved bricks. The bonding among hydration products, curing products, and fillers weakened, while pores and cracks continued to propagate, resulting in a looser structure and decreased density.

Figure 11 presents the XRD patterns and component proportions of the autoclaved bricks made with desert sand subjected to varying cycles of sulfate erosion. It can be observed in Figure 11 that after 30 cycles of sulfate erosion, the diffraction peaks corresponding to calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) within the autoclaved bricks were reduced compared with those of the non-eroded bricks, while the diffraction peak of AFt was slightly elevated, and the variation in the gypsum diffraction peak was negligible. However, after 60 cycles of erosion, there was a noticeable increase in the intensity of the gypsum diffraction peak, indicating the consumption of Ca (OH)₂ within the autoclaved bricks and the formation of gypsum and AFt erosion products. With an increase in erosion cycles, the potassium SO₄²⁻ concentration within the autoclaved bricks rose, leading to a decrease in the internal pH environment. A portion of the AFt decomposed to form gypsum under these conditions, favoring the stable presence of gypsum. Hence, the intensity of the gypsum diffraction peak was significantly enhanced.

3.3. Pore Structure of Desert Sand Autoclaved Bricks after Sulfate Erosion

Figure 12, Figure 13 and Figure 14 present the pore structure results of the desert sand-based autoclaved bricks before and after sulfate erosion, and

Table 5. Pore structure of desert sand autoclaved bricks measured using the mercury injection method.

	Porosity (%)	Average Pore Size (nm)	Total Pore Volume (mL/g)	Total Pore Area (m2/g)	Pore Volume per Unit Mass of Each Pore Size (nm/(mL/g))				Fraction of Each Pore Volume in the Total Pore Volume (%)
					<20	20~50	50~200	>200	<20	20~50	50~200	>200
Untreated	38.19	55.86	0.175	12.56	0.014	0.032	0.039	0.090	7.79	18.35	22.36	51.50
Subjected to 30 cycles of erosion	27.69	41.64	0.164	15.70	0.031	0.026	0.021	0.085	18.90	16.06	12.84	52.20
Subjected to 60 cycles of erosion	19.26	31.32	0.14	13.32	0.023	0.013	0.012	0.055	22.46	12.93	11.45	53.16

illustrates the proportional changes in pore size distribution of the bricks following exposure to sulfate erosion.

As depicted in Figure 12, the initial mercury intrusion primarily filled the larger pores, with a continuous increase in the mercury intrusion volume. Subsequently, as the finer pores and micropores gradually became filled with mercury, there was a minimal additional increase in the intrusion volume. Due to the significant energy absorption during the mercury intrusion process, the autoclaved bricks experienced volumetric expansion upon mercury withdrawal as stress was released, leading to the filling of cracks or deformed pores with mercury. Consequently, the mercury withdrawal curve in Figure 12 lies above the intrusion curve.

Figure 13 illustrates the volume of pores with different diameters within the desert sand-based autoclaved bricks. Following Mehta’s classification method, the pores were categorized into micropores (2R < 4.5 nm), mesopores (4.5 nm ≤ 2R ≤ 50.0 nm), macropores (50 nm ≤ 2R ≤ 100 nm), and macropores (2R > 100 nm) [30]. It can be observed from Figure 13 that the distribution of pore sizes within the desert sand-based autoclaved bricks was relatively broad, and the regularity was poor, with the presence of well-developed macropores throughout the bricks.

The differential pore size distribution curve of desert sand-based autoclaved bricks is depicted in Figure 14, reflecting the variation in the rate of mercury intrusion volume with the pore size. The differential curve represents the total pore volume, as indicated by the area encompassed by the horizontal axis. A higher mercury intrusion volume indicates a greater proportion of pores corresponding to that particular pore size. From Figure 14, it can be observed that the diameters of the internal pores in the specimens were primarily distributed around 50 nm, 500 nm, and 10 μm. After 30 cycles of wet-dry exposure, the most probable pore diameter was 23.41 nm, corresponding to a mercury intrusion volume of 0.082 mL/g. Following 60 cycles of wet-dry exposure, the most probable pore diameter reduced to 21.08 nm, with a corresponding mercury intrusion volume of 0.068 mL/g.

Based on Table 5, it can be observed that after 60 cycles of sulfate erosion, the porosity of the desert sand-based autoclaved bricks was 19.26%, representing a reduction of 18.93% compared with the uneroded specimens. According to the classification of pore sizes proposed by Zhongwei Wu [31], following 60 cycles of erosion, the porosity corresponding to harmless pores, few harmful pores, harmful pores, and multiple harmful pores within the desert sand-based autoclaved bricks were 22.46%, 12.93%, 11.45%, and 53.16%, respectively. Notably, the proportion of multiple harmful pores increased by 0.96% post erosion. This phenomenon arose due to the formation of erosion products such as gypsum and AFt, as well as the precipitation of sulfate salt crystals within the autoclaved bricks during sulfate wet-dry cycles. Accumulation of these erosion products led to macroscopic cracking, spalling, and a deterioration in strength.

4. Conclusions

• The water absorption rate of the desert sand-based autoclaved bricks exhibited a trend of an initial rapid increase followed by gradual slowing with an increasing soaking time. In the early stages of the water absorption test, there was a strong diffusion capacity for moisture, leading to a rapid rise in the water absorption rate of the autoclaved bricks. However, as the soaking time continued to increase, the rate of increase in water absorption slowed down. After 4 h of soaking, the water absorption rate of the autoclaved bricks reached 14.10%, approaching the saturation absorption rate of 14.74%.
• The compressive strength of desert sand-based autoclaved bricks exhibited a quadratic relationship with the water absorption rate. At a completely dry state, the compressive strength of the autoclaved bricks measured 17.60 MPa. With an increase in the water absorption rate, alterations occurred in the hydration products of the bricks, leading to an enlargement of the pore volume and coarsening of the pore size, resulting in a significant decrease in compressive strength. When the water absorption rate of the desert sand-based autoclaved bricks reached 14.10%, the compressive strength measured 11.76 MPa, representing a strength loss rate of 33.18%.
• As the number of sulfate erosion incidents for the desert sand-based autoclaved bricks increased, gypsum and AFt corrosion products were generated within the bricks, along with the precipitation of sulfate salt crystals, facilitating the generation of new pores, which were subsequently fractured by expansion stress. Simultaneously, this process resulted in a 0.96% increase in the proportion of harmful pores, leading to a degradation in strength. Through data analysis, a degradation curve of the compressive strength durability coefficient of the desert sand-based autoclaved bricks with respect to the number of sulfate erosion incidents was established.
• This study provides a theoretical basis and technical guidance for the application of desert sand autoclaved bricks in village and town buildings in Xinjiang, as well as in sulfate-rich areas. Future research will focus on improving the erosion resistance of desert sand autoclaved bricks in sulfate environments.