Experimental Study on the Strengthening Mechanism of Modified Coal Gangue Concrete and Mechanical Properties of Hollow Block Masonry

Abstract

To enhance the utilization efficiency of coal gangue aggregate, coarse aggregates are chemically modified with 5% sodium silicate solution. The effects of this modification on the compressive strength and microstructural characteristics of concrete are systematically investigated through integrated macro-testing and micro-characterization. By evaluating the compressive performance of modified coal gangue concrete blocks, the optimal mix ratio of each strength grade of blocks is determined. Experimental results indicate that the apparent density, water absorption, and crushing index of the modified coal gangue coarse aggregate exhibit better mechanical properties than the control group. The modified coal gangue coarse aggregate demonstrates improved mechanical performance, with the compressive strength of 28-day concrete showing a 15.3% increase relative to the control group. Furthermore, using a sodium silicate solution effectively enhances the interface transition zone’s performance between coal gangue coarse aggregate and cement mortar, improving the compactness of this interface. The modified coal gangue concrete blocks exhibit higher compressive strength than the original material. When the substitution rate remains constant, the compressive strength of modified coal gangue concrete decreases with increasing water–cement ratio. Similarly, at a constant water–binder ratio, compressive strength decreases with higher modified gangue aggregate replacement. Finally, compressive tests are conducted on masonry constructed with hollow blocks of strength grades MU7.5, MU10, and MU15. Then, a calculation model for the average compressive strength of modified coal gangue concrete hollow block masonry is proposed, providing theoretical support for its engineering application.

1. Introduction

Coal gangue is one of the most generated industrial solid wastes in China [1], with a stockpiled amount exceeding 7 billion metric tons. Its long-term accumulation has led to severe environmental issues, including occupation of land resources, pollution of water and soil, and geological hazards such as landslides [2,3,4,5]. To address these challenges, researchers have explored the recycling of coal gangue as a construction material [6,7,8,9,10,11]. Crushing it for use in concrete and cement-based products not only contributes to resource utilization but also reduces reliance on natural aggregates, thereby alleviating the ecological impacts of mining [12,13]. Currently, masonry construction continues to dominate in China’s building industry [14,15,16]. The development of coal gangue concrete blocks—which exhibit advantages such as lightweight properties and enhanced thermal insulation [17,18,19,20]—aligns with the national strategy of promoting green wall materials and contributes to sustainable urbanization under the “dual carbon” goals [21].

In recent years, coal gangue has made notable progress as a substitute for natural aggregates in the field of construction engineering. For instance, Dong [22] prepared coal gangue concrete and mortar using raw and calcined coal gangue as coarse and fine aggregates, respectively. The results indicated that calcination significantly upgraded the quality of coal gangue aggregates, thereby improving the performance of the resulting concrete and mortar. Similarly, Zuo [23] proposed strengthening coal gangue coarse aggregates with cement-based coatings, which effectively optimized the interfacial transition zone and enhanced the mechanical properties of the concrete. Furthermore, Mahmoud Al Khazaleh et al. [24] utilized coal gangue to replace coarse aggregates in the preparation of structural concrete, enhanced in compressive strength, splitting tensile strength, and flexural strength. In parallel, Shan Gao et al. [25] developed a predictive model for the compressive strength of coal gangue concrete, further demonstrating that coal gangue, when used as a coarse aggregate, exhibits favorable physical and mechanical performance and stands out as one of the preferred materials for concrete production. However, compared to natural aggregates, coal gangue aggregates exhibit certain drawbacks, such as a high crushing index, elevated porosity, and strong water absorption. These characteristics result in relatively poor mechanical properties of coal gangue concrete and limit its substitution rate. Moreover, the interfacial transition zone (ITZ) between the gangue aggregates and mortar is often weak. As a result, extensive research has been conducted by scholars both in China and abroad to improve aggregate quality and enhance the mechanical performance of coal gangue concrete. Ma et al. [10] prepared alkali-activated coal gangue-slag concrete using both untreated and 700 °C calcined coal gangue as coarse aggregates. The results indicated that calcined coal gangue exhibited higher compressive strength and better durability when used as coarse aggregate in alkali-activated coal gangue-slag concrete. Moreover, Fan et al. [26] demonstrated that soaking recycled aggregates in 5% sodium silicate solution maximized the compressive strength of recycled concrete, a conclusion further supported by Qiu et al. [27]. These studies collectively suggest that low-cost modification methods can significantly improve coal gangue aggregates, offering a viable pathway for the resource utilization of solid waste in cement-based products.

With the progressive improvement in the performance of coal gangue aggregates, a solid foundation has shown promising potential of coal gangue concrete at the level of structural components. Further processing coal gangue concrete into blocks not only extends the advantages of its resource utilization but also integrates the inherent benefits of masonry structures, such as design flexibility, ease of maintenance, and excellent thermal insulation performance, thereby demonstrating strong application potential. Experimental investigations by Qi et al. [28] demonstrated that recycled concrete blocks exhibit sufficient mechanical strength and frost resistance, making them suitable in seismic-resistant masonry systems. Izquierdo et al. [29,30] developed sisal fiber-reinforced concrete hollow blocks, observing a trade-off between reduced compressive strength and enhanced ductility, accompanied by a transition from brittle to progressive failure modes. In parallel, Gunasekaran et al. [31] successfully substituted natural aggregates with coconut shells, achieving compliant compressive strength while mitigating shrinkage-induced cracking through the organic material’s internal curing effects. Furthermore, Zheng et al. [32,33] investigated the influence of alkali-activated slag mortar strength on the bond performance of hollow block masonry and proposed a shear bond strength formula for design use. J.J. et al. [34] optimized masonry block geometry via finite element analysis, reducing self-weight while maintaining strength, and proposed a new strategy for lightweight masonry design. These case studies collectively substantiate that through material modification and structural optimization, masonry blocks can achieve multiple performance enhancements. However, research on the application of coal gangue in masonry blocks remains relatively limited, particularly regarding its mechanical performance and the underlying microstructural mechanisms, which still lack systematic investigation and theoretical support.

The above research provides valuable insights for the modification of coal gangue aggregates. To enhance the utilization efficiency of coal gangue and promote its high-value applications, while ensuring material cost, operational simplicity, and environmental sustainability, this study employs a 5% sodium silicate solution to modify coal gangue coarse aggregates. The microstructural characteristics of the interfacial transition zone (ITZ) in coal gangue concrete are systematically analyzed using a microhardness tester and scanning electron microscopy (SEM). Additionally, the study investigates the variation patterns in mechanical properties of modified coal gangue concrete blocks under different water-to-cement ratios and substitution rates, aiming to reveal the strengthening mechanism of the modification treatment on coal gangue concrete. At the same time, modified coal gangue concrete is used as the material for hollow blocks, and its physical and mechanical properties as well as engineering applicability are systematically tested. Upon assembling these blocks into masonry, compressive strength tests are conducted, resulting in the formulation of a bearing capacity formula tailored to the modified coal gangue masonry system. This study is conducted from two interconnected perspectives—“material modification” and “structural application”-providing a new pathway and theoretical foundation for the multi-scale and efficient utilization of coal gangue resources.

2. Mechanical Properties and Strengthening Mechanism of Modified Coal Gangue Concrete

2.1. Raw Material

(1) Cement and fly ash: The cementitious materials (Liquan Conch Cement Co., Ltd., Xianyang, China) used in the experiment were P.O42.5 ordinary Portland cement and a first-grade fly ash (Gongyi Borun Refractory Material Co., Ltd., Gongyi, China), as shown in

Table 1. Chemical Composition of Cement and fly ash/%.

Component	SiO2	Fe2O3	Al2O3	CaO	MgO	SO3
Cement	21.66	5.25	5.13	64.37	1.34	2.46
Fly ash	45.1	0.85	24.2	5.6	0.81	2.1

.

(2) Fine aggregate: The fine aggregate was made of medium particle size river sand (Xi’an, China), with a fineness modulus of 2.7, a bulk density of 1560 kg/m³, and a mud content less than 2%.

(3) Water and additives: The mixed water was ordinary tap water; the water reducer (Weihe Technology Company, Xiamen, China) was a polycarboxylate superplasticizer with reduction rate of 25%.

(4) Modified coal gangue coarse aggregate: Coal gangue was taken from the Ordos ore district in China, and its chemical composition is listed in

Table 2. Chemical Composition of Coal Gangue/%.

Component	SiO2	Fe2O3	Al2O3	CaO	MgO	K2O
Relative content	48.12	6.26	36.34	6.16	0.95	0.93

(The properties of coal gangue exhibit variations across different regions and geological strata). The coal gangue was initially screened using a square hole sieve, and the EP-3B type jaw crusher was used to crush the screened coal gangue, as shown in Figure 1. For the preparation of fine aggregate, the crushed coal gangue particles within the 0–5 mm size range were manually sieved and graded. The resulting fine particles were then proportionally blended based on the particle size distribution of natural river sand to produce coal gangue sand. As shown in Figure 2, the coal gangue sand exhibited a fineness modulus of 2.8, an apparent density of 2310 kg/m³, a bulk density of 1246 kg/m³, and a moisture content of 7.5%.

2.2. Modification of Coal Gangue Coarse Aggregate

Based on existing research findings [35,36], a 5% sodium silicate solution was selected for coal gangue aggregate modification. The modifier consisted of a commercial sodium silicate solution (Henan Luboshi New Material Technology Co., Ltd., Xinzheng, China) with chemical composition parameters of 8.15% Na₂O, 26% SiO₂, and a silicate modulus (SiO₂/Na₂O molar ratio) of 3.3. The screened aggregates were immersed in the solution, with the liquid level maintained 5–10 cm above the aggregate surface. To ensure effective surface modification while avoiding excessive dissolution of active components, a 5 h immersion time in sodium silicate solution was adopted. Preliminary trials showed that shorter immersion durations (<3 h) led to insufficient penetration, whereas prolonged immersion (>8 h) caused surface precipitation and reduced bonding efficiency. During immersion, the solution was stirred at 2 h intervals to maintain uniform concentration and prevent sedimentation. This interval was found sufficient to avoid agglomeration of silicate particles, while more frequent stirring did not yield noticeable improvements. Specimens were then cured at room temperature for 7 days to ensure complete drying of the aggregates, thereby providing stable conditions for subsequent tests. The complete modification process was schematically illustrated in Figure 3.

According to the requirements of “Pebble and Crushed Stone for Construction” (GB/T14685-2022) [37], the apparent density, water absorption, and crushing index of coal gangue coarse aggregates before and after modification were tested. The experimental results were shown in

Table 3. The physical and mechanical properties of modified coal gangue coarse aggregate.

Group Number	Apparent Density(kg/m3)	Water Absorption (%)	Crushing Index (%)
CG	2451	6.6	21.4
CG-N	2521	5.1	18.1

Note: Number CG was raw coal gangue, CG-N was modified coal gangue. The water absorption test was conducted following the requirements of “Pebble and Crushed Stone for Construction” (GB/T 14685-2022) [37]. Aggregates were dried in an oven at 105 °C ± 5 °C until constant weight was achieved. After cooling, the dry mass was recorded. The samples were then immersed in water for 24 h, surface-dried, and weighed to determine the saturated mass.

.

As quantified in Table 3, sodium silicate modification significantly enhanced the aggregate properties: the apparent density increased by 2.8%, accompanied by 22.7% and 15.4% reductions in water absorption and crushing index, respectively. These improvements were mechanistically attributed to in situ formation of sodium silicate-derived gel, which chemically bonded to pore walls and microcracks within the coal gangue. This gel-phase consolidation refined the pore structure, thereby increasing bulk density while reducing both water penetration and mechanical failure susceptibility, as corroborated by prior studies [38].

2.3. Mix Proportion of Coal Gangue Concrete

Two mix proportions of coal gangue concrete were designed, with a concrete water–cement ratio of 0.42, a coal gangue coarse aggregate substitution rate of 100%, and a fly ash content of 20%. The mix proportions of each set of coal gangue concrete were shown in

Table 4. The mix proportion of modified coal gangue concrete (kg/m³).

Group Number	Water	Cement	Sand	Modified Coal Gangue Coarse Aggregate	Raw Coal Gangue Coarse Aggregate	Fly Ash	Additive
CG	158	300	850	—	915	75	1.9
CG-N	158	300	850	915	—	75	1.9

.

2.4. Test Method

2.4.1. Mechanical Performance Test

According to the requirements of the “Standard for Test Methods of Concrete Physical and Mechanical Properties” (GB/T50081-2019) [39], 100 mm cube specimens with various ages were tested as the compressive strength of modified coal gangue concrete, for each testing group, three specimens were selected for compressive strength testing. and the measured results were multiplied by a coefficient of 0.95 to convert them into standard specimen strength [40].

2.4.2. Microhardness Test

To investigate the influence of modified coal gangue aggregate on the interface transition zone (ITZ) of concrete, the HVS-1000Z digital microhardness tester was used to test the Vickers hardness of the transition zone for coal gangue concrete before and after modification, as described below. After curing the cubic specimen for 28 days, a preliminary sample with a size of approximately 20 mm × 20 mm × 20 mm was cut using a marble cutting machine. Furthermore, the preliminary sample was soaked in anhydrous ethanol for 24 h to terminate its hydration and carbonization, and then dried in a 25 °C oven for 8 h. Afterwards, cold inlay materials were used to fix the samples for microhardness testing, and grinding and polishing were carried out on the specimens to meet the test requirements. The preparation process of the test sample was shown in Figure 4. In addition, the microhardness tester cross bench should be clean and dry, and the prepared sample should be fixed in the middle of the cross bench with double-sided tape to avoid test errors. During testing, the hardness tester indenter should be in contact with the surface of the specimen. The indenter contacted the surface under a constant load rate; after 10 s unloading should be carried out and the length of the two diagonal lines of the diamond-shaped indenter were measured. Thus the Vickers hardness of the sample was read.

2.4.3. Scanning Electron Microscope Test

The microstructure of coal gangue concrete samples before and after modification was observed using JSM-7610F field emission environmental scanning electron microscope (JEOL Ltd., Tokoy, Japan). After the cube specimen reached the curing age of 28 days, a thin sheet with size of about 10 mm × 10 mm × 3 mm and containing both gangue aggregate and mortar was selected from the damaged specimen near the center position, as shown in Figure 5a. Additionally, the selected thin sheet was soaked and cleaned in anhydrous ethanol to terminate its hydration and carbonization. After 24 h, the sample was placed in a 25 °C drying oven for 8 h. Then, the sample was stored in a sealed plastic bag and labeled with the sample number. During the experiment, the sample was attached to the sample stage and subjected to vacuum metal plating treatment using an SBC-12 ion sputtering instrument (Henan Zhongfen Instrument Co., Ltd., Shangqiu, China), as shown in Figure 5b. After the scanning electron microscope test reached the required vacuum, its microstructure was observed, as shown in Figure 5c.

2.5. Test Results and Analysis

2.5.1. Compressive Strength for Modified Coal Gangue Concrete

As shown in Figure 6, sodium silicate treatment significantly enhanced compressive strength. Compared to unmodified counterparts, the treated specimens exhibited strength increases of 10.7% (3 d), 12.3% (7 d), 15.1% (14 d), and 15.3% (28 d). This improvement mechanism was attributed to the sodium silicate solution effectively filling internal pores and healing surface cracks within the aggregates, as verified by SEM-EDS analysis in Ref. [41]. The resultant microstructural densification directly contributed to the enhanced macroscopic concrete performance.

2.5.2. Microhardness of ITZ in Modified Coal Gangue Concrete

In order to more accurately understand the distribution characteristics of microhardness in the ITZ, three specimens were selected in each group, and for each specimen, 50 random points were selected in the mortar matrix area for microhardness testing to eliminate material dispersion. Subsequently, the microhardness values of 50 points were subjected to Box-Plot statistical analysis, excluding abnormal values of excessive or insufficient values in the test results. And, the microhardness values corresponding to the upper and lower quartiles of the Box Plot were used as the standard zone values for the mortar matrix. Consequently, the boundary between the ITZ and the mortar matrix could be determined. The statistical analysis results of the microhardness for the mortar matrix of coal gangue concrete samples were shown in Figure 7.

The variation law of microhardness in the ITZ of coal gangue concrete was shown in Figure 8. It could be seen that the value decreased first and then increased from the edge of the aggregate to the mortar matrix area, and the lowest point of the microhardness value was located between 10 and 30 μm from the surface of the aggregate. The main reason for this phenomenon is that the coal gangue aggregate before and after modification still had strong water absorption. So the hydration reaction on the surface of the aggregates was more complete, filling the voids with hydration products [42,43]. Accordingly, the interface structure at the edge of the aggregate was denser, resulting in a higher microhardness value. When the distance from the surface of the aggregate was greater than 30 μm, the microhardness value of the mortar matrix gradually increased and tended to stabilize. According to the statistical distribution of microhardness values for the mortar matrix, it could be determined that the area of the mortar matrix between 0 and 50 μm away from the coal gangue aggregate was the weakest.

In the transition zone of the interface, the microhardness values of each group were as follows: CG-N>CG, the thickness of the ITZ of the modified coal gangue concrete was smaller than that of the raw one. Furthermore, the ITZ thickness of the CG-N group was 36.72 μm, which was 15.8% lower than the CG group with 43.62 μm. Overall, the sodium silicate solution could effectively improve the microhardness value of the modified concrete ITZ, and the thickness of the ITZ was reduced.

2.5.3. Microscopic Morphology of ITZ in Modified Coal Gangue Concrete

The microscopic characteristics of the ITZ of each group for coal gangue concrete were shown in Figure 9. It could be seen that there were significant cracks in the mortar matrix at the edge of the coarse aggregate for the raw coal gangue concrete, and a large amount of fly ash particles were adsorbed [44]. However, the ITZ between modified coal gangue coarse aggregate and cement mortar was relatively dense, with no obvious cracks, and no fly ash particles were found in the ITZ.

The macroscopic mechanical properties and microscopic test results demonstrated that modifying coal gangue coarse aggregates with sodium silicate solution significantly enhanced concrete performance. The improvement mechanism was attributed to the silica gel formed by sodium silicate solution hydrolysis, which filled and repaired pores and cracks within the coal gangue aggregates, thereby reducing their porosity. This pore-sealing effect reduced water absorption, thereby promoting a more complete hydration reaction of cement. Additionally, the residual sodium silicate gel on the modified coal gangue aggregate surfaces underwent hydrolysis, providing an alkali-activated environment. This environment accelerated the hydration of cement and fly ash particles while activating the pozzolanic reactivity of the cementitious materials. Consequently, a substantial amount of flocculent calcium silicate hydrate (C-S-H) and acicular/rod-shaped ettringite (AFt) phases with high strength and stability were generated. These products densely filled the microcracks in the interfacial transition zone (ITZ), enhancing the bond strength between the aggregates and the cementitious matrix. As a result, the microhardness of the ITZ increased, while its thickness decreased. The strengthening mechanism of sodium silicate solution-modified coal gangue concrete is schematically illustrated in Figure 10.

3. Mechanical Performance Test of Modified Coal Gangue Concrete Blocks

3.1. Mix Design and Preparation of Coal Gangue Concrete Blocks

According to the code “Normal Concrete Small Block” (GB/T 8239-2014) [45], standard blocks with dimensions of 190 mm × 190 mm × 390 mm and a void ratio of 43.6% were used for the test. The block dimensions are shown in Figure 11.

According to the code “Specification for Mix Proportion Design of Ordinary Concrete” (JGJ55-2011) [46], modified coal gangue concrete blocks with strength grades of MU7.5, MU10, and MU15 (MU: Masonry Unit) were prepared for testing. Based on the test results of compressive strength of modified coal gangue concrete cubes, the mix proportion of modified coal gangue concrete was optimized, and the mix proportion of modified coal gangue concrete blocks was obtained. A total of 16 groups of concrete hollow blocks were designed for the experiment, and the specific combinations for each group were presented in

Table 5. The mix proportion of modified coal gangue concrete hollow block (kg/m³).

Group Number	Water–Cement Ratio	Substitution Rate	Water	Cement	Sand	Gravel	Raw Coal Gangue	Modified Coal Gangue	Fly Ash	Additive
C42G00	0.42	0%	135	257	899	1168	0	—	64	1.6
C32G40	0.32	40%	135	388	858	670	369	—	84	2.1
C32G70	0.32	70%	135	388	858	335	647	—	84	2.1
C32G100	0.32	100%	135	388	858	0	824	—	84	2.1
C42G40	0.42	40%	135	257	899	701	387	—	64	1.6
C42G70	0.42	70%	135	257	899	350	677	—	64	1.6
C42G100	0.42	100%	135	257	899	0	967	—	64	1.6
C52G40	0.52	40%	135	208	923	720	397	—	52	1.2
C52G70	0.52	70%	135	208	923	360	696	—	52	1.2
C52G100	0.52	100%	135	208	923	0	994	—	52	1.2
C32G40N	0.32	40%	135	388	858	670	—	369	84	2.1
C32G70N	0.32	70%	135	388	858	335	—	647	84	2.1
C42G70N	0.42	70%	135	257	899	350	—	677	64	1.6
C42G100N	0.42	100%	135	257	899	0	—	967	64	1.6
C52G70N	0.52	70%	135	208	923	360	—	696	52	1.2
C52G100N	0.52	100%	135	208	923	0	—	994	52	1.2

Note: The symbol C denotes the water–cement ratio (by mass), G indicates the replacement ratio of raw coal gangue coarse aggregate, and N represents the replacement ratio of coal gangue aggregate modified by sodium silicate solution. Specifically: The group designation “C42G00” corresponds to ordinary concrete hollow blocks with 0% coal gangue content (control group); “C42G40” specifies a mix with a water–cement ratio of 0.42 and 40% replacement of coarse aggregate by raw coal gangue; “C42G40N” indicates a water–cement ratio of 0.42 and 40% replacement of coarse aggregate by sodium silicate-modified coal gangue.

.

During the test, the weighed cementitious materials, fine aggregates, coarse aggregates, and other raw materials were sequentially poured into an HJW-60 single horizontal shaft forced concrete mixer. To achieve a uniform mixture, the materials were stirred for 2–3 min. Subsequently, the water and admixture mixture was added to the mixer. After thorough blending, the mixture was transferred to a QMJ4-35B simple fixed brick-making machine (Zhengzhou Henglong Machinery Equipment Co., Ltd., Zhengzhou, China) for vibration and compression molding. Upon starting the vibration motor, the mold base (vibration platform) generated high-frequency vibrations, which were transmitted to the mixture, causing particle rearrangement and reduction in voids, achieving preliminary densification. During this process, part of the cement slurry penetrated into the particle voids, forming initial bonding. The operator manually used a lever to press the upper mold downward, applying additional compression to the mixture. The vibration pressure was 32 kN, and the vibration frequency was 2800 times per minute. Vibration continued for 10–15 s, during which the combined effect of upper pressing and bottom vibration produced bidirectional compaction, enhancing the density and strength of the bricks. After the vibration process, the pressing head was lifted, and the mold frame rose with the vibration platform, allowing the molded bricks to naturally remain on the pallet. Then, they were placed outdoors at 25 ± 3 °C, covered with cotton cloth, and watered daily for curing for 28 days. The preparation process is illustrated in Figure 12.

According to the specifications outlined in the Chinese standard “Test methods for the concrete block and brick” (GB/T 4111-2013) [47], the physical properties of the modified blocks were tested. For example, the apparent densities of C42G100 and C42G100N were 1123 kg/m³ and 1211 kg/m³, respectively, while their water absorption rates were 6.2% and 5.6%. These results are consistent with the performance trends of the aggregates used—namely, a slight increase in apparent density accompanied by a decrease in water absorption—further confirming the effectiveness of the modification treatment in enhancing aggregate compactness and reducing porosity.

3.2. Compressive Strength Test of Modified Coal Gangue Concrete Blocks

According to the standard “Test Methods for the Concrete Blocks and Brick” (GB/T4111-2013) [47], five blocks were prepared for each group. The compressive strength of the blocks was measured using a YAS-5000 computer-controlled servo pressure testing machine (JiLin Guanteng Automation Technology Co., Ltd., Changchun, China). Prior to the compressive strength test, the upper and lower bearing surfaces of the blocks were leveled with high-strength gypsum to ensure uniform load distribution. During the test, the loading rate of the testing machine was maintained at a constant speed of 5 kN/s to ensure stable and controlled loading. The experimental loading setup and its schematic diagram are shown in Figure 13.

3.3. Description of Test Phenomena of Modified Coal Gangue Concrete Blocks

During the initial loading phase, there was a slight tearing sound inside the block, accompanied by localized cracks primarily concentrated in the mid-upper sections of the blocks. These cracks generally aligned parallel to the loading direction. As the load increased, the internal cracking sounds intensified, and crack density increased. Early-stage cracks progressively extended both upward and downward, with crack widths gradually expanding, eventually forming interconnected through-cracks. With further loading, localized surface bulging and spalling occurred on the block surfaces. The failure modes of representative specimens were illustrated in Figure 14.

According to Figure 14, the failure modes of coal gangue concrete blocks (both modified and unmodified) were generally similar to those of ordinary concrete blocks. On the wide face, ordinary concrete blocks exhibited approximately vertical cracks parallel to the loading direction, while coal gangue concrete blocks displayed not only vertical cracks but also inclined cracks. In contrast, the crack distribution on the narrow face remained essentially similar across all three types of blocks, all showing vertical cracks. This difference was attributed to the tendency of coal gangue aggregates to concentrate stress under load, which initiated early cracks and failed to restrict their propagation within the cement mortar, ultimately altering the crack trajectory [8]. Modified coal gangue aggregates, however, could mitigate this issue by reducing the formation of inclined cracks, thereby improving the failure morphology of the blocks.

3.4. Compressive Strength Test Results

The compressive strength results of each group of blocks were presented in

Table 6. Compressive strength of coal gangue concrete blocks.

Group Number	Compressive Strength (MPa)					Average Compressive Strength (MPa)	Standard Deviation	Coefficient of Variation
	1	2	3	4	5
C42G00	14.74	14.53	16.80	15.84	13.42	15.07	1.30	0.0861
C32G40	13.34	15.70	14.68	14.11	16.09	14.78	1.13	0.076
C32G70	13.61	10.88	13.28	12.52	12.26	12.51	1.06	0.085
C32G100	9.91	8.71	8.23	10.62	10.85	9.66	1.16	0.120
C42G40	12.24	11.85	11.29	13.36	12.44	12.23	0.77	0.062
C42G70	10.88	9.55	10.38	11.53	12.12	10.89	1.00	0.113
C42G100	8.44	9.51	8.02	10.24	9.08	9.06	0.87	0.097
C52G40	9.70	10.89	10.14	8.13	9.38	9.64	1.02	0.105
C52G70	7.46	8.01	8.43	9.78	9.12	8.56	0.91	0.107
C52G100	7.19	6.08	8.16	7.32	7.94	7.34	0.81	0.111
C32G40N	16.37	18.47	14.17	15.75	16.12	16.17	1.54	0.095
C32G70N	12.37	12.73	14.82	13.58	13.21	13.34	0.95	0.071
C42G70N	10.19	11.45	12.80	10.72	12.17	11.46	1.06	0.092
C42G100N	10.10	9.38	11.35	9.82	11.25	10.38	0.88	0.085
C52G70N	9.84	10.32	9.02	7.93	10.03	9.43	0.97	0.102
C52G100N	7.34	7.51	8.16	9.56	9.26	8.36	1.01	0.120

. and Figure 15. As shown in Figure 15a, under constant water–cement ratios, the compressive strength of the blocks demonstrated a decreasing trend with increasing coal gangue substitution rates. Specifically: At a water–cement ratio of 0.32, blocks with 70% and 100% coal gangue aggregate substitution exhibited compressive strength reductions of 15.4% and 35%, respectively, compared to those with 40% substitution. When the water–cement ratio reached 0.42, blocks containing 40%, 70%, and 100% coal gangue aggregate showed strength decreases of 18.8%, 27.7%, and 39.9%, respectively, relative to the 0% substitution control group. At a higher water–cement ratio of 0.52, specimens with 70% and 100% substitution displayed 11.2% and 23.9% lower compressive strength compared to the 40% substitution baseline.

The primary mechanism governing the compressive strength of coal gangue concrete hollow blocks was the synergistic effect of interlocking friction between aggregates and the bonding strength at the aggregate-cement paste interface [48]. Due to the high water absorption of coal gangue aggregates, increasing their substitution rate significantly reduced cement hydration efficiency. This reduction led to decreased interlocking friction among aggregates and diminished interfacial bonding strength. Consequently, the mechanical integrity of the interfacial transition zone (ITZ) was compromised, aligning with the microstructural degradation observed in Section 1. During compression testing, a portion of coarse aggregates were pulled out, which adversely affected the compressive strength of the blocks.

The comparative analysis of compressive strength between modified and unmodified coal gangue concrete blocks was illustrated in Figure 15b. Under constant substitution rates, the compressive strength of modified coal gangue concrete blocks exhibited a progressive decline as the water–cement ratio increased. At fixed water–cement ratios, the compressive strength demonstrated a decreasing trend when the replacement ratio of modified coal gangue aggregates rose, following a similar degradation pattern to conventional coal gangue concrete blocks. Notably, sodium silicate modification enhanced compressive strength across all test groups, with measured improvements of 9.4%, 6.6%, 5.2%, 14.9%, 10.2%, and 13.9%, respectively, compared to unmodified counterparts.

This systematic enhancement demonstrates that sodium silicate treatment effectively improves the mechanical performance of coal gangue-based concrete by strengthening the aggregate–matrix interface. The improvement results from a coupled physical–chemical mechanism: a silica-rich film forms on the aggregate surface, lowering porosity and water absorption, while dissolved silicate species react with Ca(OH)₂ to generate additional C–S–H. Together, these processes produce a denser ITZ, consistent with the measured increase in microhardness and reduction in thickness.

4. Mechanical Performance Test of Modified Coal Gangue Concrete Masonry

4.1. Preparation of Coal Gangue Concrete Masonry

According to the standard “Test Methods of Basic Mechanical Properties of Masonry” (GB/T 50129-2011) [49], the dimensions of the modified coal gangue concrete masonry compressive specimens were 990 mm × 590 mm × 190 mm, with a mortar joint thickness of 10 mm, as shown in Figure 16. Specifically, three groups of compressive specimens were designed, each incorporating modified coal gangue concrete blocks with distinct strength grades. Each group consisted of six specimens, which brought a total of 18 specimens. The corresponding mix proportions are detailed in Section 3.1. Furthermore, the specific material combinations for each group are presented in

Table 7. Design of compressive test for modified coal gangue concrete masonry.

Group Number	Block Strength Grade	Mortar Strength Grade
W7.5-7.5	MU7.5	Mb7.5
W10-7.5	MU10	Mb7.5
W15-7.5	MU15	Mb7.5

Note: The designation “W7.5-7.5” in the table indicates masonry compressive specimens constructed with blocks of strength grade MU7.5 and mortar of strength grade Mb7.5.

. The mortar adopted for all specimens had a strength grade of Mb7.5. The mortar mix comprised 280 kg/m³ of water, 280 kg/m³ of cement, 914 kg/m³ of sand, and 523 kg/m³ of coal gangue fine aggregate. Notably, the substitution rate of coal gangue fine aggregate reached 40%.

4.2. Compressive Strength Test of Modified Coal Gangue Concrete Masonry

According to the standard “Test Methods of Basic Mechanical Properties of Masonry” (GB/T 50129-2011) [49], the compressive strength test of modified coal gangue concrete masonry was performed using a YAS-5000 computer-controlled servo testing machine (JiLin Guanteng Automation Technology Co., Ltd., Changchun, China). Before testing, to ensure uniform load distribution across the specimens, the loading surfaces were leveled with high-strength gypsum.

Before formal loading, cyclic preloading was applied to the masonry specimens 3 to 5 times at 10% of the estimated failure load. Subsequently, formal loading commenced only when the relative error between the axial deformation measurements on the two lateral faces was controlled within 10%, thereby ensuring the reliability of deformation data. The test employed a constant displacement loading method. Initially, the load was applied at a rate of 0.0005 mm/s. When the load reached 80% of the estimated failure load, the loading rate was reduced to 0.0002 mm/s. This adjustment was implemented to prevent brittle failure due to excessive loading speed, which could otherwise compromise the accuracy and validity of the test results. The loading device and schematic diagram of the compressive strength test setup for the masonry specimens are shown in Figure 17.

4.3. Description of Test Phenomena Test of Modified Coal Gangue Concrete Masonry

The compressive failure process of modified coal gangue concrete hollow block masonry generally resembles that of conventional concrete masonry. As illustrated in Figure 18a, the entire failure process under axial compression could be categorized into three distinct stages: the elastic stage, the plastic stage, and the failure stage. Initially, when the applied load remained within 40% of the ultimate load, the masonry exhibited a clear linear elastic behavior, characterized by minimal deformation and no visible damage. Subsequently, as the load increased to the range of 40–80% of the ultimate capacity, audible tearing sounds emerged, indicating the initiation of internal damage. Fine cracks began to appear, predominantly at the interfaces between block webs and mortar joints. These cracks progressively propagated through the block units, marking the beginning of the plastic stage. With the continuous increase in load approaching the ultimate value, crack development accelerated significantly. Vertically oriented cracks penetrated through the masonry, accompanied by localized crushing and spalling of the mortar, signifying the transition toward structural failure. Ultimately, upon exceeding the peak load, splitting sounds intensified, and crack networks rapidly expanded across the entire specimen. In some cases, individual blocks exhibited noticeable bulging and fracture. Finally, the masonry structure completely lost its load-bearing capacity, primarily due to local crushing or instability within the small columnar sections.

The compressive failure of modified coal gangue concrete hollow block masonry primarily occurred within the upper three courses of blocks and was identified as a typical brittle failure mode. This failure pattern resulted predominantly from the combined effects of boundary constraints. Specifically, the steel bearing plate at the base provided a confining effect, while frictional resistance at the interface between the specimen top and the upper bearing plate restrained lateral deformation. Consequently, stress concentrations developed in the upper and middle regions of the masonry specimen. Due to influencing factors such as irregular block geometries and uneven mortar joint thicknesses, the internal stress distribution within the masonry structure was notably non-uniform. As a result, cracks frequently emerged in the second and third courses of blocks, often accompanied by localized crushing and surface spalling. From a spatial perspective, failure patterns exhibited distinct characteristics depending on the observed surface. On the wide face of the specimen, failure predominantly occurred along the block webs, as illustrated in Figure 18b. In contrast, the narrow face typically displayed vertically penetrating cracks, as shown in Figure 18c, with additional vertical cracks occasionally observed within the internal block webs. These observations collectively confirmed the brittle nature of the failure and highlighted the role of stress concentration and local structural irregularities in initiating damage.

4.4. Compressive Strength Test Results

The results of the compressive strength tests were presented in

Table 8. Compressive strength of modified coal gangue concrete masonry.

Group Number	Serial Number	${\mathit{f}}_1$ (MPa)	${\mathit{f}}_2$ (MPa)	${\mathit{N}}_{\mathit{c}}$ (kN)	${\mathit{N}}_{\mathit{u}}$ (kN)	${\mathit{f}}_{\mathit{m},\mathit{i}}$ (MPa)	${\mathit{f}}_{\mathit{m}}$ (MPa)	${\mathit{\sigma }}_{\mathit{m}}$	Coefficient of Variation
W7.5-7.5	1	8.56	8.12	352.79	462.97	4.13	3.98	0.495	0.124
	2			312.34	420.38	3.75
	3			316.94	380.02	3.39
	4			323.42	405.80	3.62
	5			363.41	477.55	4.26
	6			389.77	532.48	4.75
W10-7.5	1	11.34		377.23	487.38	4.21	5.15	0.586	0.114
	2			380.40	569.47	5.08
	3			385.98	553.77	4.94
	4			459.55	617.67	5.51
	5			396.21	581.80	5.19
	6			484.38	668.12	5.96
W15-7.5	1	16.48		493.36	738.57	6.51	7.28	0.854	0.117
	2			706.42	949.49	8.47
	3			531.36	765.64	6.83
	4			576.37	846.36	7.55
	5			636.22	892.32	7.96
	6			444.21	709.59	6.33

Note: $f_1$ and $f_2$ represent the compressive strengths of the modified coal gangue concrete blocks and coal gangue mortar corresponding to each group of masonry specimens, respectively. $N_c$ is the initial cracking load, $N_u$ is the compressive failure load of the specimen, $f_{m,i}$ is the compressive strength of the individual specimen, $f_m$ is the average compressive strength of the masonry, and ${\sigma }_m$ is the standard deviation of the specimen.

. The compressive strength of the modified coal gangue concrete masonry varied with the block strength. Experimental data indicated that the compressive strength of masonry significantly increased as the block strength improved. The group W7.5-7.5 had an average compressive strength of 3.98 MPa with a coefficient of variation of 0.124, indicating relatively stable overall performance. The group W10-7.5 exhibited an increased average compressive strength of 5.15 MPa, corresponding to a 29.4% increase compared to the preceding group. The group W15-7.5 further increased to 7.28 MPa, approximately 83% higher than the group W7.5-7.5 and 41.4% higher than the group W10-7.5. Additionally, considering the $N_u$ value (ultimate failure load), the maximum failure load for the group W7.5-7.5 was 532.48 kN, whereas the group W15-7.5 reached 949.49 kN, further confirming the significant role of high-strength blocks in enhancing the ultimate bearing capacity of masonry. It was noteworthy that the coefficients of variation for all three groups were below 0.13, demonstrating good repeatability and representativeness of the test results.

The influence of block strength on the compressive strength of masonry was shown in Figure 19, where a clear positive correlation between masonry compressive strength and block strength was observed. Notably, the compressive performance improved significantly when the block strength grade increased from MU7.5 to MU10. Although there was still a considerable increase from MU10 to MU15, the relative growth rate slightly tapered off, indicating a marginally diminishing contribution of block strength to the overall load-bearing capacity of the masonry.

High-strength modified coal gangue blocks (e.g., MU15) possess greater compactness and crushing resistance, effectively delaying the initiation and propagation of microcracks and providing a stable foundation for masonry. In addition, the reduced surface porosity and water absorption improve mortar hydration and enhance the bond between blocks and mortar. Consequently, masonry exhibits more uniform stress distribution under compression, with delayed joint stress concentration and restrained vertical cracking, leading to increased load-bearing capacity. However, the strength gain shows diminishing returns: once block strength reaches a certain level, failure is increasingly governed by the block–mortar interface or material brittleness, limiting the additional contribution of further strength improvement.

4.5. Load-Bearing Capacity of Modified Coal Gangue Concrete Masonry

Domestic and international researchers have proposed various calculation models for the load-bearing capacity of masonry based on compressive strength tests of masonry [50,51]. Among these, the compressive load-bearing capacity calculation model for concrete hollow block masonry presented in the Chinese standard Code for Design of Masonry Structures (GB50003-2011) [52] is widely applied. The average compressive strength of masonry is calculated using the following Formula (1):

$$f_m=k_1{f}_1^{\alpha }(1+0.07f_2)k_2$$

(1)

where $f_1$ is the average compressive strength of the blocks ($\mathrm{N}/{\mathrm{mm}}^2$); $f_2$ is the average compressive strength of the mortar ($\mathrm{N}/{\mathrm{mm}}^2$); $k_1,\alpha ,k_2$ represents the correction factor, and $k_1$ is taken as 0.46, $\alpha$ is taken as 0.9, $k_2$ is taken as 1.0.

By substituting the measured compressive strengths of the modified coal gangue concrete blocks and mortar into Formula (1), the average compressive strength of the modified coal gangue block masonry was calculated. A comparison between the code-based calculated values and the experimental results is shown in Figure 20. It could be observed that the experimentally measured compressive strengths of the modified coal gangue concrete block masonry are consistently lower than the values calculated using the current national design code. This indicates that applying the existing code directly may lead to unconservative estimates of compressive strength for such masonry. Therefore, to more accurately reflect the compressive behavior of modified coal gangue block masonry, the coefficient $k_1$ in the formula is calibrated based on the experimental data presented in this study. Specifically, the coefficient $k_1$ was revised to 0.37. As shown in Figure 20, the revised calculated values closely match the experimental results, providing theoretical support for the future engineering application of modified coal gangue concrete blocks.

5. Conclusions

(1) The silicic acid gel formed through sodium silicate hydrolysis effectively filled and bound pores/cracks in coal gangue aggregates, mitigating their inherent defects. This modification improved aggregate quality by increasing apparent density, reducing water absorption, and lowering the crushing index. Consequently, the modified coal gangue significantly enhanced concrete compressive strength. Compared to raw gangue concrete, the modified variant exhibited higher strength at all tested curing ages, with increases of 10.7% (3 d), 12.3% (7 d), 15.1% (14 d), and 15.3% (28 d).

(2) The sodium silicate gel remaining on the surface of modified aggregates reacted with Ca(OH)₂ through hydrolysis. This reaction accelerated the hydration of cementitious materials, leading to the formation of additional high-strength, stable flocculent calcium silicate hydrate (C-S-H) and acicular rod-shaped ettringite (AFt) mixtures. These products filled pores and cracks in the interfacial transition zone (ITZ) of concrete and effectively improved the ITZ performance between coal gangue coarse aggregates and cement mortar. Consequently, the compactness of the interfacial transition zone was significantly enhanced.

(3) The variation pattern of compressive strength in modified coal gangue concrete blocks was fundamentally similar to that of conventional coal gangue concrete blocks. When the replacement ratio of coal gangue in modified blocks remained constant, their compressive strength gradually decreased with increasing water–cement ratio. Under fixed water–binder ratios, the compressive strength showed a declining trend as the replacement ratio of coal gangue aggregate increased. The average compressive strengths of specimen groups C32G40N, C42G100N, and C52G100N were 16.17 MPa, 10.38 MPa, and 8.36 MPa, respectively, meeting the required strength grades of MU15, MU10, and MU7.5 for experimental validation.

(4) The compressive strength of modified coal gangue concrete hollow block masonry increased significantly with the enhancement of block strength grade, with the average compressive strength of MU15 masonry being approximately 83% higher than that of MU7.5. And a calculation model for the average compressive strength of modified coal gangue concrete masonry was proposed based on compression tests.