Mechanical Properties of Fly Ash Ceramsite Concrete Produced in a Single-Cylinder Rotary Kiln

Abstract

Fly ash, as the main solid waste of coal-fired power plants, is an environmental problem that needs to be solved due to its massive accumulation. The mechanical properties and optimization mechanism of lightweight aggregate concrete prepared by using new single-cylinder rotary kiln fly ash ceramic granules as aggregate were systematically investigated. Through orthogonal experimental design, combined with macro-mechanical testing and microscopic characterization techniques, the effects of cement admixture and ceramic granule admixture on the properties of concrete, such as compressive strength, split tensile strength, and modulus of elasticity, were analyzed, and the optimization scheme of key parameters was proposed. The results show that the new single rotary kiln fly ash ceramic particles significantly improve the mechanical properties of concrete by optimizing the porosity (water absorption ≤ 5%), and its 28-day compressive strength reaches 46~50.9 MPa, which is 53.3~69.7% higher than that of the ordinary ceramic concrete, and the apparent density is ≤1900 kg/m³, showing lightweight and high-strength characteristics. X-ray diffraction (XRD) analysis shows that the new ceramic grains form a more uniform, dense structure through the synergistic effect of internal mullite crystals and dense glass phase; computed tomography (CT) scanning shows that the total volume rate of cracks of the new ceramic concrete was reduced by up to 63.8% compared with that of ordinary ceramic concrete. This study provides technical support for the utilization of fly ash resources, and the prepared vitrified concrete meets the demand of green building while reducing structural deadweight (20~30%), which has significant environmental and economic benefits.

1. Introduction

Fly ash is a typical solid waste generated by coal-fired power plants. In China, its annual output exceeds 600 million tons, with a comprehensive utilization rate of less than 80%. Large amounts of accumulated fly ash not only occupy land resources, but also their heavy metal leaching and dust pollution pose a serious threat to the ecological environment. Traditional approaches to utilizing fly ash (such as cement admixtures and roadbed materials) are plagued by issues like low added value and limited absorption capacity. In contrast, using it as the core raw material for ceramsite preparation constitutes an effective way to develop a circular economy, address fly ash absorption, and achieve high-value utilization of fly ash resources [1]. In addition, SiO₂ and Al₂O₃ in fly ash can form a porous ceramic structure in high-temperature calcination, which can give ceramic granules lightweight and high-strength characteristics [1,2], which is in line with the demand for high-value utilization of solid waste under the “dual-carbon strategy”. Therefore, the development of fly ash ceramic particles and its concrete technology is a key path to solve the contradiction between the environmental load of fly ash and the shortage of resources.

Efficient resource utilization of industrial solid waste is one of the key pathways to promote global sustainable development. As an important carrier of solid waste resource utilization, the preparation technology of fly ash ceramic granules has made significant progress in recent years in the optimization of a raw material system, molding process innovation, sintering mechanism exploration, and environmental performance regulation, etc. [2,3,4,5]. This study prepares fly ash into high-performance ceramsite concrete aggregates via the single-cylinder rotary kiln process, establishing a closed-loop utilization mode of “industrial by-products-high-value building materials”. This technical pathway directly responds to the core requirements of “Responsible Consumption and Production” in the Sustainable Development Goals (SDG), reducing the environmental load of the industrial chain by improving the utilization rate of solid waste, while minimizing ecological disturbances caused by the extraction of natural aggregates.

Meanwhile, the development of new-type fly ash ceramsite concrete provides technical support for the green transformation of the construction industry. Compared with traditional concrete, this material replaces natural aggregates with industrial solid waste, reducing carbon emissions and resource consumption during the production stage while ensuring mechanical properties. It aligns with the goal orientation of “Industry, Innovation and Infrastructure” in the SDG, providing a promotable technical paradigm for low-carbon and circular development in the field of building materials.

A major challenge of lightweight aggregate concrete lies in its relatively low strength. Since ceramsite is used as coarse aggregate in the preparation of lightweight aggregate concrete, research on its mechanical properties has long been a key focus for scholars both domestically and internationally in the field of lightweight aggregate concrete [6,7,8,9]. Pottery grains as coarse formulated lightweight aggregate concrete have an important effect on its mechanical properties. Fan et al. [10] developed a lightweight aggregate geopolymer concrete (LAGC) using shale ceramsite as raw material. The study showed that, at the curing ages of 3 d and 28 d, the compressive strength and dry density of LAGC with two different mix ratios both met the requirements of compressive strength greater than 10.0 MPa and dry density less than 1400 kg/m³. Wang et al. [11] prepared sedimentary ceramsite using Yellow River sediment and pulverized coal as main raw materials, with the mass ratio of Yellow River sediment to pulverized coal being 90% and 10%, respectively. The ceramsite belongs to the 900 density grade, and its cylinder pressure strength and water absorption rate meet the application requirements. The study indicated that the strength of masonry mortar using ceramsite as aggregate was lower than that using standard sand as aggregate, but it still met the application requirements.

By properly adjusting the water-binder ratio and ceramsite content, this can significantly improve the mechanical properties of ceramsite concrete while ensuring it is light in weight. Yang Han et al. [12] used 500 density grade coal-based solid waste ceramsite as coarse aggregate to prepare ceramsite foam concrete. The results showed that when the foam content is 43%, the water-to-binder ratio is 0.31, and the fly ash content is 15%, the concrete meets the A10 grade standard, with a high goodness of fit of the quadratic regression model (R² > 0.96). An increase in foam content leads to an increase in average pore size and roundness, while an increase in fly ash content shows the opposite trend. The established three-dimensional thermal conductivity model exhibits excellent precision. Wang Chaoqiang et al. [13] compared and evaluated the working performance and mechanical properties of LC50 fly ash microbead lightweight high-strength concrete (FLHSC) with fly ash microbeads, cement, water-reducing agent dosage, water-binder ratio, and additive types as variables. The research results show that the optimal mix proportion of FLHSC is 230 kg/m³ of fly ash cenospheres, 200 kg/m³ of ceramsite, 1200 kg/m³ of cement, 360 kg/m³ of water, and 20.4 kg/m³ of water-reducing agent. The water-binder ratio is 0.3, with type II water-reducing agent adopted, and its dosage is 1.7% of the cementitious material. The failure mode of FLHSC is close to vertical failure, with cracks all passing through lightweight aggregates; its 28-day strength is 52.4 MPa, and it has better environmental benefits compared with ordinary C50 concrete. Qi Huijun et al. [14] showed that the mechanical properties of fly ash ceramic concrete were significantly affected by the water-cement ratio, the amount of cementitious materials, and the amount of ceramic particles. It can be seen that the reasonable adjustment of the water-cement ratio and the amount of ceramic particles can significantly improve the mechanical properties of fly ash ceramic concrete while ensuring that it is lightweight. Zhang, Q. et al. [15] studied the preparation process of ceramsite foam blocks using sludge-based non-sintering ceramsite (SNSC), investigated the effects of factors such as fly ash content, SNSC volume ratio, water-binder ratio, and sodium silicate concentration on the performance of the blocks, and determined the optimal mix proportion (including cement 400 kg/m³, fly ash 100 kg/m³, etc., with a water-binder ratio of 0.43, CaCl₂ 10 kg/m³, and Na₂SO₄ 5 kg/m³), providing a reference for the preparation and optimization of such blocks. Li Xiang et al. [16] studied the synergistic modification mechanism of ceramsite sand and fly ash. They found that a combination of a 50% ceramsite sand replacement rate and 20% fly ash content can make the cubic compressive strength of concrete reach 34.5 MPa, and the splitting tensile strength rise to 3.2 MPa. This achieves an optimal balance with a lightweight coefficient of 0.82 and a strength coefficient of 1.15. Zhu Xueqing et al. [17] conducted tests on cementitious material system reconstruction. They found that the strength of the low water-binder ratio system (0.3) is 282% higher than that of the high water-binder ratio system (1.2). This is due to the dense reconstruction of the spatial distribution of hydration products. Studies on the pozzolanic effect of fly ash showed that the 56d strength growth rate of the 45% content group reaches 38.7%, a significant increase compared with the 28 d strength. XRD analysis confirmed that the content of Friedel’s salt increases to 5.3% at this time, indicating that the secondary hydration reaction forms a new reinforcing phase.

The size of the water-cement ratio is an important factor affecting the strength of lightweight aggregate concrete. Zhao Feiyang et al. [18] found that an increase in the water-cement ratio led to a decrease in the fluidity of the ceramic concrete, a significant increase in the degree of layering of large-size ceramic particles uplift, and a decrease in the uniformity of the concrete. The compressive strength decreased with the increase of the water-cement ratio, and the increase of the water-cement ratio led to a weak interfacial transition zone and decrease of splitting tensile strength. Yan Chunhao et al. [19] pointed out that the axial compressive strength and splitting tensile strength of concrete were significantly increased by adding ceramic particles, and the water-cement ratio was controlled at 0.33–0.37 to ensure the high strength and durability of concrete. Prokopski et al. [20] discussed the results of fracture toughness studies of natural gravel aggregate concrete with different water-cement ratios (W/C = 0.33, 0.43, 0.53, and 0.63), no silica fume, and silica fume added. Fracture toughness study results found that as the water-cement ratio increased, the structural porosity in the aggregate-cement paste transition zone increased, crack extension accelerated, and crack surface roughness increased. Li et al. [21] found that in lightweight high-strength concrete (LWHSC) incorporating ceramsite, a lower water-cement ratio and more mineral admixtures (such as fly ash and slag powder) significantly improved the compressive strength.

The sand ratio is also an important reason for the strength of vitrified concrete. Liu Qiang et al. [22] found that the strength of high-strength shale vitrified concrete increases with the increase of sand rate when the sand rate is adjusted within a certain limit. Mufti et al. [23] found that the more significant the sand addition in the concrete mix, the lower the compressive strength was through the compressive strength test study. Tang Xiao [24] in his study pointed out that optimization of sand rate can improve the overall performance of concrete, and the 28-day compressive strength was 53.2 MP at a 40% sand rate, which is the optimum sand rate range.

The incorporation of mineral admixtures has a greater impact on the mechanical properties of concrete. Feiyu Tao et al. [25] adopted orthogonal tests, with ceramsite content, water-cement ratio, and fly ash content as research parameters, to optimize the mix proportion of ceramsite foam concrete and improve its strength, taking compressive strength as the main evaluation index. The results show that fly ash content has the greatest influence on the compressive strength of ceramsite foam concrete, followed by ceramsite content, and the water-cement ratio has the smallest influence. The recommended mix proportion is a ceramsite content of 10∼20%, fly ash dosage of 10∼15%, and water-cement ratio of 0.4∼0.5. Yuping Li [26] systematically investigated the effect of adding different contents of mineral admixtures on the compressive strength of shale vitrified concrete and found that with a total admixture of 10% (mass fraction) and a ratio of 1∶2 of biotite kaolin and fly ash, the compressive strength at 3 d, 7 d, and 28 d increased by 417%, 267%, and 250%, respectively. Shannag [27] found that the use of silica fume with 5–15% by weight in lightweight concrete (LWC) instead of cement increased the compressive strength and modulus of elasticity by 57% and 14%, respectively, compared to the mixture without silica fume. Zhao et al. [28] studied the mechanical behavior and autogenous deformation of concrete with the addition of pre-wetted ceramsite (PWC) or CaO-based expansive agent (CEA). The experimental results showed that the addition of PWC enhanced the compressive and splitting tensile strengths, while the opposite results were obtained with CEA addition. Zhu et al. [29] investigated the use of full lightweight ceramsite concrete to repair ordinary concrete and found that this method could make the tensile strength of the repaired composite structure reach about 84.5% of that of the original concrete structure.

In order to clearly present the current research status of vitrified concrete, the relevant research literature is categorized and organized as follows according to the research perspective (

Table 1. Comparison table of related studies on fly ash concrete.

Research Entry Angle	Research Literature	Core Research Content	Difference with This Study
Reasonable adjustment of water-cement ratio and ceramic granule admixture	Yang Han et al. [12], Chaoqiang Wang et al. [13], Huijun Qi et al. [14], Zhang, Q.Y. et al. [15], Xiang Li et al. [16], Xueqing Zhu et al. [17]	Exploring the effects of water-cement ratio, ceramic granule dosing/substitution rate, and mineral admixture synergism on performance	Ceramics are mostly of common type or non-fly ash-based. Fly ash is only used as an admixture with low dosage, not as the core raw material of ceramic granules, and high-value utilization; the preparation process of fly ash ceramic granules is not involved; only the optimization of the mixing ratio is carried out, and the closed-loop of solid waste resource utilization is not constructed
Size of water-cement ratio	Zhao Feiyang et al. [18], Yan Chunhao et al. [19], Prokopski et al. [20], Li et al. [21].	To analyze the effect of the water-cement ratio on flowability, homogeneity, durability, fracture toughness, and strength of	Pottery is not fly ash-based; only single-parameter analysis, not associated with efficient use of fly ash
Sand ratio	Qiang Liu et al. [22], Mufti et al. [23], Xiao Tang [24].	To study the effect of sand rate on strength and comprehensive performance	Ceramic granules are mostly natural raw materials such as shale; only the sand rate was optimized, and no innovation in the fly ash ceramic granule preparation process was involved
Incorporation of mineral admixtures	Feiyu Tao et al. [25], Yuping Li [26], Shannag [27], Zhao et al. [28], Zhu et al. [29]	Analyzing the strengthening effect of mineral admixtures (fly ash, silica fume, etc.) on strength	Fly ash is only used as a concrete admixture, not as a core raw material for ceramic granules; some ceramic granules are not fly ash-based, not reflecting the high value of solid waste

).

In summary, existing studies either use natural aggregates (such as shale ceramsite) or only take fly ash as a concrete admixture, mostly focusing on the impact of single parameters (e.g., water-cement ratio, sand ratio, admixture proportion) on performance. This study uses fly ash (industrial solid waste) as raw material to prepare ceramsite through the industrialized process of a single-cylinder rotary kiln. With “new-type fly ash ceramsite prepared by single-cylinder rotary kiln” as the core, it breaks through the traditional mode of “low-value-added utilization of fly ash” and constructs a full-chain resource recycling loop of “solid waste-high-value aggregate-concrete”. It systematically explores the synergistic laws of workability and mechanical properties under different mix ratios and conducts mechanical property tests and microscopic analysis on the designed new-type fly ash ceramsite concrete with different strengths. While optimizing practical parameters, it solves the interconnected issues of “solid waste disposal-performance improvement-green transformation” through technological innovation, which to a certain extent makes up for the deficiencies of existing studies and provides a reference for research on similar solid waste-based materials.

2. Test Overview

2.1. Test Material

P.O 42.5 grade silicate cement produced by Jidong Cement Company (Tangshan, China) is used in this test, and all the performance indexes are in accordance with the standard requirements of “General Silicate Cement” (GB 175-2007) [30]. The basic properties of silicate cement are shown in

Table 2. Physical property indexes of silicate cement.

Cement Variety	Fineness/%	Initial Setting Time/min	Final Setting Time/min	Stability	Burning Vector
P.O 42.5	≤10	≥45	≤600	Qualified	≤5.0

. Mechanized sand from Caofeidian Hengxu Co., Ltd. (Tangshan, China) is used, and the fineness modulus of 2.77 (medium sand), mud content of 1.9%, loose bulk density of 1620 kg, and good particle grading are determined according to the test of “Sand for Construction” (GB/T 14684-2001) [31].

The common ceramic granule used in this test is the fly ash ceramic granule produced by Datang Tongzhou Science and Technology Co., Ltd. (Beijing, China). The particle size is generally 5~20 mm, and the maximum particle size is 22 mm, and its physical properties are shown in

Table 3. Physical properties of ceramic granule indexes.

Type of Ceramic Granule	Gradation/mm	Bulk Density/(kg m−3)	Cylinder Compression Strength/MPa	Water Absorption Rate/%
Fly ash granule	5–20	990	≥6.5	16

, and the external and internal morphology diagrams are shown in Figure 1.

The new single-drum rotary kiln fly ash ceramic granules used in this test are prepared by taking fly ash as the main raw material and adding some additives such as binder (red mud, gangue, clay, bentonite) and pore-forming agent (gravel coal, sludge, rice bran). Differing from the traditional ceramic granules, through improved design and optimized process, the new single-cylinder rotary kiln fly ash ceramic granules have the advantages of high strength, being green and friendly, and low energy consumption, and their physical properties are shown in

Table 4. Physical property index of new single-cylinder rotary kiln fly ash ceramic granule.

Type of Ceramic Granule	Gradation/mm	Bulk Density/(kg m−3)	Cylinder Compression Strength/MPa	Water Absorption Rate/%
Fly ash granule	5–20	1000	10.8	4.8

, and the external morphology is shown in Figure 2.

The polycarboxylic acid water-reducing agent produced by Caofeidian Hengxu Concrete Co., Ltd. (Tangshan, China) was used in the test, and the specific performance indexes are shown in

Table 5. Performance index of water-reducing agent.

Appearance and Shape	Water Reduction Rate/%	pH Value	Gas Content/%	Total Alkali Content/%
Colorless liquid	30	7	3	≤0.2

.

2.2. Test Design

Lightweight aggregate concrete is prepared in accordance with the Technical Specification for Lightweight Aggregate Concrete (JGJ 51-2002) [32]: compressive strength, modulus of elasticity, and axial compressive strength of prismatic cylinders of lightweight aggregate concrete are carried out in accordance with the methods stipulated in the Standard for Mechanical Properties of Ordinary Mixed Soil Test Methods (GB/T 50081-2019) [33].

The mechanical properties of new single-drum rotary kiln ceramic lightweight aggregate concrete were investigated by three factors: ceramic granule mixing, water-cement ratio, and cement mixing. The orthogonal test ratio method was used to determine the mixing ratios, and the variable factors of water-cement ratio (the amount of water in the water-cement ratio excluding the additional water consumption) were determined to be 0.31, 0.35, and 0.39; the dosage of ceramic granules was 712 kg/m³, 752 kg/m³, and 792 kg/m³, and the dosage of cement was 440 kg/m³, 480 kg/m³, and 520 kg/m³. The mixing ratio of concrete should be designed in terms of the mass of various materials per 1 m³ of concrete. The design should be expressed in terms of the mass of various materials in each 1 m³ of concrete; the new single-cylinder rotary kiln ceramic concrete mix ratio is shown in

Table 6. New type single-cylinder rotary kiln ceramic concrete mixing ratio.

No.	Cement/ (kg m−3)	Ordinary Sand/(kg m−3)	Ceramic Granule/ (kg m−3)	Water-Reducing Agent/(kg m−3)	Water/ (kg m−3)	Water-Cement Ratio
1	440	667	712	12	175	0.31
2	520	620	752	12	150	0.31
3	480	620	792	12	165	0.31
4	520	620	712	12	180	0.35
5	480	620	752	12	195	0.35
6	440	620	792	12	180	0.35
7	480	620	712	12	180	0.39
8	440	620	752	12	180	0.39
9	520	620	792	12	180	0.39

.

3. Test Methods

3.1. Compressive Strength Test and Split Tensile Strength Test

In this study, a total of 9 groups of different mixing ratio schemes were designed (Table 6), and 18,100 mm × 100 mm × 100 mm cubic specimens were prepared in each group, totaling 162 specimens that were molded. The following process was strictly followed during the specimen preparation: firstly, the fly ash ceramic granules were presoaked for 4 h, and after the surface water drained naturally, they were added into the other cementitious materials that had been preliminarily mixed, and then mixed and stirred for 5 min in a forced mixer. The mixture was poured into the mold in layers, and after 24 h of molding, the mold was dismantled and transferred to the standard maintenance room for constant temperature and humidity maintenance at (20 ± 2) °C and relative humidity ≥ 95%. The test program was designed as follows: each group of specimens was tested for mechanical properties at three ages: 3 d, 7 d, and 28 d, and three specimens were selected for the cubic compressive strength test and three specimens for the split tensile strength test at each age. During the testing process, a WAW-1000 microcomputer-controlled electro-hydraulic servo universal testing machine was used, and the loading rate was strictly controlled in the range of 0.5 MPa/s. For each set of data, the arithmetic average of the test results of three valid specimens was taken as the final test value.

3.2. Axial Compressive Strength Test, Modulus of Elasticity Test, and Poisson’s Ratio Test

In accordance with the nine groups of different proportioning schemes in Table 6, a total of 135 lightweight aggregate concrete specimens were produced for testing. In total, 81 prismatic specimens with dimensions of 150 mm × 150 mm × 300 mm were subjected to an axial compressive strength test; 27 prismatic specimens with dimensions of 150 mm × 150 mm × 300 mm were subjected to an elastic modulus test; and 27 prismatic specimens with dimensions of 150 mm × 150 mm × 300 mm were subjected to Poisson’s ratio test, which included 300 mm prisms for Poisson’s ratio test. Each group of parameters was under the conditions of the design of 15 specimens, of which 9 specimens tested axial compressive strength, 3 specimens tested Poisson’s ratio, and 3 specimens tested the modulus of elasticity E, respectively, taking the average of three data as the test results. During the casting of test specimens, fly ash ceramsite was soaked in a water tank in advance, after which the excess surface moisture was drained off. Subsequently, the fly ash ceramsite was added into a mixer where other materials had been uniformly mixed; the mixture was then stirred for 5 min before being poured into molds for forming. The formed specimens were placed indoors for standard curing for 28 days. Tests were carried out in accordance with Standard for Test Methods of Mechanical Properties of Ordinary Concrete (GB/T 50081-2019).

4. Test Results and Analysis

4.1. Compressive Strength

4.1.1. Compressive Strength of Ceramic Concrete of New Single-Cylinder Rotary Kiln

The test results are shown in Figure 3, and the compressive strength statistics of the specimens are shown in

Table 7. Compressive strength of ceramic concrete of novel single-cylinder rotary kiln.

No.	Compressive Strength/MPa
	3 d	7 d	28 d
1	30.40	41.61	50.92
2	28.50	39.55	46.65
3	35.34	40.05	49.97
4	34.39	43.32	48.54
5	33.44	40.54	46.93
6	30.65	36.67	47.59
7	28.60	35.34	47.12
8	28.03	34.39	47.88
9	26.22	39.14	47.46

.

As can be seen from Figure 3, the compressive strengths of ceramic concrete with different mix ratios show a significantly increasing trend with age, which is in line with the hydration reaction law of silicate cement-based materials. Among them, the 28-day compressive strength of No. 1 reaches 50.92 MPa, which is the optimal value for each group, with an increase of 67.5% compared with the 3-day strength, indicating that its later hydration reaction is sufficient and the microstructure densification process is good. The 3-day strength of No. 9 was the lowest (26.22 MPa), but its 28-day strength still increased to 47.46 MPa, an increase of 80.9%, reflecting that vitrified concrete generally possesses excellent potential for long-term strength development.

Comparison of the strength differences between different mix ratios revealed that the extreme difference in 3-day strength was 9.12 MPa, and the extreme difference in 28-day strength was narrowed to 4.27 MPa, indicating that the early strength was more significantly affected by the mix ratio. For example, the leading 3-day strength of No. 3 may be related to the higher proportion of early strength components in its cementitious material, while the high late strength of No. 1 may be attributed to the optimization of the volcanic ash effect of the fly ash with the ceramic interface transition zone. It is worth noting that the sudden increase in strength of No. 4 at 7 days of age may be related to its aggregate gradation or water-cement ratio design to promote the mid-term hydration reaction.

4.1.2. Sensitivity Analysis of Compressive Strength Factors

For the compressive strength of new single-cylinder rotary kiln ceramic concrete, further polar analysis to explore the impact of the test factors on the new single-cylinder rotary kiln ceramic concrete, the results are shown below:

As can be seen from

Table 8. Extreme difference analysis of compressive strength of a new type of monoclonal rotary kiln ceramic concrete.

Factor	3 d Compressive Strength/MPa			7 d Compressive Strength/MPa			28 d Compressive Strength/MPa
	Water-Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water- Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water- Cement Ratio	Pottery Granule Mixing Amount	Cement Dosage
K1	94.24	93.39	89.08	121.20	120.27	112.67	147.54	146.59	146.40
K2	98.48	89.97	97.38	120.53	114.48	115.93	143.07	141.46	144.02
K3	82.25	92.21	89.11	108.87	115.86	122.01	142.46	145.03	142.66
k1	31.41	31.13	26.69	40.40	40.09	37.56	49.18	48.86	48.80
k2	32.83	29.99	32.46	40.18	38.16	38.64	47.69	47.15	48.01
k3	27.41	30.74	29.70	36.29	38.62	40.67	47.49	48.34	47.55
R	5.42	1.14	5.77	4.11	1.93	3.11	1.69	1.71	1.26
R Comparison		R3 > R1 > R2		R1 > R3 > R2			R2 > R1 > R3

, for 3 d compressive strength, the sensitivity ranking of the factors is cement admixture (R = 5.77) > water-cement ratio (R = 5.42) > ceramic granule admixture (R = 1.14). The extreme deviation of cement dosage was significantly higher than other factors, indicating its dominant role in early strength. An increase in cement dosing accelerates the hydration reaction and generates more hydration products in the short term, which leads to rapid strength enhancement. The effect of the water-cement ratio is second; a lower water-cement ratio can enhance the compactness by reducing the porosity, but the early hydration is not yet complete, and its role is weaker than that of cement dosage. Ceramic admixture has the least extreme difference, probably due to the low strength of the ceramic itself and its limited contribution to the concrete skeleton in the short term. At the age of 7 d, the order of sensitivity shifts to water-cement ratio (R = 4.11) > cement admixture (R = 3.11) > ceramic admixture (R = 1.93). The sensitivity of the water-cement ratio increased significantly as the hydration reaction advanced. A lower water-cement ratio reduces the free water content, resulting in a denser cement paste structure with reduced porosity, which in turn exhibits greater strength modulation at mid-age. The effect of cement dosing diminished, probably due to the slowing down of the hydration rate and its incremental marginal benefit on strength. The effect of ceramic dosing remained weak, but the polar deviation increased slightly, indicating that the interfacial bond between the ceramic and matrix started to play a role. The sensitivity ranking further changed to ceramic dosing (R = 1.71) > water-cement ratio (R = 1.69) > cement dosing (R = 1.26) at the age of 28 d. The effect of ceramic dosing on the strength was also stronger at the middle age. After long-term maintenance, the ceramic admixture becomes the dominant factor. The increase in the admixture of vitrified granules as a lightweight aggregate decreases the concrete density, but the optimization of the long-term interfacial transition zone (ITZ) and the increase in stress transfer efficiency may enhance the overall strength. The effect of the water-cement ratio persists, but the extreme difference is further reduced, indicating that the paste structure has stabilized. The lowest sensitivity of cement dosage indicates that the hydration reaction is close to completion, and its contribution is gradually saturated.

4.2. Splitting Tensile Strength

4.2.1. Splitting Tensile Strength of New Single Rotary Kiln Ceramic Concrete

The splitting tensile strength of concrete at 3 d, 7 d, and 28 d was carried out for the determined mix ratio. The test results are shown in Figure 4, and the split tensile strength statistics of the specimens are shown in

Table 9. Splitting tensile strength of new single-cylinder rotary kiln ceramic concrete.

No.	Splitting Tensile Strength/MPa
	3 d	7 d	28 d
1	1.54	2.75	3.96
2	1.61	3.01	3.84
3	1.64	3.39	3.80
4	2.10	3.08	3.93
5	1.50	3.20	3.78
6	1.57	3.17	3.53
7	1.41	3.13	3.96
8	1.72	3.11	3.81
9	2.01	2.71	3.79

.

The test results show that the split tensile strength of each specimen of each proportion shows a significant growth trend with the age extension, which is in line with the characteristics of the hydration reaction of cementitious materials. Among them, No. 4 reached 2.1 MPa at 3 days of age, significantly higher than other groups, which may be related to the optimization of the early-strength component or aggregate grading in its cementitious materials, while No. 7 had the lowest 3-day strength, but the 28-day strength jumped to 3.96 MPa, with an increase of 181%, reflecting that some of the mixes have outstanding potential for late strength development. Comparing the 28-day strengths, No. 1 and No. 7 tied for the highest, with increases of 157% and 181%, respectively, over the 3-day strengths, indicating that the continuous optimization of the interfacial bonding properties between the ceramic grains and the matrix has a decisive role in the long-term tensile strength.

Analysis of the strength development curves revealed that the rate of strength growth in the early stage (3~7 days) was generally higher than that in the later stage (7~28 days); e.g., the 7-day strength of No. 1 was enhanced by 78.6% compared with that of 3 days, whereas the 28-day strength increased by only 44.0% compared with that of 7 days, which was in line with the staged character of the hydration reaction rate of cement. It is noteworthy that No. 9 has a higher strength at 3 days but a relatively low increase in 28-day strength, which may imply that the early-strength component in its proportion has an inhibitory effect on later hydration. In addition, the strength of No. 3 peaked at 7 days but slightly decreased to 3.80 MPa at 28 days, so it is necessary to analyze whether there exists an interfacial microcrack expansion phenomenon in combination with the microstructure.

4.2.2. Damage Pattern of Split Tensile Strength Test

This study systematically reveals the damage pattern characteristics and failure mechanism of new single rotary kiln ceramic concrete through splitting tensile strength tests. The test shows that the damage process of the specimen presents typical brittle fracture characteristics, which can be divided into three stages: the initial loading stage (elastic deformation), the microcrack sprouting stage (ITZ damage), and the main crack penetration stage (brittle splitting). In the elastic stage, the stress distribution inside the specimen was uniform, and no macroscopic damage was observed; when the load was increased to 60~80% of the critical value, the interfacial transition zone (ITZ) took the lead in the emergence of radial microcracks due to stress concentration, which was closely related to the difference in the mechanical properties between the ceramic granule porous surface and the cement matrix. After the formation of the main crack, its extension path is regulated by the distribution density of ceramic grains and matrix strength, often extending along the aggregate-paste interface (Figure 5) and locally bifurcating or offset due to ceramic grains blocking, resulting in the damage surface showing a “stepped” uneven characteristic.

Comparison of the damage morphology of the two types of fly ash vitrified concrete (Figure 6) revealed that the damage surface of the new vitrified specimens was relatively intact, and the proportion of vitrified particles shedding was reduced by about 30%, indicating that its optimized surface roughness enhanced the aggregate-matrix mechanical occlusion. On the other hand, the ordinary ceramic specimens showed obvious particle fragmentation and interfacial peeling, reflecting the weak interfacial bond of unmodified ceramic. It is noteworthy that the load–displacement curves of both types of concrete showed steeply decreasing segments, but the residual strength of the new ceramic specimens increased by 15~20%, which may be related to the enhanced ceramic bridging effect during crack extension. Further analysis revealed that the water absorption rate of the ceramic grains had a significant effect on the damage mode: the pre-soaked treated ceramic grains effectively inhibited the directional growth of calcium hydroxide crystals in the ITZ region by reducing the interfacial water migration, thus improving the interfacial microstructural densification. It provides an important basis for the optimization of the tensile properties of vitrified concrete.

4.2.3. Sensitivity Analysis of Splitting Tensile Strength by Factors

Consistent with the compressive strength analysis, for the new single rotary kiln ceramic concrete splitting tensile strength further polar analysis, the results are shown in

Table 10. Extreme difference analysis of splitting tensile strength of a new type of single-drum rotary kiln ceramic concrete.

Factor	3 d Compressive Strength/MPa			7 d Compressive Strength/MPa			28 d Compressive Strength/MPa
	Water-Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water- Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water- Cement Ratio	Pottery Granule Mixing Amount	Cement Dosage
K1	4.79	5.05	4.83	9.15	8.96	9.03	11.6	11.85	11.3
K2	5.17	4.83	4.55	9.45	9.32	9.72	11.24	11.43	11.54
K3	5.14	5.22	5.72	8.95	9.27	8.8	11.56	11.12	11.56
k1	1.60	1.68	1.61	3.05	2.99	3.01	3.87	3.95	3.77
k2	1.72	1.61	1.52	3.15	3.11	3.24	3.75	3.81	3.85
k3	1.71	1.74	1.91	2.98	3.09	2.93	3.85	3.71	3.85
R	0.12	0.13	0.3	0.17	0.12	0.31	0.12	0.24	0.08
R Comparison		R3 > R2 > R1			R3 > R1 > R2			R2 > R1 > R3

.

The sensitivity of the split tensile strength of new single rotary kiln ceramic concrete shows significant dynamic characteristics with the curing time. At the 3 d age, cement admixture (R = 0.30) dominates the sensitivity, which accelerates the hydration reaction to enhance the matrix compactness; at the 7 d stage, the role of water-cement ratio (R = 0.17) is enhanced, and the low water-cement ratio optimizes the pore structure and indirectly strengthens the interfacial bond; after 28 d of long-term curing, the amount of ceramic granules (R = 0.24) becomes the key factor. After 28 d of long-term maintenance, the dosage of ceramic grains (R = 0.24) becomes a key factor; excessive ceramic grains will weaken the interfacial stress transfer efficiency, while a reasonable dosage of ceramic grains can improve the overall tensile properties through the densification of the interfacial transition zone (ITZ). The effect of cement dosage gradually decreases with hydration saturation (R = 0.08), indicating that its contribution tends to be limited.

This dynamic law provides a phased optimization strategy for engineering applications: early (3 d~7 d), focus on increasing the cement dosage and controlling the water-cement ratio to meet the construction strength requirements; long-term (28 d), the dosage of ceramic granules should be precisely regulated (≤30% is recommended) and supplemented with surface modification to balance the lightweighting and tensile properties. Through the orthogonal test of comprehensive coordination of water-cement ratio, ceramic mixing amount, and the global optimization of material properties can be realized, providing theoretical support for the proportioning design and process improvement of lightweight high-strength concrete.

4.3. Axial Compressive Strength

4.3.1. Axial Compressive Strength of New Single Rotary Kiln Ceramic Concrete

The axial compressive strength test was carried out for the determined proportion, and the axial compressive strength of concrete was measured at 3 d, 7 d, and 28 d. The test results are shown in Figure 7, and the axial compressive strength statistics of the test blocks are shown in

Table 11. Axial compressive strength of ceramic concrete of a new single-cylinder rotary kiln.

No.	Axial Compressive Strength/MPa
	3 d	7 d	28 d
1	30.24	33.56	47.48
2	31.45	33.64	43.13
3	30.89	33.13	46.22
4	31.82	35.69	45.46
5	29.14	34.18	43.31
6	29.83	35.65	43.87
7	31.32	35.87	44.74
8	30.89	33.08	44.26
9	30.79	35.93	42.95

.

The results of the axial compressive strength test of new single-cylinder rotary kiln vitrified concrete show that its strength development presents a significant age dependence. From Table 11 and Figure 7, it can be seen that the average compressive strengths at 3 d, 7 d, and 28 d were 30.61 MPa, 34.53 MPa, and 44.50 MPa, respectively, and the strengths continued to increase with age, in which the average increase from the 3 d to 7 d stage was 12.8%, and the average increase from the 7 d to 28 d stage further reached 28.9%. This trend indicates that the rate of strength development of vitrified concrete accelerates significantly at the later stage, which may be related to the gradual stabilization of the internal pore structure of the vitrified grains and the continuous advancement of the cement hydration reaction. It is worth noting that the dispersion of 28 d strength data is relatively high (standard deviation 1.58 MPa), and the strength difference between individual specimens such as No. 1 (47.48 MPa) and No. 9 (42.95 MPa) is up to 10.5%, which suggests that the uniformity of the distribution of the ceramic aggregate or the strength of the interfacial transition zone may have local fluctuations, which requires further optimization of the proportioning process.

From the individual performance of the specimen, the overall early strength is relatively stable, the coefficient of variation is only 2.7%, but the 3 d strength of No. 5 (29.14 MPa) and No. 6 (29.83 MPa) is significantly lower than the average value, which may be related to the compactness of the specimen molding or the local defects. In the 7 d strength data, No. 7 (35.87 MPa) and No. 9 (35.93 MPa) are outstanding, which are different from No. 3 (33.13 MPa) and No. 9 (35.93 MPa). In the 7 d strength data, No. 7 (35.87 MPa) and No. 9 (35.93 MPa) formed an obvious contrast with No. 3 (33.13 MPa), with a difference rate of 8.4%, which indicated that the condition of maintenance or the water-absorbing property of ceramic grains may have a key influence on the medium-term strength development. The 28 d strength did not appear in the ultra-high performance characteristics (both lower than 50 MPa), although it met the engineering requirements in general.

4.3.2. Damage Pattern of Axial Compressive Strength Test

The main damage patterns in the axial compressive strength tests are shown in Figure 8.

In the axial compressive strength test, significant brittle damage occurred. With the continuous application of pressure, the stresses inside the fly ash vitrified concrete gradually accumulated. Accompanied by some sound of concrete crumbling, hairline cracks appeared on the surface of the specimen. Due to the porous structure of fly ash ceramic concrete, there are a large number of microcracks and pores inside it, and these microcracks will gradually expand and connect under pressure, forming macroscopic cracks to form vertical cracks from top to bottom. The crack path is affected by the distribution of ceramic grains showing irregular bifurcation, and when the pressure reaches a certain level, these cracks will expand rapidly, leading to the sudden destruction of concrete. Eventually, sudden brittle fracture occurs under peak load, and the damage surface may show a composite morphology of a conical crush zone and longitudinal splitting. The brittle nature of fly ash ceramic concrete makes it difficult to undergo significant plastic deformation during compression, but rather sudden damage occurs when the stress reaches a certain value. In addition, the strength of fly ash ceramic concrete is relatively low, and the ceramic grains inside it are prone to breakage during compression, which exacerbates the damage of the concrete.

4.3.3. Sensitivity Analysis of Axial Compressive Strength by Factors

For the new single-tube rotary kiln ceramic concrete axial compressive strength further extreme difference analysis, the results are shown in

Table 12. Axial compressive strength of ceramic concrete from a new single-cylinder rotary kiln.

Factor	3 d Axial Compressive Strength/MPa			7 d Axial Compressive Strength/MPa			28 d Axial Compressive Strength/MPa
	Water-Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water-Cement Ratio	Dosage of Ceramic Granule	Cement Dosage	Water-Cement Ratio	Pottery Granule Mixing Amount	Cement Dosage
K1	92.58	93.38	90.96	100.33	105.12	102.29	136.83	137.78	135.61
K2	90.79	91.48	91.35	105.52	100.9	103.18	132.64	130.70	134.27
K3	93	91.51	94.06	104.88	104.71	105.26	131.95	133.04	131.54
k1	30.86	31.12	30.32	33.44	35.04	34.10	45.61	45.93	45.20
k2	30.26	30.49	30.49	35.17	33.63	34.39	44.21	43.57	44.76
k3	31.00	30.50	31.35	34.96	34.90	35.09	43.98	44.35	43.85
R	0.74	0.63	1.03	1.73	1.41	0.99	1.63	2.36	1.35
R Comparison		R3 > R1 > R2			R1 > R2 > R3			R2 > R1 > R3

.

According to the results of the polar analysis of Table 12, the sensitivity of the axial compressive strength of new single-cylinder rotary kiln ceramic concrete varies significantly with the age of curing. At the age of 3 d, the sensitivity ranking of each factor is R3 > R1 > R2 (the extreme deviation is 1.03, 0.74, and 0.63, respectively), which indicates that the early strength is mainly dominated by the material proportion, and the role of environmental factors is weak. By the age of 7 d, the sensitivity ranking changed to R1 > R2 > R3 (extreme difference of 1.73, 1.41, and 0.99), and the influence of factor 1 was significantly enhanced, probably due to the accelerated period of cement hydration, and the contribution of its dosage difference to the strength accumulation was more prominent. At this time, the sensitivity of factor 2 was elevated to the second place, reflecting that the moderating effect of maintenance conditions in the middle period gradually appeared.

By the age of 28 d, the sensitivity ranking further evolved to R2 > R1 > R3 (extreme difference of 2.36, 1.63, and 1.35), indicating that factor 2 became the dominant factor for long-term strength. This is due to the fact that the late strength development of concrete relies on the continuous hydration reaction, and environmental conditions such as humidity and temperature directly affect the stability of the hydration process. In contrast, the weakened effects of factors 1 and 3 indicate that the initial differences in material proportioning gradually converge with age, while the long-term effects of curing conditions are more critical. In summary, the sensitivity of axial compressive strength shows a dynamic change law: early on, it is dominated by the material proportion; middle cement hydration highlights the role of cement hydration; and late curing conditions become the core influencing factors, and this law provides a theoretical basis for the optimization of the ceramic concrete preparation process.

4.4. Modulus of Elasticity and Poisson’s Ratio

As shown in

Table 13. Modulus of elasticity and axial compressive strength of a new single-cylinder rotary kiln vitrified concrete.

No.	Axial Compressive Strength /MPa	Modulus of Elasticity × 10−4/MPa	Apparent Density kg/m3	Poisson’s Ratio	Calculated Value × 10−4/MPa
1	47.48	2.18	1866.47	0.204	2.69
2	43.13	1.93	1864.55	0.217	2.57
3	46.22	2.14	1859.66	0.198	2.66
4	45.46	2.06	1911.45	0.234	2.69
5	43.31	1.96	1879.63	0.225	2.60
6	43.87	2.06	1922.15	0.211	2.68
7	44.74	2.04	1889.69	0.236	2.62
8	44.26	2.01	1903.34	0.233	2.66
9	42.95	1.98	1915.77	0.219	2.67

, the modulus of elasticity of the new single-tube rotary kiln vitrified concrete shows a significant positive correlation with the axial compressive strength, but the correlation is affected by the material proportion and compactness. The specimens with higher axial compressive strength (e.g., Group 1: 47.48 MPa) corresponded to higher modulus of elasticity test values (2.18 × 10⁴ MPa), whereas the specimens with lower strength (e.g., Group 9: 42.95 MPa) also had lower modulus of elasticity test values (1.98 × 10⁴ MPa). This trend suggests that compressive strength enhancement can indirectly increase material stiffness by enhancing matrix compactness and interfacial bonding. However, the effect of fluctuating apparent density (1864.55~1922.15 kg/m³) on the elastic modulus needs to be further explored. For example, Group 6, with the highest apparent density (1922.15 kg/m³), did not significantly outperform the other groups in terms of axial compressive strength (43.87 MPa) and modulus of elasticity (2.06 × 10⁴ MPa), which may be due to the interfacial defects resulting from too high ceramic granule admixture that counteracted the densification advantage.

As can be seen from Table 13, the measured value of Poisson’s ratio ranges from 0.198~0.236, with an average value of 0.220, which is slightly higher than that of ordinary concrete (usually 0.1~0.2), indicating that the material has enhanced lateral deformation capacity when axially pressurized. Among them, the No. 7 specimen has the highest Poisson’s ratio, with a difference of 19.2% from the lowest value, which may be related to the uneven distribution of ceramic grains inside the specimen or the randomness of microscopic defects in the interfacial transition zone (ITZ). It is worth noting that the Poisson’s ratios of numbers 4, 7, and 8 are all more than 0.23, and their corresponding modulus of elasticity (2.06 × 10⁻⁴~2.04 × 10⁻⁴ MPa) is relatively low, which reflects the negative correlation characteristics between the material stiffness and lateral deformation capacity, which is in line with the flexibility effect introduced by porous ceramic grains.

The Poisson’s ratio statistics measured for the test blocks are shown in Figure 9. From the distribution characteristics of Figure 9, the Poisson’s ratio data are concentrated in the interval of 0.20~0.24 (accounting for 77.8%), which is in line with the law of normal distribution, but No. 3 and No. 7 are significant outliers. Further analysis reveals that the higher modulus of elasticity of specimen No. 3 may be due to its denser ceramic-matrix interface bonding, which suppressed the transverse strain, while specimen No. 7 showed a combination of low modulus and high Poisson’s ratio, implying that its internal microcracks expanded at the beginning of the loading period, which resulted in the intensification of the transverse deformation.

4.5. Relationship Between Cube Compressive Strength and Axial Compressive Strength

Based on the theory of constraint effect, the difference between cubic and axial compressive strength of vitrified concrete originates from the gradient decay phenomenon of end-face friction constraint. According to the article 4.1.3 of GB 50010-2010 [34], the strength conversion factor of ordinary concrete takes the base value of 0.76, while the factor of vitrified concrete is raised to the interval of 0.81~0.86 (Δ = 6.5%~13.2%), which is directly related to its brittle fracture characteristics: when the Poisson’s ratio is 0.18~0.21, the friction constraint contribution rate of the bearing plate η decreases to 61%, resulting in a significant weakening of the shear expansion effect (crack extension angle θ decreases from 45° to 32°).

As shown in

Table 14. Existing vitrified concrete f_c and f_cu ratios.

Literature	Aggregate Variety	Concrete Grade	fc/fcu	Mean Value
Chen Yan [35]	Spherical shale ceramic granule	LC40	0.940	0.940
Shaanxi Construction Research Institute [36]	Fly ash ceramic granule	LC20 LC30 LC50	0.866 0.972 0.956	0.938
Liu Hanyong [37]	Rounded shale pellets Shale ceramic granule	LC50	0.900	0.900
Tianjin Light Aggregate Research Group [36]	Fly ash Ceramic granule	LC20 LC30 LC40	0.932 0.943 0.907	0.911
Structural Institute of China Academy of Building Research [36]	Shale Ceramic Granules Clay	LC10 LC30	0.912 0.866	0.889
Niu Jianguang [38]	Fly ash Ceramic granule	LC30~LC35	0.804	0.804
Shanghai Construction Research Institute [36]	Clay granule Shale Ceramic Granule	LC30 LC15	0.897 1.006	0.952
Fan Zhiyong [39]	Pumice	LC30 LC35 LC40	0.880	0.880
Yang Ying [40]	Ceramic concrete	LC20 LC30 LC40	0.865 0.831 0.793	0.825

, some scholars in China have studied the ratio of axial compressive strength to cubic compressive strength of vitrified concrete, but a unified calculation ratio has not yet been developed.

Substituting the measured data of cube compressive strength into the empirical model in Table 14, the theoretical axial compressive strength values were calculated and compared with the experimental results, as shown in

Table 15. Theoretical values based on empirical models.

	Chen Yan	Structural Institute of China Academy of Building Research	Tianjin Lightweight Aggregate Research Group	Fan Zhiyong	Shanghai Institute of Building Research	Ying Yang	Shaanxi Provincial Institute of Building Research	Liu Hanyong	Niu Jiangang
1	47.864	45.268	46.388	44.810	48.479	42.009	47.610	45.828	40.940
2	43.851	41.472	42.498	41.052	44.411	38.486	43.618	41.985	37.507
3	46.972	44.423	45.523	43.974	47.571	41.225	46.722	44.973	40.176
4	45.632	43.156	44.224	42.720	46.215	40.050	45.390	43.691	39.030
5	44.114	41.721	42.753	41.298	44.677	38.717	43.880	42.237	37.732
6	44.739	42.312	43.359	41.884	45.310	39.266	44.501	42.836	38.266
7	44.293	41.890	42.926	41.466	44.858	38.874	44.057	42.408	37.884
8	45.007	42.565	43.619	42.134	45.582	39.501	44.768	43.092	38.496
9	44.614	42.194	43.238	41.7667	45.184	39.156	44.377	42.719	38.159
Mean value	45.232	42.778	43.836	42.345	45.810	39.698	44.991	43.308	38.688

and

Table 16. Ratio of theoretical to measured values based on empirical modeling.

	Chen Yan	Structural Institute of China Academy of Building Research	Tianjin Lightweight Aggregate Research Group	Fan Zhiyong	Shanghai Institute of Building Research	Ying Yang	Shaanxi Provincial Institute of Building Research	Liu Hanyong	Niu Jiangang
1	1.008	0.953	0.977	0.943	1.021	0.885	1.003	0.965	0.953
2	1.016	0.961	0.985	0.951	1.030	0.892	1.011	0.973	0.962
3	1.016	0.961	0.985	0.951	1.029	0.892	1.011	0.973	0.961
4	1.003	0.949	0.973	0.940	1.017	0.881	0.998	0.961	0.961
5	1.018	0.963	0.987	0.954	1.032	0.894	1.013	0.975	0.963
6	1.019	0.964	0.988	0.955	1.033	0.895	1.014	0.976	0.964
7	0.990	0.936	0.959	0.927	1.002	0.869	0.985	0.947	0.936
8	1.017	0.962	0.985	0.952	1.030	0.892	1.011	0.974	0.962
9	1.039	0.982	1.01	0.972	1.052	0.912	1.033	0.995	0.982
Mean value	1.014	0.959	0.983	1.067	1.027	0.890	1.009	0.971	0.959
Variance	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001

.

The tabulated data show that there is a stable linear relationship between the axial compressive strength f_c and the cubic compressive strength f_cu of vitrified concrete, with the ratio (f_c/f_cu) usually fluctuating between 0.80 and 0.95, depending on the material and process. Spherical shale aggregates perform best in high-strength concrete (e.g., LC40~LC50), with the ratio stabilized at 0.90~0.94, which is attributed to the smooth surface that reduces the interfacial stress concentration; fly ash aggregates can reach the ratio of 0.90~0.94 in medium-strength concrete (LC30~LC40), but the ratio decreases to 0.86~0.90 after the strength is increased to LC50. Pumice aggregate always keeps the ratio around 0.88 due to its internal porous structure that absorbs energy, which is suitable for scenarios that require high lightweight and impact resistance.

The theoretical prediction accuracy of the existing ceramic concrete empirical model is high, but there are some differences: the experimental data show a high similarity with the Chenyan model (theoretical average value of 45.232 MPa, measured average value of 44.602 MPa); the error is only +1.41%, due to the use of rounded shale ceramic particles similar to the experimental shape of the ceramic particles and the optimization of interfacial stress calculations, especially for LC40~LC50 high-strength concrete (e.g., LC50 high-strength concrete), this model is particularly suitable for LC40~LC50 high-strength ceramsite concrete (e.g., the error of Test 1 is only +0.81%). For the Shaanxi Provincial Institute of Construction Science and Research Model (theoretical mean value: 43.308 MPa), the calculated error is −2.90%, and it exhibits better applicability to fly ash ceramsite scenarios. The Shanghai CRRI model (theoretical mean value 45.810 MPa) systematically overestimated the theoretical value by +2.71% (e.g., Test 6 overestimated by 3.3%). The Fan Zhiyong model (theoretical mean value 42.345 MPa) underestimates the strength by −5.06% (test 9 underestimates by 2.8%). However, due to the different applicable materials, the empirical formula is now proposed for the axial compressive strength and cubic compressive strength of ceramic concrete for the new single-tube rotary kiln:

$$f_c=0.985f_{cu}-2.79$$

$$f_c=0.927f_{cu}$$

In the formula:

$f_c$—compressive strength of vitrified concrete prismatic specimen, MPa;

$f_{cu}$—compressive strength of ceramic concrete cube specimen, MPa.

The linear relationship of the regression curve is good, R² = 0.86.

4.6. Comparison of the Mechanical Properties of the Two Types of Concrete

The basic mix proportion was determined based on the optimal performance ranges of new-type single-cylinder rotary kiln ceramsite concrete and ordinary fly ash ceramsite concrete. Specimens were produced using the same mix proportion with ceramsite selected within a particle size range of 5~15 mm. The reference mix proportion of the concrete is shown in

Table 17. Basic mix ratio.

Sand/ (kg m−3)	Water/ (kg m−3)	Cement/(kg m−3)	Ceramic Granule/ (kg m−3)	Water-Reducing Agent/ (kg m−3)	Water-Cement Ratio
627	163	480	750	12	0.34

. Compressive strength and splitting tensile strength tests were conducted on cubic specimens at 3 d, 7 d, and 28 d.

The experimental results are shown in

Table 18. Comparison of mechanical properties.

Type	Compressive Strength/MPa			Split Tensile Strength/MPa			Mix Working Performance
	3 d	7 d	28 d	3 d	7 d	28 d
Ordinary fly ash ceramic granule	21.6	25.4	34.6	1.58	2.1	2.8	Better
New type single-cylinder rotary kiln ceramic granule	27.1	35.6	46.2	1.68	2.4	3.6	good

. The new single-drum rotary kiln ceramic concrete shows significant mechanical property advantages compared with ordinary fly ash ceramic concrete. In terms of compressive strength, the new ceramic granules reached 27.1 MPa, 35.6 MPa, and 46.2 MPa at the ages of 3 d, 7 d, and 28 d, respectively, which was 25.5~35.5% higher than that of the ordinary ceramic granules (21.6 MPa, 25.4 MPa, and 34.6 MPa). This difference stems from the optimized water-cement ratio and cement dosing of the new ceramic granules, which accelerates the early hydration reaction and enhances the matrix densification, while its single-cylinder rotary kiln process may improve the surface roughness of the ceramic granules and strengthen the bonding efficiency of the aggregate-slurry interfacial transition zone (ITZ). Regarding the split tensile strength, the 28 d strength of the new ceramic granules reached 3.6 MPa, which was 28.6% higher than that of the ordinary ceramic granules (2.8 MPa), indicating that their interfacial stress transfer capability was significantly enhanced. Compared with ordinary fly ash ceramsite concrete, the compressive and tensile strength growth rate of the new single-cylinder rotary kiln ceramsite concrete is greater; especially after 7 days, the performance improvement is accelerated, reflecting the synergistic effect of material ratio and process optimization. In general, the new ceramsite concrete has more potential for engineering application in the balance between lightweight and mechanical properties.

4.7. XRD Test and Analysis

According to the XRD pattern analysis, Figure 10 shows that the diffraction peaks appearing at angles of 11.6°, 20.1°, 21.2°, and 23.2° in the common fly ash ceramic grain samples can be attributed to the typical mineral phases in the fly ash ceramic grains: quartz (SiO₂, 2θ ≈ 26.6°), mullite (Al₆Si₂O₁₃, 2θ ≈ 16.4°, 21.0°), calcium feldspar (CaAl₂Si₂O₈, 2θ ≈ 22.0°, 27.5°), and hematite (Fe₂O₃, 2θ ≈ 33.2°). Among them, the peaks near 20.1° and 27° may correspond to the characteristic peaks of calcium feldspar (CaAl₂Si₂O₈) and quartz (SiO₂), respectively, while the peaks at 29.6° and 50.5° may be associated with hematite (Fe₂O₃) or spinel phases (e.g., FeAl₂O₄). The multiple peaks of the common fly ash pellet samples indicate that the composition of the crystalline phases is complex, including incompletely reacted raw material phases (e.g., quartz) and high-temperature stabilized phases, such as mullite and calcium feldspar, formed during sintering, but interfacial defects between the crystalline phases may be present, which leads to a loose structure. The XRD patterns of the new single-tube rotary kiln ceramic grain samples show that the peaks near 11.7° are significantly enhanced (probably related to the broadening peaks superimposed on the amorphous glass phase), while the peaks of the crystalline phases such as 20.1°, 21.2°, and 50.5° are significantly weakened or even disappear. This indicates that the proportion of amorphous glass phases in the new single-tube rotary kiln ceramic pellet samples increased significantly, and the crystalline phases (e.g., quartz, calcium feldspar) partially melted and participated in the formation of glass phases at higher sintering temperatures or better ratios. The increase in the glassy phase facilitates the sintering of the liquid phase between particles and the formation of a dense and continuous network structure, thus enhancing the mechanical properties. Specifically, the disappearance of the quartz peak (2θ ≈ 26.6° corresponds to the weakening of the peak at 27°) indicates that SiO₂ reacts with Al₂O₃, CaO and other components to form a low melting point aluminosilicate glassy body at high temperatures, which facilitates liquid-phase sintering. Meanwhile, the reduction in the hematite phase (e.g., disappearance of the 50.5° peak) may indicate the partial incorporation of Fe³⁺ into the glassy phase or the formation of a more stable spinel structure with the aluminosilicate, further optimizing the interfacial bonding.

From the performance correlation analysis, the mechanism of the improved barrel compressive strength and reduced water absorption of the new single-tube rotary kiln ceramic grain samples can be attributed to the following two points: (1) the increase in the glass phase content caused the internal pores of the ceramic grains to be filled by the high-viscosity liquid phase, which resulted in a reduction of the porosity (reduced water absorption), and at the same time, the liquid phase bridged the grains to form a more homogeneous dense structure (increased barrel compressive strength); (2) the reduction of the crystalline phase lowered the concentration of stresses at the grain boundaries. The residual high-temperature phases, such as mullite (e.g., near 11.7° may correspond to a weak peak of mullite), can still provide skeletal support, while the continuity of the glassy phase enhances the overall compressive capacity. In contrast, unreacted free quartz (high hardness but brittle) and calcium feldspar (difference in coefficients of thermal expansion) in ordinary fly ash ceramic grain samples may lead to microscopic crack extension and weaken the mechanical properties. Therefore, the new single-tube rotary kiln ceramic grain samples achieved synergistic enhancement of the crystalline phase and glass phase by optimizing the sintering process, which is in line with the structural design principle of high-performance ceramic grains.

4.8. CT Test and Analysis

The test is shown in Figure 11. In order to make the results of scanning more clear, the cubic specimen is made.

From the pore volume data in

Table 19. Pore volume.

No.	Pore Volume
	Maximum Value/(mm3)	Minimum Value/(mm3)	Average Value/ (mm3)	Total Volum/(mm3)	Percentage of Total Volume
Ordinary fly ash	58.25	0.0025	0.092	2736.67	0.00760458
Single-drum rotary kiln	188.34	0.0035	0.25	2297.51	0.00642272

and the comparison of 3D pore structure in Figure 11a,b, it can be seen that there is a significant difference in pore morphology and distribution between ordinary ceramic concrete and new ceramic concrete. The total pore volume percentage of ordinary fly ash vitrified concrete is 0.0076, and its maximum value of pore is much lower than that of new vitrified concrete, but the minimum value is close to that of new material. This indicates that the pore size distribution of ordinary concrete is more concentrated, while the pore size of the new material spans a larger range, and some large-size pores may exist. Combined with the 3D pore images, the pore distribution of plain concrete may be denser and more uniform, whereas the novel concrete has fewer but larger sized pores and slightly lower overall porosity. This difference may stem from the optimization of the preparation process; for example, the single-cylinder rotary kiln process improves the densification of the ceramic grains through high-temperature sintering, which reduces the generation of tiny pores, but a small number of large pores may be formed locally.

The crack volume data in

Table 20. Crack Volume.

No.	Crack Volume
	Maximum Value/(mm3)	Minimum Value/(mm3)	Average Value/ (mm3)	Total Volume/(mm3)	Percentage of Total Volume
Ordinary fly ash	22.46	0.010	0.276	3271.72	0.00909134
Single-drum rotary kiln	30.42	0.010	0.44	1171.97	0.00327629

and the three-dimensional fracture structure in Figure 11c,d show that the difference in fracture development characteristics between the two types of concrete is more significant. The total crack volume percentage of ordinary fly ash ceramic concrete is as high as 0.0091, while that of the new concrete is only 0.0033, which is a decrease of 63.8%. From the extreme value, the mean value of cracks in ordinary concrete is significantly lower than the latter, indicating that the number of cracks in ordinary concrete is large and dominated by small scale, while the number of cracks in the new material is small, but a small number of large-scale cracks exist. Combined with the 3D fracture images, the fracture network of ordinary concrete is more complex, and penetrating microcracks may be formed, while the fracture distribution of new concrete is sparse, and the large-scale cracks are mostly isolated.

From the 3D visualization results, the microstructural differences between ordinary fly ash ceramsite concrete and the new-type ceramsite concrete exhibit a remarkable synergistic characteristic in terms of “morphology-quantity-distribution”:

Regarding cracks (Figure 11c,d), the cracks in ordinary concrete show a “network-like penetration” feature. Fine cracks are densely interwoven, and their total volume fraction reaches 0.0091, which is consistent with the statistical results of “low average value and large quantity” in Table 19, suggesting that there are more stress concentration points inside it, which may be a key factor leading to its relatively weak macroscopic crack resistance. In contrast, the cracks in the new-type concrete are mainly “isolated and large-sized”, with a total volume fraction of only 0.0033. The characteristic of sparse distribution directly reflects the crack-resistance advantage achieved by the single-cylinder rotary kiln process in optimizing the interfacial transition zone of ceramsite through high-temperature sintering.

In terms of pore structure (Figure 11a,b), the pores in ordinary concrete present the characteristics of “small size, high density, and uniform distribution” (Table 18, with a maximum value of 58.25 mm³ and a total volume fraction of 0.0076). Although such a structure may improve the water permeability of the material, an excessive number of tiny pores are likely to become weak points in strength. The pores in the new-type concrete, however, show the characteristics of “large size, low frequency, and discrete distribution” (with a maximum value of 188.34 mm³ and a total volume fraction of 0.0064). It can be seen that this pore morphology not only reduces the weakening effect on the overall strength but also retains a certain pore functionality through local large pores, reflecting the “strength-porosity” balance mechanism.

5. Conclusions

This paper takes the new single-cylinder rotary kiln fly ash ceramic concrete as the research object, systematically explores the influence of law and micro-mechanism on its mechanical properties, and combines orthogonal experimental design, macro-mechanical testing, and microscopic characterization techniques to reveal the optimization path of material properties. The main conclusions are as follows:

(1) The amount of ceramic grain doping is the dominant factor affecting the compressive strength and axial compressive strength; excessive ceramic grains (780 kg/m³) caused a weak interface due to insufficient slurry package, and the strength decreased significantly; the water-cement ratio has a significant effect on the early compressive strength (3 d, 7 d), and the low water-cement ratio is better than the high water-cement ratio; the extreme difference in cement doping is the smallest, which indicates that the adjustment of the cement dosage has a limited effect. The split tensile strength increased nonlinearly with age, and the 28 d strength reached 3.96 MPa. The synergistic optimization of ceramic granule admixture and water-cement ratio can significantly improve the densification of the interfacial transition zone (ITZ) and inhibit the expansion of microcracks.

(2) The mean value of the modulus of elasticity of the new vitrified concrete is 2.04 × 10⁻⁴ MPa, which is about 40% lower than the predicted value of the existing specification formulas (e.g., GB 50010-2010), and the discrepancy stems from the fact that the computational model does not adequately reflect the effect of microcracks or non-homogeneity at the vitrified-matrix interface. The average value of Poisson’s ratio is 0.220, which is higher than that of ordinary concrete, indicating that the material has enhanced lateral deformation capacity, which is related to the porous characteristics of the ceramic grains and the random distribution of interfacial micro-defects.

(3) The compressive strengths of the new vitrified concrete at 3, 7, and 28 days were 27.1 MPa, 35.6 MPa, and 46.2 MPa, respectively, which were significantly higher than those of ordinary vitrified concrete at 21.6 MPa, 25.4 MPa and 34.6 MPa, with an increase of 25.5%, 40.2%, and 33.5%, respectively. The splitting tensile strengths of the new vitrified concrete at 3, 7, and 28 days were 1.68 MPa, 2.4 MPa, and 3.6 MPa, respectively, which were better than those of ordinary vitrified concrete of 1.58 MPa, 2.1 MPa, and 2.8 MPa; especially at 28 days, the splitting tensile strength of the new vitrified concrete was 28.6% higher.

(4) XRD test results show that the new single-barrel rotary kiln ceramsite sample achieves the synergistic enhancement of the crystalline phase and glass phase by optimizing the sintering process, which is in line with the structural design principle of high-performance ceramsite. The CT scan showed that the internal crack volume rate of the new vitrified concrete was 0.328%, which was significantly lower than that of ordinary vitrified concrete, which was 0.909%. The new vitrified concrete has lower porosity and water absorption, and the internal structure is more dense, which reduces the formation of microcracks.