Experimental Study on the Properties and Hydration Mechanism of Gypsum-Based Composite Cementitious Materials

Abstract

In order to achieve the resourceful, large-scale and high-value utilization of bulk industrial solid wastes such as flue gas desulfurization gypsum (FGDG), fly ash (FA) and ground blast furnace slag (GGBS), and to reduce the dosage of cementitious materials, orthogonal experimental methods were used to prepare composite cementitious materials based on the principle of synergistic coupling and reconstruction of multi-solid wastes. Through the method of extreme difference and ANOVA, the influence law of different factor levels on the performance of the cementitious materials was studied, and the maximum compressive strength of cementitious materials was reached when the ordinary Portland cement (OPC) dosage was 20%, the FGDG dosage was 56%, the FA dosage was 19.2% and the slag dosage was 4.8%, and the W/B was 0.55. The hydration products and microscopic morphology of the cementitious materials were analyzed by means of XRD, SEM and MIP techniques, so as to elucidate the complex synergistic hydration mechanism, and then to determine the more optimal group distribution ratio. The results show that the hydration reaction between FGDG and OPC can be synergistic with each other, and C-A-H further generates AFt under the action of SO₄²−, and at the same time, it plays the role of alkali-salt joint excitation for FA–GGBS, generates a large amount of cementitious materials, fills up the pores of the gypsum crystal structure, and forms a dense microstructure.

1. Introduction

Currently, Ordinary Portland Cement (OPC) is widely used in the construction industry as a cementitious material with good properties. However, China is the world’s largest developing country, and as OPC production rises it leads to a continued increase in carbon emissions from the cement industry [1,2]. Data show [3,4] that CO₂ emissions from OPC production account for about 15% of the country’s total carbon emissions, with each tonne of cement produced releasing 0.7 tonnes of CO₂, second only to high carbon-emitting industries such as power and steel. In addition, a large amount of energy and resources are consumed, and 92.6 Mt of SO₂, 878 Mt of NO_X and 72.9 Mt of PM [3,4], which leads to climate warming, environmental pollution and human health risks. Therefore, finding ways to reduce the carbon emissions of the cement industry and realize a carbon peak and carbon neutral transformation path have attracted widespread attention.

At the present stage, China produces a huge amount—and wide variety—of industrial solid waste [5,6,7,8], mainly including iron tailings (IOTs), ground granulated blast furnace slag (GGBS), fly ash (FA) and steel slag (SS), etc., with a complex composition. Treatment is quite difficult, the comprehensive utilization rate is generally low, and the main treatment is mainly by piling up, which not only encroaches on a large amount of land resources, but also causes great harm to the natural ecological environment and the social and economic environment. As an alternative to conventional cement, the preparation of alkali-inspired cementitious materials (AAMs) [9,10,11] and supplementary cementitious materials (SCMs) [12,13,14,15,16,17,18] from bulk industrial solid wastes has become a focus of research. They have lower environmental pollution, energy consumption and production costs compared to OPC. With the in-depth study of AAMs and SCMs, some industrial solid wastes with silica–aluminum-based components have potential pozzolanic activity, but the lower activity limits their widespread use [19]. Numerous studies have shown that the reactivity of precursors can be improved using appropriate methods. Luciano et al. [12] activated IOTs by heat treatment, which increased their pozzolanic activity index and optimized the pore structure of the cement paste. Yao et al. [20] investigated the effect of different mechanical activation times on tailings and showed that mechanical grinding reduced the relative crystallinity of the mineral phases in tailings. Wang et al. [21] reduced the crystallinity and surface binding energy of iron tailings by wet milling, which improved the reactivity and facilitated the hydration of reactive silica–aluminum phases in SS by MIOT participation, and improved the mechanical properties of the slurry. On the other hand, excitation using chemical activators [22,23,24,25,26] is also an effective means to induce chemical bond rupture by dissolving and corroding the internal crystal structure, and reactive SiO₂ and Al₂O₃ are effectively released, resulting in the generation of a large amount of calcium silicate hydrate (C-S-H) gel and ettringite (AFt), which is in agreement with the findings of the studies of references [10,24,25,27]. Although the above activation modes changed the gelling properties of the precursors, they were not favorable for large-scale use. In addition, a new mechanism of action has been proposed, and the use of multi-solid waste synergistic coupling can improve the reactivity and obtain good performance.

Flue gas desulfurization gypsum (FGDG) is an industrial by-product generated from the treatment of pulverized coal combustion flue gas in power plants [28], which is characterized by low strength and poor water resistance, and is mainly used as a cement retarder, gypsum building material products, and to improve saline and alkaline soils, with low added value [29]. Duan et al. [30] investigated the effect of substitution of natural gypsum by FGDG as a cement retarder on the workability, compressive strength and durability properties of cement. Wang et al. [31] investigated the effect of FGDG on slag geopolymer, and the results showed that the incorporation of FGDG can combine with activated aluminates to produce AFt, fill the pores of the internal structure, and improve the compressive strength. It has been shown [32] that the incorporation of OPC and volcanic ash materials in FGDG can generate a large number of C-S-H gel and AFt to improve the mechanical properties and water resistance of FGDG.

In order to study the influence of multi-solid waste coupling on gypsum-based composite cementitious materials, this paper takes FGDG, FA and GGBS as solid waste cementitious materials and OPC as active cementitious material, analyses them by orthogonal experimental method and optimizes the design of the proportion. XRD, SEM and MIP were used to investigate the hydration products, micro-morphology, pore structure and synergistic hydration reaction mechanism.

2. Materials and Experiments

2.1. Raw Materials

Ordinary Portland cement: P.O 42.5 cement produced by Dalian Onoda Cement Co. (Dalian, China); Desulphurisation gypsum: construction desulphurisation gypsum with light yellow color provided by Liaoning Xinmeijia Building and Decoration Materials Co. (Shenyang, China); Fly ash: provided by Yatai Group Shenyang Building Materials Co. (Shenyang, China); Ground granulated blast furnace slag: S95 slag provided by Liaoning Anshan Iron and Steel Group (Anshan, China). With reference to the Chinese standard GB/T 14506.28-2010 [33], the chemical composition of the raw materials was determined using X-ray fluorescence spectrometry (XRF), and the results are shown in

Table 1. Main chemical components of raw materials (wt%).

	SiO2	Al2O3	CaO	MgO	Fe2O3	K2O	Na2O	SO3	TiO2
OPC	25.53	9.01	54.01	3.29	3.36
DG	2.24	0.55	48.476	0.71	0.462			46.481	0.033
FA	56.2	18.67	9.89	2.12	6.08	1.23		1.15	2.95
GGBS	33.98	15.22	36.91	9.27	0.62	0.41	0.39	1.81

. The particle size distribution (PSD) of the raw material was tested using a laser particle size analyzer with reference to the Chinese standard GB/T 19077-2016 [34], and the results are shown in Figure 1. With reference to the Chinese standards JY/T 0587-2020 [35] and JY/T 0584-2020 [36], the mineral phase composition and micro-morphology of the raw materials were examined using X-ray diffractometer (XRD) and scanning electron microscope (SEM), and the results are shown in Figure 2.

Figure 2 shows that GGBS has a distinct hump, which indicates that it has a large amount of amorphous phases SiO₂ and Al₂O₃, whereas FA has a large amount of quartz and mullite, which makes them potentially water-hard, which is necessary for the preparation of gelling materials. In addition, FGDG particles, on the other hand, are prismatic dominated by monoclinic crystal system and have a more rough-hewn surface, whereas FA exists mostly in spherical structure [37], which not only acts as a roller ball to improve the fluidity and water retention of the gelling material [38], but also fills up the pores of the sample matrix [39,40].

2.2. Test Programme

2.2.1. Mixing Ratio Design

In this paper, based on the results of previous studies, a gypsum-based composite cementitious material system was prepared with two parts: solid waste adhesive materials (FGDG, FA, GGBS) and active coagulation materials (OPC). On the one hand, FGDG and OPC can be used as cementitious materials and at the same time can provide sulphate and an alkaline environment for the hydration of FA and GGBS, which is conducive to the generation of a large number of hydration products. On the other hand, FA and GGBS can be used as active admixtures to promote the secondary hydration of cement while improving the flow properties of the composite slurry. Based on the resource utilization of bulk industrial solid waste and the improvement of desulfurization gypsum performance, the content of solid waste cementitious material in the control system is not less than 70%, and the content of gypsum in solid waste cementitious material is not less than 50%. Combined with the previous test results, the L4 (4⁴) orthogonal test was designed, in which W/B is the ratio of water and cementitious material to investigate the influence of different components on the performance of cementitious material. The design of test factor level is shown in

Table 2. Factor level table of orthogonal test for composite cementitious materials.

	Factors	A	B	C	D
Levels		OPC/(FGDG + FA + GGBS)	FGDG/(FA + GGBS)	FA/GGBS	W/B
1		15:85	50:50	1:1	0.55
2		20:80	60:40	2:1	0.6
3		25:75	70:30	3:1	0.65
4		30:70	80:20	4:1	0.7

, and the specific test method is shown in

Table 3. Orthogonal test ratios for composite cementitious materials.

Number	OPC (%)	FGDG (%)	FA (%)	GGBS (%)	W/B
S1	15	42.5	21.3	21.2	0.55
S2	15	51	22.7	11.3	0.6
S3	15	59.5	19.1	6.34	0.65
S4	15	68	13.6	3.4	0.7
S5	20	40	26.7	13.3	0.65
S6	20	48	16	16	0.7
S7	20	56	19.2	4.8	0.55
S8	20	64	12	4	0.6
S9	25	37.5	28.1	9.4	0.7
S10	25	45	24	6	0.65
S11	25	52.5	11.3	11.2	0.6
S12	25	60	10	5	0.55
S13	30	35	28	7	0.6
S14	30	42	21	7	0.55
S15	30	49	14	7	0.7
S16	30	56	7	7	0.65

.

2.2.2. Sample Preparation

The raw materials described in Section 2.1 were dried in a drying oven at 80 °C before preparing the samples of the cementitious material. The preparation process of the composite cementitious materials was divided into three main stages: powder mixing, slurry preparation, and molding and maintenance. Firstly, weigh the mentioned mass of OPC, FA and GGBS according to Table 2 and mix them fully using cement clean slurry mixer, and add them into water for 90 s of low-speed mixing; then, weigh the corresponding mass of FGDG and add it into the slurry for 30 s of low-speed mixing, and after stopping the mixing for 15 s, mix it for 120 s of high-speed mixing, to obtain the composite cementitious material slurry. The slurries with different ratios were injected into a 40 mm × 40 mm × 160 mm long mold, vibrated 120 times by a shaker, closed the sample using cling film and demolded after 24 h of maintenance under standard conditions, and the cured samples were put into the maintenance room for maintenance under standard conditions (20 ± 2 °C, humidity ≥ 95%). The specific preparation process is shown in Figure 3.

2.3. Test Methods

2.3.1. Mechanical Performance

Mechanical properties test according to the Chinese standard GB/T 17671-2021 [41], using WDW-20 microcomputer-controlled electronic universal testing machine for all the specimens 7 d and 28 d strength test, the specimens will be placed in the center of the press under the pressure plate. The specimens were placed in the center of the lower platen of the press, and the loading rate of flexural strength was 0.5 N/s, and the loading rate of compressive strength was 2.4 N/s. The average of the flexural results of three prisms in each group was taken as the test result, and the average of the six compressive strengths obtained from a group of three prisms was taken as the test result.

2.3.2. Dry Density

Test method refers to the Chinese standard GB/T 23451-2009 [42], take three specimens as a group, put the specimen at a temperature of 50 °C drying oven drying the mass difference between the two times does not exceed 0.2% until cooled to room temperature and weigh the mass, and calculate the volume of the specimen and the dry density.

2.3.3. Water Absorption

Test method refers to the Chinese standard GB/T 23451-2009, take three specimens as a group, dry the specimens to a constant weight and weigh them, soak them in water at (20 ± 5) °C, add water to 20 mm above the sample, take them out after 2 d, use a towel to wipe off the water on the surface of the specimen and weigh them.

2.3.4. Softening Factor

Test method refers to the Chinese standard GB/T 23451-2009, take two groups of drying specimens, one group to test the compressive strength after drying, and the other group soaked in water for 48 h and then removed to test the compressive strength of the samples after water absorption.

2.3.5. X-ray Diffraction Test (XRD)

The core of the sample after the test damage in Section 2.3.1 was immersed in anhydrous ethanol to abort hydration, dried in a drying oven at 40 °C for 24 h and then ground into a powder passing through a 75 μm sieve. An X-ray diffractometer (X Pert PRO MPD, Panalytical, Almelo, The Netherlands) was used to characterize the hydration products of the samples, with a scanning angle ranging from 5~90° (2θ) and a scanning speed of 5 °/min in increments of 0.02°.

2.3.6. Scanning Electron Microscopy Test (SEM)

The microstructures of the different samples were analyzed using a scanning electron microscope (Sigma 300, Carl Zeiss AG, Oberkochen, Germany) with a resolution of 1 nm and an accelerating voltage of 3 kV. Sample pre-treatment was carried out in the same way as in Section 2.3.5, with the dried samples being cut into small pieces of 1 mm diameter and thickness.

2.3.7. Mercury Impression Test (MIP)

The microscopic pore structure characteristics of samples are closely related to their mechanical properties and durability [43], such as pore size distribution, pore volume, pore size, most probable aperture size and pore surface area. The microstructure of the samples preserved for 28 d was analyzed using an AutoPore Iv 9510 (Micromeritics, Shanghai, China) high performance fully automated mercuric piezometer. All MIP samples were taken from cores damaged by compressive testing and immersed in anhydrous ethanol to abort hydration, dried to constant weight at 40 °C, and then cut into 3~5 mm thick pieces with pore sizes ranging from 3 nm to 1000 μm and a mercury contact angle of 130°.

3. Results and Discussions

3.1. Visual Analyses

The gypsum-based composite cementitious material specimens were tested and the test results are shown in

Table 4. Experimental results of the performance of samples with different mixing ratios.

Number	Flexural Strengths (MPa)		Compressive Strengths (MPa)		Softening Coefficient (%)		Water Absorption (%)	Dry Density (kg/m3)
	7d	28d	7d	28d	Flexural	Compressive
S1	2.75	5.16	13.54	23.96	0.95	0.94	8.26	1505.2
S2	2.01	4.64	9.02	19.17	0.94	0.93	13.98	1443.9
S3	1.31	3.71	4.68	12.20	0.92	0.91	31.65	1315.0
S4	1.03	3.47	4.43	12.08	0.89	0.87	23.04	1223.2
S5	1.64	4.97	8.93	20.36	0.93	0.93	13.71	1411.5
S6	1.63	3.41	8.29	17.67	0.91	0.90	11.16	1348.9
S7	2.89	5.35	9.66	23.96	0.96	0.97	12.80	1460.5
S8	2.11	4.71	7.81	19.34	0.95	0.95	16.81	1430.8
S9	1.31	3.68	6.18	14.57	0.92	0.91	21.34	1230.1
S10	1.27	3.86	5.62	15.37	0.92	0.92	18.97	1318.5
S11	2.11	4.55	8.25	19.64	0.94	0.94	12.69	1406.3
S12	2.05	4.50	9.15	20.83	0.97	0.95	13.62	1462.2
S13	1.97	4.23	8.32	17.13	0.92	0.93	17.91	1309.7
S14	2.44	4.86	9.86	20.38	0.93	1.05	21.31	1366.8
S15	1.05	3.65	5.45	14.71	0.90	0.85	24.96	1251.6
S16	1.43	3.50	5.96	14.26	0.92	0.86	30.98	1214.2

. As can be seen from Table 4, the magnitude of the W/B plays a decisive role in the performance of the cementitious materials, which is in agreement with the findings in the literature [44,45]. With the gradual increase of the W/B, the strength, softening coefficient and adiabatic density of the specimen gradually decrease. At the same time, the water absorption rate gradually increases, which is due to the W/B being too large, and the free water exceeds the mixing water required for the hydration reaction process. The specimen contains a large number of capillary pores. The structure of such a porous hardened body for the water promotes the erosion of the gypsum dihydrate, to provide a large number of channels that destroy the combination of the crystal structure of the inter-micro-unit, thus resulting in the internal structure of the specimen becoming more porous [46]. This causes the degree of densification to decrease, which intensifies the detrimental effect on the strength. In addition, the compressive strengths of the samples all exhibited a large increase with the increase of the curing time, which was attributed to the pozzolanic activity and the filling effect of FA and GGBS particles [47]. In the early stages of hydration, the volcanic ash reaction of FA particles is slow and mainly plays the role of accumulation filling, and the hydration process is dominated by OPC and FGDG reactions [48], which is consistent with the findings of other researchers [38,49]. At the later stage of the hydration reaction, FA and GGBS particles were able to act as nucleation sites to promote the secondary hydration of OPC and FGDG [50], generating a large number of C-S-H gel and AFt, which further grew and developed in the pristine water-filled space between OPC and FGDG particles and acted as pore fillers.

In order to further elucidate the degree of influence of each factor on the performance of the gelling material, some of the experimental results were subjected to extreme variance analysis, and the results are shown in

Table 5. Polar analysis of orthogonal test results.

	Factors	k1	k2	k3	k4	R
28 d Compressive strengths (MPa)	A	16.85	20.33	17.6	16.62	3.72
	B	19.0	18.15	17.62	16.63	2.38
	C	18.88	18.77	16.62	17.13	2.26
	D	22.28	18.82	15.55	14.76	7.52
Compressive softening coefficient (%)	A	0.91	0.94	0.93	0.95	0.04
	B	0.96	0.95	0.92	0.91	0.05
	C	0.91	0.91	0.96	0.95	0.05
	D	0.98	0.96	0.9	0.88	0.1
Water absorption (%)	A	0.19	0.14	0.17	0.24	0.1
	B	0.15	0.16	0.21	0.21	0.06
	C	0.16	0.17	0.23	0.18	0.07
	D	0.14	0.15	0.24	0.2	0.1
Absolute dry density (kg/m3)	A	1384.4	1412.9	1354.3	1285.6	127.35
	B	1376.6	1369.6	1358.3	1332.6	43.99
	C	1381.2	1392.3	1335.7	1327.9	64.35
	D	1461.2	1397.7	1314.8	1263.5	197.7

and as shown in Figure 4. Specifically, the R value represents the extreme deviation of the factors, reflecting the magnitude of the change in the test indicators when the level fluctuates, and the larger the R, the greater the influence of the factor on the test indicators.

From Table 5 and Figure 4a, it can be seen that the magnitude of W/B has a more significant effect on the intensity of the samples, while the extreme values of the effects of the levels of the other factors are very close to each other, based on the magnitude of the R-value to judge the magnitude of the effect of each factor is: W/B > OPC/(FGDG + FA + GGBS) > FGDG/(FA + GGBS) > FA/GGBS. In addition, with the increase of water–cementitious ratio, the strength of the specimen decreased rapidly from 22.28 MPa to 14.76 MPa, which was due to the fact that the amount of cementitious material per unit volume was inversely proportional to the water–cementitious ratio, and the free water evaporated continuously during the maintenance of the specimen, resulting in the increase of porosity, the decrease of structural compactness, and the final strength reduction.

Table 5 and Figure 4c show the order of magnitude of the effect of each factor on the water absorption of the specimens as follows: OPC/(FGDG + FA + GGBS) > W/B > FGDG/(FA + GGBS) > FA/GGBS, in which the ratio of OPC to the cementitious material of the solid waste had a significant effect on the water absorption of the specimens, and the water absorption rate was increased from 0.14% to 0.24%, which was attributed to the fact that when too little OPC is added, less calcium hydroxide (Ca(OH)2, CH) is produced by hydration, which leads to the insufficient hydration reaction between FA and GGBS particles. The pore spacing between the crystal structures then becomes larger, which leads to the increase of water absorption of the specimen.

Each factor on the dry density of the sample is in the order of W/B > OPC/(FGDG + FA + GGBS) > FGDG/(FA + GGBS) > FA/GGBS. The size of the W/B directly determines the specimen unit volume of cementitious materials in the mixing, which in turn affects the generation of hydration products, while the excess water evaporation after the formation of internal pore structure reduces its dry density.

3.2. Analysis of Variance

Analysis of variance (ANOVA) can not only calculate the primary and secondary order of the influence of each factor on the performance of the specimen, but also derive the size of the inevitable error in the test, and it can be deduced that the differences in the test data corresponding to the different levels of the factors are caused by the experimental error or by the different levels of the factors, which makes up for the deficiencies of the extreme variance analysis method in a better way. The main influencing factors of the sample performance were further determined by ANOVA and combined with the results of polar analysis, as shown in

Table 6. ANOVA results of samples with different mixing ratios.

	Factors	Square Sum	Mean Square	F Value	Significance
28 d Compressive strengths (MPa)	A	34.928	11.643	7.718	0.064
	B	15.703	5.234	3.47	0.167
	C	11.866	3.955	2.622	0.225
	D	141.812	47.271	31.336	0.009
	Inaccuracy	4.256	1.509	/	/
	Aggregate	5307.88	/	/	/
Compressive softening coefficient (%)	A	0.003	0.001	2.694	0.114
	B	0.007	0.002	1.565	0.361
	C	0.007	0.002	1.529	0.368
	D	0.025	0.008	5.965	0.088
	Inaccuracy	0.004	0.001	/	/
	Aggregate	13.959	/	/	/
Water absorption (%)	A	0.024	0.008	16.374	0.023
	B	0.01	0.003	6.862	0.074
	C	0.012	0.004	7.891	0.062
	D	0.022	0.007	14.902	0.026
	Inaccuracy	0.001	0	/	/
	Aggregate	0.607	/	/	/
Absolute dry density (kg/m3)	A	35857.8	11952.62	6.272	0.083
	B	4471.3	1490.432	0.782	0.578
	C	12434.9	4144.984	2.175	0.27
	D	92055	30685.04	16.101	0.024
	Inaccuracy	5717.5	1905.82	/	/
	Aggregate	29712538.7	/	/	/

. In ANOVA, the larger the F-value, the smaller the significant value, indicating that the factor has a greater influence on the sample performance index. From the results of ANOVA in Table 6, it can be seen that the degree of influence of each factor on the 28 d compressive strength of the specimen in descending order is as follows: W/B > OPC/(FGDG + FA + GGBS) > FGDG/(FA + GGBS) > FA/GGBS, indicating that the size of the W/B has a significant effect on the mechanical properties of the cementitious materials, which is consistent with the results of the analysis in Section 3.1, and the main effects of the other properties are not to be repeated one by one.

3.3. Multiple Linear Regression Model Analysis

Multiple linear regression model analysis refers to a method of statistical analysis that identifies interdependent quantitative relationships between two or more variables. Based on the least squares method, the orthogonal experimental test results were linearly fitted to obtain the following multiple linear regression model for $Y_1-Y_4$:

$$Y_1=47.1-0.02X_1+0.001X_2+0.283X_3-50.602X_4$$

$$Y_2=1.476-0.002X_1+0.002X_2+0.004X_3-0.73X_4$$

$$Y_3=0.06-0.002X_1-0.002X_2-0.008X_3+0.513X_4$$

$$Y_4=1713.7+5.5X_1+4.874X_2+12.22X_3-1325.19X_4$$

where $X_1$, $X_2$, $X_3$ and $X_4$ denote the actual values of FGDG, FA, GGBS and W/B, respectively.

In order to assess the accuracy and reliability of each model, the models were tested for significance through goodness of fit (R value), overall significance (F value) and regression coefficients (t value) and the predicted values were analyzed against the experimental values, and the results are shown in

Table 7. Analysis of multiple linear regression equations for composite cementitious materials.

	Model			Model Coefficients (t Value)
	R Value	F Value	Significance	$X_1$	$X_2$	$X_3$	$X_4$
28 d Compressive strengths (MPa)	0.881	9.534	0.001	−0.024	0.007	0.366	−0.783
Compressive softening coefficient (%)	0.889	10.335	0.001	−1.54	1.059	−1.973	−5.314
Water absorption (%)	0.812	2.825	0.078	−0.623	0.667	−2.337	2.044
Absolute dry density (kg/m3)	0.93	17.624	0.001	2.735	1.844	4.487	−6.884

and Figure 5. Specifically, larger R value, F value and t value indicate more significant models. The R value and F value for the 28 d compressive strength, compressive softening coefficient, water absorption and adiabatic density models were 0.881, 0.889, 0.812, 0.93 and 9.534, 10.335, 2.825, 17.624, respectively, all p values were <0.05, and the significance of the models was not more than 0.1, which shows that each regression equation model is accurate and all are statistically significant in this study. As can be seen from Figure 5, the model predicted values and experimental values can be better matched, indicating that the model fit is high and the fitting effect is good [51]. The process of practical application can be based on the value of one or several variables, through the multiple linear regression model of gypsum-based composite cementitious materials mechanical properties and waterproof performance prediction, the raw material doping and composition of the optimization of the matching, to achieve the multi-objective dynamic regulation of the performance of the sample.

3.4. X-ray Diffraction Test (XRD)

A large number of studies have shown that the incorporation of OPC and volcanic ash materials in FGDG can improve its mechanical properties and water resistance. In order to elucidate the hydration reaction mechanism of gypsum-based composite cementitious materials, the hydration products of the samples with different mixing ratios were determined by XRD, and the results are shown in Figure 6. As can be seen from Figure 6, the hydration products of gypsum-based composite cementitious materials mainly include gypsum dihydrate (CaSO₄·2H₂O), C-S-H gel, AFt and CH, etc. However, due to the low crystallinity of C-S-H gel, it does not affect the intensity of the diffraction peaks, but rather the width of the diffraction peaks [52]. In addition, the increase in the amount of FA and GGBS particles incorporated led to an increase in the diffraction peaks of quartz (SiO₂) and AFt [53,54], a phenomenon that suggests that FA and GGBS promote the hydration reaction. On the other hand, unreacted tiny particles provide the main quartz phase and act as a microaggregate filling effect [55], filling the pores in the matrix to improve the compactness of the structure.

When the samples were conditioned to an age of 7 d, CaSO₄·2H₂O, quartz and CH had higher diffraction peak intensities, while the AFt crystalline phase was weaker due to the hydration of mainly FGDG and OPC at an early stage. The intensity and number of diffraction peaks of CaSO₄·2H₂O, CH and quartz decreased and the intensity of diffraction peaks of AFt increased with the increase of the conservation time, which was attributed to the fact that gypsum acted as an excitatory agent and was able to promote the secondary hydration reaction of OPC, which was transformed from calcium aluminate hydroxide (C-A-H) to AFt [28]. Meanwhile, FA and GGBS particles contain a large amount of amorphous SiO₂ and Al₂O₃, which provide nucleation sites for the formation of AFt and C-S-H gel. Under the action of CH and SO₄²−, the silica–oxygen tetrahedra or aluminum–oxygen tetrahedra undergo a depolymerization reaction [56], which generates a large number of hydration products, leading to the weakening of the crystalline phases of CH and quartz, which further fills up the pore cracks in the microstructures, and promotes the development of compressive strength in the gelling materials.

3.5. Scanning Electron Microscopy Test (SEM)

The hydration products and micro-morphology of the samples with different mixing ratios at 7 d and 28 d age were characterized by SEM, and the test results are shown in Figure 7. From Figure 7a, it can be seen that the microstructure of the samples of group S6 is looser and the pore spacing is relatively large when the samples are maintained up to the age of 7 d. This is due to the large W/B and the evaporation of free water, which leads to the presence of a certain amount of microcracks in the samples, resulting in their lower macro-mechanical properties. In addition, the reaction is mainly dominated by FGDG and OPC hydration at the early stage of the reaction, and the FA and GGBS particles react with CH and CaSO₄·2H₂O hydration to a lower extent, and the tiny particles are uniformly distributed in the matrix, which mainly play the role of filling the pores.

In this study, it can be clearly seen that there is a large amount of spherical FA in the system, with some AFt attached to the surface, which is due to the slower hydration reaction of FA particles under ambient condition, which is consistent with the results of the reference studies [47,57]. With the gradual progress of the hydration reaction, Figure 7b reflects that the FA particles in the samples act as nucleation sites, the dense oxide shell layer on the surface of the spherical vitreous body undergoes dissolution corrosion, which releases the active substances, and the hydration reaction generates a large number of flocculent C-S-H gel as well as needle and acicular AFt, the internal pores are filled and the structure is gradually densified, which results in the enhancement of the macroscopic mechanical properties of the specimens.

From Figure 7c, it can be seen that the hydration products of the samples in group S7 were mainly dominated by CaSO₄·2H₂O with a large number of unhydrated FA particles at the age of 7 d during the maintenance to 7 d, whereas the content of AFt was significantly less than that of the specimens in group S6, which was due to the fact that the samples in group S6 had a larger admixture of GGBS particles, which resulted in the generation of a larger quantity of C-S-H gel and AFt at the early stage of hydration. On the other hand, although the samples have fewer hydration products, their internal structure laps are relatively dense, and the number of pores present is significantly smaller than that of the group S6 specimens, which is due to the smaller W/B, and therefore their macroscopic strength is relatively high. In addition, Figure 7d shows the microscopic morphology of the samples at 28 d of sample conditioning, and it can be seen that the FA and GGBS particles act as nucleation sites, and the vitreous undergoes depolymerization under the excitation of SO₄²− and CH, further hydration generates a large number of AFt and C-S-H gel, with a reduction in the number of pores and an increase in the degree of structural densification, which leads to the higher macro-mechanical properties of this group of specimens.

In summary, it can be seen from the microscopic morphology diagrams of the specimens in groups S6 and S7 that the hydration products of gypsum-based composite cementitious materials mainly include CaSO₄·2H₂O, C-S-H gel, AFt and CH, etc., and the doping amount of FA and GGBS particles affects the amount of generation of hydration products at the early stage of the reaction, while the magnitude of W/B creates an important change in the microscopic morphology of the specimens. This, in turn, had a decisive influence on the macroscopic mechanical properties, which is consistent with the polar and variance analyses of the mechanical properties in Section 3.1 and Section 3.2. In addition, combined with the micro-morphology of the specimens at the age of 7 d and 28 d, it is not difficult to analyze the synergistic hydration of FGDG–OPC and the synergistic stimulation of FA–GGBS hydration, which is also consistent with the analysis in Section 3.4.

3.6. Mercury Impression Test (MIP)

Numerous studies have shown that cementitious materials have a complex spatial structure and uneven pore distribution [58,59,60,61], The pore distribution and total porosity of the samples with different mixing ratios were tested by MIP at 28 d, and the results are shown in Figure 8. According to the results of academician Zhongwei Wu, all pores are classified into four types: pores with diameters less than 20 nm are considered to be harmless pores, produced by gel pores with a C-S-H interlayer structure [62,63,64,65]: less hazardous pores (d = 20~100 nm); more hazardous pores (d = 100~200 nm); and more hazardous pores, with diameters more than 200 nm.

From Figure 8, it can be seen that the pore size of S6 sample is concentrated in 0~1000 nm, and the most probable aperture size (MPA) is 384.02 nm, and the pore size of S7 sample is mainly distributed in 0~200 nm, and the most accessible pore size is 37.05 nm, and the distribution of pore size of the collodion material slurry is mainly closely related to the size of W/B. In addition, with the increase of FA and GGBS particles doping, the harmless pore volume of the cementitious material samples in the range of 0~20 nm decreases, but the pore volume in the range of 20 nm~10 um undergoes enlargement, which is mainly due to the role of FA and GGBS in optimizing the pore space. Typically, an increase in the number of innocuous pores provides reaction space and reaction drive to promote FA and GGBS hydration rates [66], and this result suggests that an increase in the volume of innocuous pores facilitates the development of mechanical properties. This result indicates that the increase in the volume of harmless pores favors the development of mechanical properties. An interesting phenomenon is that the mechanical properties of the samples of group S6 are not satisfactory despite the small total and harmless pore volumes, mainly because the increase in the number of harmful pores adversely affects the compressive strength of the samples, leading to a reduction in their strength.

3.7. Hydration Mechanism Analysis

According to the hydration products and micro-morphological analysis in Section 3.5 and Section 3.6, the hydration reaction process of gypsum-based composite cementitious materials was elucidated, as shown in Figure 9, and the main reaction processes were shown in Equations (1)~(6). As can be seen from Figure 9, during the preparation of gypsum-based composite cementitious materials, the minerals undergo relatively complex hydration reactions, such as the hydration of FGDG, the hydration of OPC, the synergistic hydration of FGDG–OPC and the synergistic stimulation of the FA–GGBS to undergo hydration, which are divided into the following stages:

(1) As shown in Equation (1), in the formula 0.5 means that 1 mol of CaSO₄·0.5H₂O contains 0.5 mol of water of crystallization, while 1.5 means the ratio of the amounts of the substances CaSO₄·0.5H₂O and H₂O.Due to the faster hydration rate of FGDG, it first reacts with water to crystallize to produce CaSO₄·2H₂O to generate strength and form the first supporting skeleton, but due to the irregular shape of the hydration product, the crystals are more scattered, and the products are staggered and lapped. The structural skeleton formed has more pores.

(2) As shown in Equations (2)~(4), the hydration of C₃S and C₂S in OPC generates flocculent C-S-H gel and flaky CH, which can regulate the pH value of the slurry to an environment suitable for the growth of the crystal form, which is conducive to the growth of gypsum crystals, so that the dense structure formed by hydration products lapped on top of each other reduces the porosity, and at the same time, the hydration of C₃A generates C-A-H in the AFt at the same time, the hydration of C₃A generates C-A-H under the action of gypsum, which further generates AFt and fills in the pores of the internal structure, thus enhancing the strength of the specimen.

(3) As shown in Equations (5) and (6), the chemical composition of GGBS has a high content of CaO and SiO₂ fractions, and the content of both SiO₂ and Al₂O₃ in FA is high, all of which are typical calcium–aluminum–silica materials capable of undergoing a volcanic ash reaction in an alkaline environment. With the gradual hydration reaction, polar water molecules and alkaline molecules dissolved and corroded the surface structure of the vitreous body, and reacted with the active Si⁴⁺ molecules in the FA and GGBS, which accelerated the dispersion and dissolution to generate a large number of flocculated C-S-H gel. In addition, gypsum played the role of sulfate excitation to active Al³⁺ and promoted the generation of AFt. Meanwhile, the C-S-H gel and AFt played the role of coating and filling to the CaSO₄·2H₂O grain morphology and contact site, and formed the second supporting skeleton, which improved the macroscopic properties of the specimen. On the other hand, the unreacted FA can also fill the pores of the system due to the fine FA grain size, making the structure denser and thus improving the strength and softening coefficient of the material.

In summary, through the synergistic hydration reaction and mutual excitation of the components, the performance of gypsum-based composite cementitious materials has been significantly improved compared with that of single-component gypsum.

$$CaS{O}_4\cdot 0.5H_{2}O+1.5H_{2}O\to CaS{O}_4\cdot 2H_{2}O$$

(1)

$$3CaO\cdot Si{O}_2+n{H}_{2}O\to xCaO\cdot Si{O}_2\cdot y{H}_{2}O+(3-x)Ca{(OH)}_2$$

(2)

$$2CaO\cdot Si{O}_2+n{H}_{2}O\to xCaO\cdot Si{O}_2\cdot y{H}_{2}O+(2-x)Ca{(OH)}_2$$

(3)

$$3CaO\cdot A{l}_2{O}_3+26H_{2}O+3CaS{O}_4\cdot 2H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 3CaS{O}_4\cdot 32H_{2}O$$

(4)

$$Si{O}_2+xCa{(OH)}_2+y{H}_{2}O\to xCaO\cdot Si{O}_2·(x+y) H_{2}O$$

(5)

$$A{l}_2{O}_3+3Ca{(OH)}_2+3CaS{O}_4\cdot 2H_{2}O+23H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 3CaS{O}_4\cdot 32H_{2}O$$

(6)

4. Conclusions

In order to clarify the multivariate composite form of gypsum-based composite cementitious materials, OPC was used as active cementitious material, FGDG, FA and GGBS were used as solid waste cementitious materials in this paper. The effects of various components on the physical and mechanical properties of cementitious specimens were analyzed. Through the analysis of hydration products and microstructure of cementitious materials, the synergistic hydration mechanism was clarified, which provided a reference for the comprehensive utilization of industrial solid waste and the application of gypsum-based cementitious materials in non-structural components. The main conclusions are as follows:

(1) Through the analysis of the sample performance of the extreme deviation, variance and regression model, it is concluded that the main factors affecting its ranking: W/B > OPC/(FGDG + FA + GGBS) > FGDG/(FA + GGBS) > FA/GGBS. The size of the water–cementitious ratio plays a decisive role in influencing the performance of cementitious materials, with the increase of the water–cementitious ratio of the specimen and the compressive strength of the specimen being decreased at all ages.

(2) The increase of FA and GGBS particle content promoted the secondary hydration of OPC and FGDG, and the content of C-S-H gel and AFt in the cementitious material system increased significantly.

(3) The microstructure and pore size of the samples are mainly affected by the size of W/B. A large W/B leads to the evaporation of excess free water, the formation of a large number of pores and microcracks, which reduces the macroscopic mechanical properties of the cementitious material system.

(4) The hydration reaction in the gypsum-based gelling material system is more complicated, including the hydration reaction of FGDG and OPC and the hydration reaction of FA and GGBS excited by alkali salts jointly, and the hydration products of the gelling material mainly include CaSO₄·2H₂O, AFt, CH, and C-S-H gel, in which the columnar crystals of gypsum dihydrate are irregularly lapped, and the flocculent C-S-H gel as well as the needle and rod-like AFt fill the internal pores to form a relatively dense crystal structure.