Preparation and Performance of Ultra-Fine High Activity Composite Micronized Powder from Multi-Solid Waste

Abstract

The composite micronized powder is prepared by using blast furnace slag (BFS), water-quenched manganese slag (WQMS), manganese tailing slag (MTS) and desulfurization gypsum (DG) and grinding aid (GA) through orthogonal test optimization design. The effect of the doping amount of each solid waste on the fluidity, activity at different ages and resistance to chloride ion penetration of the composite micropowder was studied systematically, and the exothermic characteristics of hydration of the composite micropowder with the optimal ratio were tested. The results showed that the amount of MTS dosing was the most significant factor among the four factors on the activity index of composite micronized powder at 7 d and 28 d. The activity index at 28 d decreased and then increased with the increase in MTS dosing; the amount of BFS dosing was the most significant factor affecting the fluidity and chloride ion permeation resistance of composite micronized powder. With an increase in BFS dosing, the fluidity ratio of composite micronized powder increased and then decreased; the electric flux of the matrix decreased, and the chloride ion permeation resistance increased. The optimal ratio of composite powder with the highest 28 d activity is 35% BFS, 30% MTS, 0.3% GA, 5% DG and 30% WQMS. The hydration rate and cumulative heat release of the slurry prepared with the optimal ratio of composite micronized powder to cement (1:1) are lower than those of pure cement slurry. The microstructure of the mortar test block prepared with a 1:1 composite of cement is more compact than that of the pure cement mortar test block, and the pores are fewer.

1. Introduction

As solid industrial waste, the inventories of blast furnace slag (BFS), manganese tailing slag (MTS), water-quenched manganese slag (WQMS) and desulfurization gypsum (DG) gradually increase with time. It not only occupies a large amount of land but also contains pollutants such as acid radical ions and heavy metal ions, which will cause huge pollution to air, water and soil during the stacking process. Therefore, how to recycle these solid wastes with high efficiency and high added value has become an urgent problem to be solved.

BFS is a solid waste produced in the process of blast furnace ironmaking. It mainly contains CaO, SiO₂, Al₂O₃ and other oxides. The total amount of these oxides is generally more than 90%, which has a high potential activity [1]. A large number of studies have shown that BFS is the most common solid waste material for preparing composite micronized powder. Replacing a part of ordinary Portland cement (OPC) with BFS can not only reduce costs but also improve the durability and long-term mechanical properties of concrete [2]. Guneyisi studied the influence of adding slag powder and fly ash to concrete and found that the workability and mechanical properties of concrete have been improved [3]. Chen also used the composite powder prepared with slag and fly ash in a ratio of 1:1 to replace 30% of the cement, which improved the mechanical properties of concrete [4].

MTS is an industrial solid waste produced after manganese ore beneficiation, which mainly contains minerals such as dihydrate gypsum, quartz and hydrated calcium silicate. MTS can replace a part of cement to prepare concrete, which can change the pore structure of concrete and improve the impermeability, frost resistance and durability of concrete [5,6]. Wang Yong prepared aerated concrete with an excellent performance by mixing lime, gypsum, cement, MTS and aluminum powder in a certain ratio [7].

WQMS is a kind of slag waste formed by water quenching in the high-temperature melting state during the smelting process of manganese ore. The main mineral composition is mainly glass phase (about 85%), which contains active or potential hydrosetting and pozzolanic substances, such as dicalcium silicate and magnesium rose pyroxene [8]. Therefore, WQMS can be used as a concrete admixture and a cement admixture. Liu’s research found that WQMS plays an important role in improving the durability of concrete and believes that it can be used as mineral powder in offshore and marine environment concrete projects [9].

DG is an industrial by-product obtained after wet flue gas desulfurization. The main component is CaSO₄·2H₂O. At present, the annual output of desulfurization gypsum in China has reached 80 million tons [10]. DG can be used as a cement retarder and can reduce the drying shrinkage of cementitious materials [11]. Gu et al. found that compared with natural gypsum, the addition of DG not only prolongs the setting time of cement but also increases the mechanical properties of cement [12].

Solid-waste-based composite micronized powder is a kind of ultra-fine powder with small particle size and high activity prepared from industrial solid waste. It has the characteristics of low hydration heat, high activity and environmental protection. Based on the above contents, it is shown that BFS, MTS, WQMS and DG can be used to prepare composite micronized powder for cement and concrete. Using the characteristics of these solid wastes to prepare composite micronized powder with performance meeting the requirements of S95 grade mineral powder through complementary advantages can not only benefit society and improve the ecology but also have significant economic benefits.

2. Materials and Methods

2.1. Material Sources

(1) Cement: Conch P-O 42.5 ordinary Portland cement, density 3.12 kg/m³ at 20 ± 2 °C, specific surface area of 400 m²/kg; (2) standard sand: standard sand produced in Xiamen; (3) DG, BFS, WQMS, MTS: all from the local area of Quanzhou, Guilin, where the density of BFS is 2.90 kg/m³, DG is 2.33 kg/m³ and WQMS is 2.77 kg/m³ at 20 ± 2 °C; (4) water: tap water; (5) grinding aid: homemade alcohol amine grinding aid. The chemical composition of raw materials is shown in

Table 1. Main chemical composition of raw materials.

Raw Material	Mass Fraction/%
	Al2O3	CaO	Fe2O3	K2O	MgO	MnO	Na2O	P2O5	SiO2	TiO2	S
BFS	13.60	33.18	0.03	0.38	6.29	0.39	0.64	0.04	34.84	1.60	1.04
WQMS	22.43	11.44	0.01	2.72	2.03	12.25	0.73	0.03	29.93	0.26	0.95
MTS	1.67	0.64	2.82	0.01	0.62	3.37	0.14	0.08	89.72	0.03	0.01
DG	3.78	27.82	0.63	0.27	3.81	0.09	0.46	0.15	17.58	0.12	24.36

.

2.2. Experimental Method

According to the orthogonal test design, each industrial solid waste and GA were weighed and ground with different ratios of industrial solid waste using a small ball mill (SM-500 × 500) to prepare composite micronized powder. Then, according to the Chinese standard GB/T 51003-2014 “Technical code for application of mineral admixture” [13], the composite micronized powder was compounded with OPC at a ratio of 1:1, and the mortar test block and the electric flux test block were prepared with a water-binder ratio of 0.5. After curing at room temperature for 24 h, the prepared specimens were demolded and then cured to the corresponding age under the standard conditions of 20 ± 2 °C and relative humidity greater than 95%. According to GB/T 2419-2005 “Test method for fluidity of cement mortar” [14], the sand mix mortar jump table flow is determined. According to GB/T 17671-2021 “cement mortar strength test method (ISO method)” [15], a YAW-300 (Shanghai Sansi Vertical and Horizontal Machinery Manufacturing Co., Ltd., Shanghai, China) pressure testing machine is used to determine the molding size of 40 mm × 40 mm × 160 mm specimens for 7 d and 28 d compressive strength. The hardened slurry in the center of the specimen was selected and put into anhydrous ethanol to terminate the hydration. Then, the representative hardened slurry was selected for gold spraying, and the microscopic morphology was analyzed by a JSM-6380LV scanning electron microscope (Japan Electronics Co., Ltd., Kumagaya-shi, Japan). The heat of hydration of the cementitious material mixed with a superfine admixture was determined using an I-CAL4000/8000 type heat of hydration tester (Calmetrix, Boston, MA, USA). The chloride ion penetration resistance of the cement mortar test block was determined by the electric flux method according to the ASTM C1202 testing method of electric flux [16].

2.3. Orthogonal Test Proportioning Design

Based on the principle of mathematical statistics, an orthogonal experiment with 4 factors and 4 levels was designed to explore the effects of BFS, MTS, DG and GA on the fluidity, mechanical properties and chloride ion penetration resistance of mortar cementitious materials. A total of four different factors, A (BFS admixture amount), B (MTS admixture amount), C (GA admixture amount) and D (DG admixture amount), and an L16 (4⁴) orthogonal table were used. According to the team’s previous experimental exploration, the design of each factor level is shown in

Table 2. Table of factors and levels of orthogonal test.

Level	Factor
	A (BFS)/%	B (MTS)/%	C (GA)/%	D (DG)/%
1	20	15	0	3
2	25	20	0.1	5
3	30	25	0.2	8
4	35	30	0.3	10

. The amount of WQMS is determined by the amount of other industrial solid waste and the total amount of cementitious materials. The orthogonal test mix ratio is shown in

Table 3. Orthogonal experimental design.

Number	A (BFS)/%	B (MTS)/%	C (GA)/%	D (DG)/%	WQMS/%
F1	20	15	0	3	62
F2	20	20	0.1	5	55
F3	20	25	0.2	8	47
F4	20	30	0.3	10	40
F5	25	15	0.1	8	52
F6	25	20	0	10	45
F7	25	25	0.3	3	52
F8	25	30	0.2	5	40
F9	30	15	0.2	10	45
F10	30	20	0.3	8	42
F11	30	25	0	5	40
F12	30	30	0.1	3	37
F13	35	15	0.3	5	45
F14	35	20	0.2	3	42
F15	35	25	0.1	10	30
F16	35	30	0	8	27

.

3. Results and Discussion

3.1. Discussion of Orthogonal Experimental Results

According to the orthogonal experiment design, the activity index of composite micronized cement mortar, the fluidity ratio of the mix and the electric flux at 28 d were tested and calculated under standard maintenance conditions at 7 d and 28 d. The correlation results are given in

Table 4. Results of orthogonal experiment.

Number	Fluidity (mm)	Fluidity Ratio (%)	Electric Flux (c)	Compressive Strength/MPa		Activity Index/%
				7 d	28 d	7 d	28 d
Control	195	100	\	33.1	50.8	100	100
F1	180	92.3	2877	20.4	39.3	61.6	80.3
F2	180	92.3	2750	23.9	36.7	72.2	75.0
F3	188	96.4	2013	31.1	45.0	94.0	92.1
F4	195	100	1921	31.4	44.3	94.9	90.6
F5	185	94.9	2126	28.3	40.2	85.6	82.2
F6	190	97.4	2410	27.8	36.9	83.8	75.5
F7	194	99.5	2518	30.1	42.8	90.9	87.4
F8	186	95.4	1964	32.0	48.0	96.8	98.1
F9	180	92.3	2018	31.7	40.6	95.7	83.0
F10	174	89.2	1926	30.6	39.1	92.4	79.9
F11	183	93.8	2552	33.2	44.0	100.3	90.0
F12	186	95.4	2289	29.0	44.7	87.6	91.3
F13	185	94.9	1799	29.0	46.4	87.6	94.8
F14	180	92.3	2234	27.8	37.9	84.0	77.5
F15	177	90.8	1554	34.3	43.5	103.5	89.0
F16	178	91.3	1869	31.0	44.2	93.6	90.3

. The extreme difference analysis of the effect of four factors (BFS admixture amount A, MTS admixture amount B, GA admixture amount C, DG admixture amount D) on the activity index, mix fluidity ratio and electric flux of the composite micronized cement mortar test block is shown in

Table 5. Range analysis of orthogonal experiment.

Performances		Factor
		A (BFS)	B (MTS)	C (GA)	D (DG)
Activity index/%	7 d	13.3	14.6	7.8	13.4
	28 d	3.4	15.6	4.1	5.3
Fluidity ratio/%		4.5	2.7	2.6	2.2
Electric flux/c		526	319	386	504

.

3.1.1. Effect of Factors on the Fluidity Ratio of Composite Micronized Powder

As can be seen from Table 5, the order of the influence of each factor on the fluidity ratio of composite micronized powder is A > B > C > D. Figure 1 shows the effect curve of four factors on the fluidity ratio of composite micronized powder. It can be seen that, with the increase in BFS doping, the composite micronized powder fluidity ratio increases first and then decreases; with the increase in MTS and GS doping, the fluidity ratio of composite micronized powder roughly shows an increase. Furthermore, the fluidity ratio of composite micronized powder decreases with the amount of DG and then increases. When the ratio is A2B4C4D4, the compound micronized powder has the largest fluidity ratio, i.e., 25% BFS, 30% MTS, 0.3% GS, 10% DG and 35% WQMS.

Yang [17] et al. found that BFS powder contains a large number of glass beads or needle-like glass bodies with dense and smooth surfaces, which do not easily adsorb water molecules in the gelling system, causing a tumbling effect and easily producing a smooth sliding surface, thus improving its fluidity. In addition, in the gelling system, with water filled in the spaces between the particles, the surfaces of the particles are water-wrapped, forming a water film layer. The thickness of the water film layer and the quantity of surface-layer water are the main factors affecting the fluidity of the slurry. Since the particle size of BFS powder is smaller than that of cement particles, it can displace the filling water between cement particles and release the water within the gaps among the cement particles, which accordingly increases the thickness of the water film layer and the quantity of surface-layer water, thus improving the fluidity of the slurry [17]. However, since its particle size is smaller than the cement particles, resulting in a larger specific surface area, the mixing process requires more water than cement. As a result, an excessive amount of BFS powder will instead deteriorate the slurry’s fluidity, which also explains the phenomenon that the fluidity ratio increases first and then decreases with the increase in the amount of BFS compound micronized powder [18]. The viscosity of the slurry is the main factor affecting the fluidity, and the resistance is high when the viscosity is too high, leading to reduced flow. The addition of MTS can properly reduce the viscosity of the slurry, thus improving the mortar fluidity [19]. The role of GA is to improve the ease of grinding solid waste materials, ensuring a more uniform mixture and concentrated fineness. The slurry prepared with the composite micro powder exhibits reduced sliding resistance, thus increasing the fluidity of the slurry [20]. The structure of DG is a dense structural network formed by short columnar crystals in contact with each other, and the increase in its doping quantity will reduce the flow of the system, but when its doping quantity is greater than a certain reasonable range, the flow of the system increases due to its higher water content [21].

3.1.2. Influence of Factors on the Performance of Composite Micronized Powder against Chloride Ion Penetration

As can be seen from Table 5, the main law of the influence of each factor on the substrate resistance to chloride ion penetration is A > D > C > B. Figure 2 shows the effect curve of each factor on the substrate electric flux. From Figure 2, it can be seen that with the increase in BFS, DG and GA doping, the electric flux of the matrix decreases and the resistance to chloride ion penetration increases; conversely, with an increase in MTS doping, the electric flux of the matrix first increases and then decreases, and the resistance to chloride ion penetration of the matrix first decreases and then increases. The matrix prepared with the composite micronized powder has the best resistance to chloride ion penetration when the ratio is A4B4C4D4, i.e., 35% BFS, 30% MTS, 0.3% GA, 10% DG and 25% WQMS.

Table 6. Grading of concrete resistance to chloride ion penetration (electric flux method).

Grade	Q-I	Q-II	Q-III	Q-IV	Q-V
Electric flux (C)	QS ≥ 4000	2000 ≤ QS < 4000	1000 ≤ QS < 2000	500 ≤ QS < 1000	QS < 500

shows the grading of the chloride ion permeation resistance of concrete by the electric flux method. Combined with the results of the orthogonal experiments in Table 4, it can be seen that the chloride ion permeation resistance of the composite micropowder matrix reaches Q-II grade, with some reaching Q-III grade.

The ability of BFS and DG to enhance the chloride ion penetration resistance of the matrix can be attributed to their two effects in the cementitious system, namely, the volcanic ash effect and the filling effect [22,23]. The volcanic ash effect of BFS and DG is manifested by the secondary reaction of glass phase SiO₂ and Al₂O₃ in BFS and DG with cement hydration product Ca(OH)₂ in the alkaline environment of the cementitious system, and the formation of cementitious materials such as hydrated calcium silicate and hydrated calcium aluminate improves the compactness and volume stability of the matrix, and it is more challenging for chloride ions to penetrate. The filling effect of BFS and DG is shown by the fact that during the hydration process of cement, BFS and DG micropowder particles with smaller particle size than cement, which are not involved in the reaction, are evenly dispersed in the pores and cementitious body, playing the role of filling capillary pores and pore cracks, improving the pore structure and the compactness of the matrix, thus reducing the chloride ion permeability. The mechanism of action of the two effects can be summarized as the incorporation of BFS and DG leads to a denser microstructure with a matrix having fewer interconnected pores [24]. Most of the MTS is also in an unstable vitreous state, and its induced volcanic ash reaction enhances the resistance of the matrix to chloride, but when it is mixed in small amounts, its potential activity is not fully stimulated, but slightly reduces the resistance of the matrix to chloride ion penetration. In terms of the influence of GA on the matrix’s anti-chlorine ion permeation performance, as mentioned above, the higher the amount of GA within a reasonable range, the better its grinding effect. This results in finer powder, a denser matrix, lower porosity and better anti-chlorine ion permeation performance.

3.1.3. Effect of Factors on the Activity of Composite Micronized Powder

As can be seen from Table 5, the major and minor laws of the effect of each factor on 7 d activity index were B > D > A > C, and the major and minor laws of the effect of each factor on 28 d activity index were B > D > C > A. Figure 3 shows the effect curves of each factor on the mortar test blocks at different ages. Combined with Figure 3 and Table 5, it can be seen that the 7 d activity index increases first and then decreases with the increase in BFS, MTS and GA doping, increasing with the amount of DG doping. When the ratio is A3B3C3D4, the 7 d activity index of composite micronized powder is the highest. The 28 d activity index increases with the increase in BFS and GA, decreases with the increase in MTS and with an increase in the amount of DG, it increases first and then decreases. When the ratio is A4B4C4D2, it has the highest 28 d activity index, i.e., 35% BFS, 30% MTS, 0.3% GA, 5% DG and 30% WQMS.

BFS, MTS and WQMS represent the main active materials of the composite micronized powder, among which BFS is a granular active material with calcium silicate and calcium aluminate as the main components. It is discharged from iron-making and formed by water-quenching and rapid cooling treatment, generally containing 80–90% in a glassy state, in a sub-stable state, with high reactivity, and is a potential water-hardened gelling material [25]. Although the main chemical composition of MTS and WQMS and their relative amount are similar to BFS, the glass body occupies most of it but is lower than that in BFS. As a result, its activity is lower. However, the aqueous solution of MTS is weakly acidic, which can adjust the PH value in the gel system to have an effect on hydration. Moreover, its fluctuation of admixture has a greater effect on the activity of composite micronized powder. The phenomenon in the experiment shows that the activity index of composite micronized powder increases with the BFS and MTS doping in a certain range, and the MTS doping is the most significant factor affecting the composite micronized powder activity index among the four factors. However, because the total amount of composite micronized powder is fixed, excessive amounts of both substances will lead to reduced dosing of DG, which reduces the effect of early DG on the alkaline excitation of mineral powder, thus the 7-d activity decreases when excessive amounts of both are dosed.

The reason why the 7 d activity of composite micronized powder increases with a certain range of DG admixture is that the gypsum dihydrate in DG can promote the hydration of C3S in cement clinker, while the presence of its soluble impurity ions and CaCO₃ is conducive to accelerating the hydration of cement, which increases the concentration of Ca(OH)₂ in the liquid phase compared with the hydration system without DG, providing conditions for the alkaline excitation of BFS powder and prompting the hydration reaction of BFS powder. On the other hand, the DG in the system can react directly with the active Al₂O₃ in the BFS powder in an alkaline environment to form calcium alumina, which has an alkaline excitation effect on the BFS powder, thus contributing to the early strength of the system [26]. However, because the volcanic ash effect of BFS and MTS mainly occurs in the later stage and absorbs a large amount of Ca(OH)₂ crystals in the secondary hydration reaction, the reaction generates hydrated calcium silicate and hydrated calcium aluminate with higher strength and better stability, so the activity effect in the later stage of hydration gradually appears and dominates over the effect of DG on the alkaline excitation of mineral powder. As a result, the greater the dosing of both, the greater the later contribution to the compressive strength of colluvium [27,28]; thus, the 28 d activity index of composite micronized powder increases with the increase in BFS and MTS dosing, while it increases first and then decreases with the increase in DG dosing.

The source of activity of the composite micronized powder is also reflected in its grinding fineness, and it is usually believed that the finer the grinding, the higher its activity. As the amount of GA mixed within a reasonable range increases, the better the grinding aid effect and the finer the powder. Consequently, the overall activity index of the composite micronized powder shows an increase with the increase in GA doping.

3.2. Effect of Composite Micronized Powder on the Performance of Cementitious Materials

Integrating the above orthogonal experimental results and giving priority to the influence of each factor on the 28 d activity index of the composite micronized powder, the best ratio for the preparation of composite micronized powder is A4B4C4D2, i.e., 35% BFS, 30% MTS, 0.3% GA, 5% DG and 30% WQMS. The specific surface area of the composite micronized powder is 460 m²/kg. Figure 4 shows the particle size distribution of OPC and composite micronized powder, and it can be seen that the particle size range of composite micronized powder is smaller. Figure 5 shows the XRD pattern of the composite micronized powder, which shows an obvious dispersion peak, indicating that the main component of the composite micronized powder is an amorphous high-activity substance. On this basis, the cementitious material was prepared by compounding the composite micronized powder, prepared using the best mix ratio with OPC at a 1:1 ratio. The fluidity and activity of the cementitious material are given in

Table 7. The test results of the optimal ratio of composite micronized powder.

Fluidity (mm)	Activity Index/%
	7 d	28 d
185	94.6	101.3

; compared with Table 4, it can be observed that it has the highest 28 d activity. In addition, the effect of the composite micronized powder on the hydration heat release characteristics and microstructure of the cementitious material was also tested.

3.2.1. Effect of Composite Micronized Powder on the Hydration Characteristics of Cementitious Materials

Figure 6 shows the exothermic properties of hydration of cementitious materials mixed with 50% composite micronized cementitious materials and pure cementitious materials. Figure 6a shows the exothermic rate curve of hydration of different cementitious materials, and the input image provides details of the exothermic rate curve of hydration within 0–0.6 h. Figure 6b shows the cumulative exothermic curve of hydration of different cementitious materials. Compared with the maximum hydration rate of pure cement paste, the maximum hydration rate of cementitious material mixed with 50% composite micronized powder was reduced by 37.08%. Compared with the 3 d cumulative heat release of pure cement paste, the 3 d cumulative heat release of cementitious material mixed with 50% composite micronized powder was reduced by 24.69%.

The experimental results show that the hydration rate and cumulative heat release of the cementitious material mixed with 50% composite micronized powder are much lower than those of the pure cementitious material. One of the main reasons for this is the significantly greater reduction in the heat of hydration of the cementitious material due to the halving of the cement dosage [29]; secondly, it is because there are many kinds of powder particles in the composite micronized powder, in which the unreacted various inert particles will hinder and delay the chemical reaction between Ca(OH)₂, the hydration product of cement and the active material in BFS, WQMS and MTS, and absorb part of the exothermic heat.

3.2.2. Effect of Composite Micronized Powder on the Microscopic Morphology of Gelling Materials

Figure 7a shows the microscopic morphology of the pure cementitious mortar specimen cured for 7 d. Figure 7b shows the microscopic morphology of the mortar specimen cured for 7 d, derived from the 1:1 composite of composite micronized powder and cement. As shown in Figure 7a, there are a large number of needle-rod calcium alumina crystals in the pure cement hardened slurry, and the structure is loose and distributed with a large number of pores. As shown in Figure 7b, the microscopic surface structure of the test block doped with 50% composite micronized powder is dense, and the main hydration products, flaky Ca(OH)₂ crystals, are closely packed with fine particles. Only a small amount of pores are distributed around the flaky hydration products and fine particles. It is again shown that the concentration of Ca(OH)₂ in the DG system is higher, and the microstructure of the specimens mixed with 50% composite micronized powder is tighter and has finer and fewer pores compared with the pure cement specimens after 7 d of curing [30].

Figure 8 shows the microscopic morphology of pure cementitious mortar specimens and specimens mixed with 50% composite micronized mortar cured for 28 d. In comparison with Figure 7, the microstructure of different cementitious materials specimens became more dense as the curing age increased. The main hydration products of pure cement specimens change from the loose needle and rod form attached to the slurry to the form of flocculent gel and flaky crystals cemented to each other; the microstructure of the mortar test block mixed with 50% compound micronized powder changed from the form of accumulation of sheet hydration products and fine particles to the form of continuous sheet lamellae composed of hydration products and unreacted particles. It shows that the vitreous form of SiO₂ and A1₂O₃ in the composite micronized powder reacts with the cement hydration product Ca(OH)₂ in a secondary reaction to produce continuous dense hydrated calcium silicate, hydrated calcium aluminate and other cementitious substances [31]. After 28 d of curing, the microstructure of the mortar specimens mixed with 50% composite micronized powder was more continuous, and the bond between the cementitious material and the aggregate was tighter when compared with the pure cementitious mortar specimens.

4. Conclusions

In this work, the effects of BFS, MTS, DG and GA on the fluidity, mechanical properties and chloride ion penetration resistance of composite micronized powder cementitious materials were studied by orthogonal test. The ratio of the composite micronized powder with the highest 28 d activity was obtained. The hydration characteristics and microscopic morphology of cementitious materials, specifically the composite micronized powder which has the the optimal ratio mixed with OPC, were compared with those of pure OPC. The conclusions are as follows:

(1)

The amount of MTS is the most significant factor among the four factors affecting the 7 d and 28 d activity index of the composite micronized powder, and the 28 d activity index decreases first and then increases with the increase in the amount of MTS. When the ratio is A4B4C4D2, it has the highest 28 d activity index, i.e., 35% BFS, 30% MTS, 0.3% GA, 5% DG and 30% WQMS.

(2)

The amount of BFS is the most significant factor among the four factors affecting the composite micronized powder fluidity and resistance to chloride ion penetration. With an increase in BFS doping, the fluidity ratio of composite micronized powder initially increases, then decreases. Simultaneously, the electric flux of the matrix decreases, resulting in an increased resistance to chloride ion penetration. When the ratio is A2B4C4D4, the composite micronized powder has the largest fluidity, i.e., 25% BFS, 30% MTS, 0.3% GA, 10% DG and 35% WQMS; when the ratio is A4B4C4D4, the matrix prepared with composite micronized powder has the best chloride ion permeation resistance, i.e., 35% BFS, 30% MTS, 0.3% GA, 10% DG and 25% WQMS.

(3)

The ratio with the highest 28 d activity index was selected as the best ratio of composite micronized powder, i.e., 35% BFS, 30% MTS, 0.3% GA, 5% DG and 30% WQMS. The hydration rate and accumulated heat release of the slurry mixed with 50% of the best ratio of composite micronized powder were lower than those of pure cement slurry; after 7 d and 28 d of curing, the microstructure of the mortar specimens mixed with 50% composite micronized powder of the best ratio appeared more compact and less porous than that of the pure cement mortar specimens cured at the same age.