1. Introduction
Industrial carbon emissions are a topic of global concern [
1,
2,
3,
4]. Research shows that the cement industry contributes approximately 7% to the total global carbon emissions [
5,
6,
7]. It is evident that this issue has a detrimental effect on the sustainable development of cement production. In recent years, extensive research has been conducted on the utilization of industrial solid waste, such as fly ash and slag, in construction materials [
8,
9,
10,
11,
12]. This measure offers significant environmental and economic benefits, including the reduction of CO
2 emissions and the conservation of natural resources like limestone [
13,
14]. Consequently, the investigation of using industrial solid waste as a substitute or supplement for cement has emerged as a prominent research focus in efforts to mitigate carbon emissions [
15,
16,
17].
Currently, there are two significant approaches to address the issue of carbon emissions in cement production: the utilization of industrial solid waste for the preparation of geopolymers and their use as mineral admixtures [
18,
19,
20]. Among these, the preparation of geopolymers without cement clinker holds immense potential as a low-carbon alternative to cement [
21,
22,
23]. However, the process of preparing geopolymers is complex, and their performance is unstable, necessitating further comprehensive and in-depth research [
24,
25,
26]. The partial replacement of cement with industrial solid waste, such as fly ash and slag, is widely accepted and employed as a low-carbon strategy by cement manufacturers and concrete mixing plants [
27,
28,
29]. Nevertheless, the low reactivity of these mineral admixtures results in their limited usage [
30,
31,
32,
33]. Even so, the mineral admixtures commonly used in cement are still high-quality solid waste.
In order to increase the content, the activity of the mineral admixture is usually increased by grinding [
34]. However, due to its dense vitreous structure, the mineral admixture primarily serves as a filler, with volcanic ash providing supplementary effects [
35,
36]. Therefore, the actual utilization rate of industrial solid waste remains quite low. Similarly, the utilization of clinker in ordinary silicate cement is also inefficient. Ordinary silicate cement, with a specific surface area of about 300 m
2/kg, has an effective utilization rate of less than 50% for one year [
36,
37,
38]. The part of the cement particle that is not fully hydrated acts only as a microaggregate.
Existing studies [
39,
40] have shown that the utilization of superfine cement can exceed 80%. Shondeep L. Sarkar [
41] pointed out two main drawbacks of refined cement particles, namely, late strength regression and faster setting time. However, these adverse effects of refined cement particles can be mitigated by replacing 20% of the cement with ultrafine fly ash. Dale P. Bentz [
42] conducted computer simulations to analyze the strength development of different particle size cements, demonstrating that coarse cement lags behind in strength development compared to fine cement. A.K.H. Kwan [
43] further showed in their research that incorporating a small amount of ultrafine cement into the voids of ordinary cement can improve the packing density of the cement, thus reducing the water requirement for filling the voids. Mingjuan Zhou [
44] utilized a slurry of ultrafine cement to suppress deformation and damage of surrounding rocks in tunnels.
Taken together, it is evident that superfine cement is an efficient cementitious material for enhancing cement performance. However, current research primarily focuses on the application of high dosages of superfine cement, with mineral admixtures only serving as additives to regulate the properties of superfine cement. Therefore, if the roles of cement and mineral admixtures are interchanged, using superfine cement to fill the voids left by mineral admixtures, it is possible to simultaneously enhance the utilization of cement and mineral admixtures by increasing the reactivity of the mineral admixtures. This approach provides a new pathway for the development of low-carbon cementitious materials.
In this paper, superfine cement was produced through the process of ultra-fine grinding. The low-quality industrial solid waste was activated by mechanical grinding, sulfate activation, and alkali activation. By incorporating superfine cement with industrial solid waste, low-carbon cementitious materials were prepared, ensuring that the resulting compressive strength was not lower than that of the control group (without mineral admixture). Additionally, the impact of superfine cement on the microstructure of HMCM was investigated using a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furthermore, based on the particle size distribution characteristics of superfine cement, a reverse filling mechanism for the mineral admixture in the presence of superfine cement under different activation conditions was proposed.
4. Mechanism Analysis
4.1. Filling Mechanism of Mineral Admixture
Figure 17 shows the filling mechanism of the mineral admixture. As seen in the figure, the particle size of the mineral admixture is much smaller than that of ordinary cement particles, which can fill in the voids between cement particles to increase the density of concrete. However, the voids between cement particles are limited. It is difficult to improve the amount of mineral admixture. It is because the activity of the mineral admixture is low before activation, and to increase the amount of mineral admixture is not good for the establishment and development of the mechanical strength of the cementitious material. Similarly, cement particles with a large particle size are less hydrated and waste a lot of cement resources. Moreover, alkalinity provided by the cement paste is low, which makes it difficult to activate the potential activity of the mineral admixture. Therefore, most of the mineral admixtures only play the role of filler and are not fully utilized.
4.2. Reverse Filling Mechanism of Superfine Cement
Figure 18 shows the reverse filling mechanism of superfine cement. As seen in the figure, the particle size of superfine cement is much smaller than that of the mineral admixture, which can better fill in the gaps of the mineral admixture and improve its density. Meanwhile, superfine cement has a small particle size and fast hydration rate, which can be fully hydrated in a relatively short time. Therefore, cement clinker is ground to make its particle size smaller than that of the mineral admixture, which has the effect of reverse filling the void of the mineral admixture particles, as well as improve the hardening paste density and the effective utilization of cement clinker.
Superfine cement can be fully hydrated in a short time and generate a large amount of hydration products. Therefore, the content of superfine cement has a great influence on its mechanical properties. When the superfine cement is not enough to fill the voids between the mineral admixtures, its hydration products are also difficult to bind all mineral admixtures tightly. Therefore, the mechanical properties of the hardened paste are low. When the superfine cement is just enough to fill the voids between the mineral admixtures, its hydration products can also bind all the mineral admixtures together. Therefore, the mechanical properties of the hardened paste can be fully improved. When the content of superfine cement exceeds the demand of the voids between the mineral admixtures, the mechanical properties of the hardened paste will be greatly improved and develop towards ultra-high performance.
In summary, the reverse filling effect of superfine cement has the following four main effects: improving the paste density; improving the degree of hydration of cement particles; increasing the utilization rate of cement clinker significantly; and increasing the amount of mineral admixture significantly. Among them, the increase of solid waste mineral admixture not only depends on the reverse filling effect of superfine cement, but also on the activity of the mineral admixture itself. Therefore, improving the activity of the mineral admixture based on the reverse filling effect of superfine cement is another important way to improve the mineral admixture content.
4.3. Sulfate Activation Principle Based on Reverse Filling Action
Mineral admixtures usually have a dense glass structure, which makes it difficult to activate their potential activity by grinding. Sulfate can destroy the dense glass structure by reacting with alumina (Al
2O
3) and calcium oxide (CaO) in the glass body. Therefore, sulfate has the effect of activating the mineral admixture. However, sulfate is not enough to decompose the glass body sufficiently, and a combination of a certain alkaline environment (pH > 12) is required to promote the dissolution of the other major component of the glass body (SiO
2) [
64,
65].
The principle of sulfate activation based on reverse filling action is shown in
Figure 19. It can be seen from the figure that the superfine cement improves the density of the matrix. The hydration of the mineral admixture increases with sulfate activation. The reaction process of sulfate activation is shown in Equation (1) [
66].
In Equation (1), Al2O3 is from the mineral admixture and cement, Ca(OH)2 is from hydration products of cement, and CaSO4·2H2O is from desulfurization gypsum. The hydration product is ettringite. From Equation (1), it can be seen that the main reason for sulfate to improve the activity of mineral admixture is to promote the dissolution of Al2O3. Both ettringite and hydrated calcium silicate have a cementation effect, which promotes the establishment of mechanical properties of HMCM.
4.4. Alkali-Activated Principle Based on Reverse Filling Action
The activation effect of sulfate on the mineral admixture depends on a certain alkaline environment. However, the low solubility of calcium hydroxide does not substantially increase the alkalinity of the paste. In contrast, the low modulus liquid sodium silicate not only provides high alkalinity but is also commonly used cementitious material itself.
The principle of alkali-activated based on reverse filling action is shown in
Figure 20. When the liquid sodium silicate makes contact with superfine cement mixture, it first transforms to silica gel and releases a large amount of OH
−. The OH
− penetration into the glass body of the mineral admixture causes covalent bonds such as Si–O–Si, Al–O–Al, and Si–O–Al to break. The breakage of the chemical bond promotes the dissolution of the mineral admixture. The dissolved silica-alumina component reacts with the calcium component in the liquid phase to produce a large amount of hydration products such as hydrated calcium aluminosilicate. It forms a three-dimensional network structure different from the traditional structure of hydrated calcium silicate. The two structures interweave, resulting in a significant increase in the density and mechanical properties of the hardened cement paste.