1. Introduction
With the continuous promotion of global infrastructure construction, cement and concrete as the main building materials, its demand is increasing, and with it comes the problem of serious environmental pollution and increasing energy consumption. Therefore, the cement and concrete industry needs to adjust the development mode, change the direction of development, reduce energy consumption, reduce environmental pollution, and take the road of sustainable development [
1]. The decisive material in cement and concrete is the cementitious material, and the traditional cementitious material is usually cement-based, fly ash, mineral powder, silica fume and other industrial by-products as mineral admixtures [
2,
3,
4,
5]. The construction industry consumes a large amount of steel and cement, especially in the process of cement production, which emits gases such as CO
2, SO
2, SO
3 and NO
2 that cause serious pollution to the environment. According to statistics, each ton of cement produced emits about one ton of gases, and reducing the production of cement will make an important contribution to reducing emissions. For this reason, the EU government has committed to reduce 30% of the gases affecting the greenhouse effect by 2020 and 95% by 2050 [
6]. Use the advantages of the cement industry to effectively absorb waste, increase the resource utilization of various solid wastes, achieve low pollution and low emissions in the cement industry, and promote the cement industry to become a circular economy industrial system with coordinated and sustainable development of resources, environment and human society [
7,
8].
The role of industrial waste, mining solid waste and municipal waste as mineral admixtures in cementitious materials to control energy consumption and reduce environmental pollution is becoming more and more prominent. The accumulated solid waste not only causes waste of resources, along with the waste of land resources and serious pollution of the environment, endangering the ecosystem. Mineral admixture can not only alleviate the problem of energy consumption, but can also play a positive role in protecting the environment, and is the road to sustainable development of today’s green high-performance concrete [
9,
10,
11]. Fly ash, silica fume and mineral powder, as common mineral admixtures, are severely limited in their capacity due to energy saving and emission reduction requirements [
12,
13,
14]. Therefore, we need to find new mineral admixtures to meet the market demand and follow the trend of energy saving and emission reduction and green development.
Iron tailings (IOTs) are mining solid wastes, and there are a large amount of IOTs produced every year in China. The accumulation of a large amount of IOTs wastes land resources and pollutes the environment. Additionally, IOTs are potential secondary resources, besides containing valuable metals, vein minerals and other materials are the main mineral composition of IOTs, and oxides of silicon, calcium and aluminum are the main chemical components. However, IOTs are inert materials that do not have high cementing activity [
15]. To effectively utilize IOTs, they must first be activated, thus stimulating the activity. Single-doped IOTS has a negative impact on compressive strength [
16]. Over the years, many scholars have conducted various studies on IOTs as SCMs. Zhao et al. [
17] investigated the effects of particle size and conditioning temperature on the properties of IOTs-doped cement slurry and found that partial replacement of cement by IOTs accelerated the hydration of cement at the initial stage. Yang et al. [
18] used IOTs as concrete admixture to study the activity optimization of cementitious materials and found that mechanical activation was an effective method to improve the activity of IOTs. Han et al. [
19] studied the early hydration characteristics of composite binder containing IOTs powder and found that the early hydration of IOTs was extremely low, but the gradation of tailings and cement particles was good, and the compressive strength of IOTs was not significantly reduced by increasing the admixture of IOTs. Yao Lei et al. [
20] prepared C40 concrete with compressive strength higher than normal concrete by using IOTs as raw material after grade optimization of natural fine sand, artificial coarse sand and IOTs. However, in these studies, the activity index of the activated IOTs still did not meet the standards for blending [
21], and the tailings blending was low.
China is the largest producer of ceramics in the world and produces about 10 million tons of ceramic (CP) waste each year, which leads to a serious waste of land resources and environmental pollution. The raw materials of ceramic products are clay, quartz and feldspar, which are then fired at high temperature to form a CP after grinding [
22]. Others [
23] have shown by the test, after grinding to a certain degree of fineness of the CP has obvious volcanic ash effect, if at room temperature, moisture and with a certain degree of alkali environmental conditions, the later can be with Ca(OH)
2 secondary hydration reaction, thus generating a large number of C-S-H and C-A-H and other substances with a certain degree of cementation, and hardening to produce strength, and it has been demonstrated that CP has no significant positive effect on the early strength development of cement mortar. It mainly acts as a microfiller, resulting in a low early strength of cement mortar. However, due to the volcanic ash activity of CP [
24], the late strength development of concrete incorporated with CP was enhanced.
Fly ash (FA) is the tiny fine ash particles collected from the flue gas produced by burning coal, and is the main solid waste from coal-fired power plants. The use of FA mixed into the original concrete to replace part of the cement can optimize the performance of concrete and improve the strength of concrete, because FA can not only save the amount of raw materials, but also can play the “volcanic ash effect” and “morphological effect” and “micro-aggregate effect”. In general, FA is similar to CP in that it is less efficient in the initial stage of cementation and acts more as a microfiller, but the volcanic ash properties prove to be effective in later stages, leading to a significant increase in strength [
25]. It has been documented that FA has certain volcanic ash reactivity, and when it is mixed into cement as admixture, its active components SiO
2 and Al
2O
3 can react with Ca(OH)
2 produced during the hydration of cement clinker to produce hydration products such as C-S-H and C-A-H [
26], which is one of the main bases for FA to function in cement and concrete, respectively. Another study [
27] pointed out that FA play a “morphological effect” and “micro-aggregate effect” can play a lubricating role in the material, filling the gaps in the concrete to increase the density of concrete and improve the working properties of concrete.
IOTs, CP, and FA are three materials used as SCM and have been the subject of a lot of research, but still cannot achieve a large consumption of solid waste, and there are many problems. It has been pointed out that there is a synergy between different materials that can be used to adjust the working properties, mechanical properties and particle gradation of concrete through multiple components [
5]. This synergistic effect combining these multiple components together is more effective than using them individually. CP and FA are aluminum-rich waste, in the system of secondary hydration can produce C-A-S-H, so that the hydration products are more abundant, enhance the density of the matrix, and IOTs can fill in the pores to further improve the density, and theoretically should be able to achieve the synergistic purpose.
In this study, we study the ternary SCMs, analyze the co-hydration process of IOTs, CP, FA and cement, clarify the physical phase changes, microscopic morphological structure evolution and macroscopic mechanical property development law during the hydration process, and elucidate the effects of multi-solid waste characteristics on the hydration process and hydration mechanism of the SCM system.