1. Introduction
Riverside silty soft clay (SC) has difficulty satisfying construction requirements due to its high water content, relatively large pores, high compressibility and poor bearing capacity [
1,
2,
3]. Therefore, it is particularly important to solidify SC quickly and efficiently so as to improve these properties [
4,
5]. Benefiting from the advantages of high in situ clay utilization, low environmental impact and adaptability, a binder is widely used for solidifying SC [
6]. Ordinary portland cement (OPC) and lime are commonly used in the preparation of the binder to improve the properties of problematic soils [
7]. The OPC-based binder is capable of undergoing a series of physico-chemical interactions with SC to optimize the properties of solidified soils [
8,
9,
10]. However, the production process of OPC is energy-intensive and emits large amounts of pollutants as well as greenhouse gases, which poses a huge environmental burden [
11,
12]. There is an urgent need to develop an environmentally friendly binder with excellent performance to replace OPC.
The alkali-activated binder consists of an alkali activator, precursors and auxiliary materials. Precursor particles, mainly calcined natural minerals (e.g., metakaolin (MK)) or industrial solid wastes (e.g., granulated blast furnace slag (GBFS) and fly ash (FA)), contain reactive silica–alumina oxides that can be stimulated by alkali activators to produce C-S-H and C(N)-A-S-H gels through a series of dissolution–diffusion–polymerization reactions [
13,
14]. These gels are able to connect the soil particles and fill the pores to improve the microstructure of SC, thus improving the properties of SC [
13,
15]. Furthermore, the alkali-activated binder reduces carbon emissions by 80% during production compared to the traditional OPC-based binder [
16]. Researchers are gradually focusing on the use of the alkali-activated binder as an alternative to OPC for solidifying SC. Lang et al. [
17] explored the use of an alkali-activated binder instead of OPC for solidifying dredged sludge (DS) with different water contents. The results of their study showed that the change in water content significantly affected the unconfined compressive strength (UCS) of the solidified DS. Composite activators were more effective than single activators in activating GBFS to achieve a higher UCS of solidified DS. Murmu et al. [
18] investigated the feasibility of solidifying black cotton soil (BCS) using FA geopolymer. The results showed that FA geopolymer exhibited a good solidifying effect on BCS, even at a lower alkali solution concentration (5 M sodium hydroxide). Zhang et al. [
19] verified the feasibility of MK-based geopolymer as a binder for solidifying SC from multiple perspectives. The results indicated that with increasing geopolymer concentrations, the compressive strength, failure strain and Young’s modulus of the solidifying SC specimens increased, and shrinkage strains during curing decreased. Miraki et al. [
20] investigated the potential of using an alkali-activated GBFS-volcanic ash (VA) binder for solidifying SC from multiple perspectives. The results showed that the combination of VA and GBFS provided sufficient calcium, silicon and aluminum to promote the formation of N-A-S-H and C-(A)-S-H gels. The incorporation of GBFS resulted in remarkably superior resistance against wet–dry and freeze–thaw cycles, as well as low carbon footprints. Chen et al. investigated the effect of reactive ions on the early properties of rice husk ash (RHA)-solidified soils. The results showed that the use of RHA alone could only improve the UCS of soft soils to a certain extent, but not the soaked strength of solidified soils. The UCS, shear strength and soaked strength of the solidified soils were significantly improved by modifying RHA using calcium carbide slag and MK as activators. From the above studies, it can be seen that the use of the alkali-activated binder for solidifying SC has been fruitful, but most of the studies have focused on a few materials. Broadening the pipeline of precursor materials for the preparation of an alkali-activated binder is promising [
12].
Coal gangue (CG), a type of solid waste separated from coal mining and washing, is a mixture of various rocks. China’s CG annual emissions account for about 25% of all categories of industrial solid waste [
21]. In 2021 alone, 743 million tons of CG were generated, of which nearly one-third spontaneously combusted under natural conditions to form self-combusted coal gangue (SCCG) [
22,
23]. It has been reported that the spontaneous combustion process can stimulate the activity of silicon oxide and aluminum oxide in CG to some extent [
24,
25]. Li et al. [
24] prepared a SCCG-based alkali-activated foam with low bulk density, high total porosity, acceptable compressive strength and low thermal conductivity using the fast microwave foaming method. Liu et al. [
26] compared SCCG-based/calcined CG-based geopolymer concretes, and the results showed that SCCG-based geopolymer concretes had good mechanical properties with good economic and cost benefits. Qin et al. [
27] developed an OPC-CG blend paste by carbonation curing. The results showed that carbonation curing improved the compressive strength of OPC-CG pastes with a higher incorporation of CG. The specimens also showed better resistance to chloride ion penetration. The annual production of PS in China has reportedly reached 8 million tons [
28]. The accumulation of PS not only leads to the waste of resources but also affects the ecological environment. Therefore, a large amount of PS is in urgent need of resource utilization. Thanks to the fact that the main chemical components of PS are SiO
2 and Al
2O
3, it has the potential to be a precursor [
29]. Wang et al. [
30,
31] systematically investigated the relationship between hydration, microstructure and compressive strength of alkali-activated PS. The results showed that the alkali activator content and modulus of the alkali activator had both positive and negative effects on PS hydration, and the overall effect depended on their relative magnitudes. The hydration of alkali-activated PS experienced a transformation of NASH first into CNASH (low Ca) and then into CNASH (high Ca) due to the stronger polarization ability of Ca
2+ over Na
+, and the gel transformation was accompanied by dealumination. Yang et al. [
28] found that the poor performance of the PS-based geopolymer was due to its aluminum deficiency. The results also indicated that ultrafine FA and more activators contributed to the Al and high alkalinity environments, which positively induced the production of more geopolymer gels, thus releasing more heat and optimizing the pore structure of the matrix. A similar conclusion was reached in the study of Zhang et al. [
32]. PS mixed with GBFS to prepare alkali-activated composite cementitious materials can overcome the problem of the poor early compressive strength of PS-based geopolymers.
This literature review demonstrates that the utilization of the SCCG/PS-based geopolymer is mainly focused on construction. Relatively few studies have been conducted on the preparation of an alkali-activated binder using SCCG or PS for application in solidifying SC. Based on this, the aim of this paper is to investigate the feasibility of developing a SCCG-GBFS-PS (SGP) alkali-activated ternary binder using SCCG, GBFS and PS for solidifying SC. The range analysis of the orthogonal experiment was first used to optimize the alkali activator content, modulus of the alkali activator and mass ratio of the GBFS and PS of the SGP ternary binder. The effects of the SGP ternary binder content, initial water content of SC and types of additives on the 7 d and 28 d USC of solidified soils were subsequently explored. Finally, the solidifying mechanism of the SGP-solidified soil was analyzed by conducting microstructural property analysis.
4. Solidification Mechanism of the SGP-Solidified Soil
The solidification mechanism of the SGP-solidified soil was mainly attributed to the geopolymerization and ion-exchange reactions of the SGP-solidified soil. Increasing the dosage of the SGP ternary binder, adjusting the initial water content of SC and mixing additives were conducive to increasing the degree of hydration of the SGP-solidified soil to form more gels, while the hydration products can promote the exchange of ions and enhance the agglomeration effect of the soil particles, thus improving the strength.
Geopolymerization reaction of the SGP-solidified soil: firstly, under the alkaline environment provided by Na2SiO3 and NaOH, the Si-O-T (T = Si or Al) of active SiO2 and Al2O3 in the SGP ternary binder and SC were broken, so that the silicon–oxygen tetrahedron and aluminum–oxygen tetrahedron were released into the system. Then, oligomers were formed through ionic polymerization and dehydration condensation. Meanwhile, under the excitation of the alkali activator, the content of Na+ ions in the system increased, and GBFS and PS released a large amount of Ca2+, which further recombined with the oligomers to generate gels such as hydrated C-S-H, N-A-S-H and C-A-S-H gels. Moreover, excess Ca2+ will combine with OH− to produce Ca(OH)2 flake crystals, which will absorb CO2 from the air in an alkaline environment to carbonize to produce CaCO3 crystals. The Ca2+ in the system would replace part of the Na+ in the N-A-S-H gel product by ion exchange, thus promoting the conversion of N-A-S-H gels to C(N)-A-S-H gels. The gels generated by hydration bound the soil particles, filled the pores, enhanced the compactness of the soil and thus promoted the development of UCS.
Ion exchange reaction of the SGP ternary binder: Ca2+ dissolved from CaO in the SGP ternary binder was enriched on the surface of soil particles, replacing Na+ and K+ ions adsorbed on the surface of soil particles, reducing the thickness of the double electric layer, decreasing the repulsive force between soil particles and making the connection between soil particles more tightly connected, which promoted the agglomeration and flocculation of soil particles, ultimately decreasing the porosity of the soil body and optimizing the strength of the SGP-solidified soil.
5. Conclusions
In this paper, a novel SGP ternary binder was prepared by using SCCG, GBFS and PS. Firstly, alkali activator content, modulus of alkali activator and mass ratio of GBFS and PS were optimized by orthogonal experiments. Then the effects of the SGP ternary binder content, initial water content of SC and types of additives on the UCS of the SGP-solidified soil were analyzed. Finally, the solidification mechanism of the SGP ternary binder on SC was investigated by combining XRD analysis and SEM-EDS analysis. The main conclusions are shown below:
(1) The results of orthogonal experiments showed that the mass ratio of GBFS and PS was the main factor affecting the USC, and the alkali activator content and modulus of the alkali activator were the secondary factors, among which the modulus of the alkali activator had the least effect on the strength. The optimal combination of the SGP ternary binder was A2B2C3, i.e., the alkali activator content was 13%, the modulus of the alkali activator was 1.3 and GBFS:PS = 2:1. At this time, the 7 d and 28 d UCS were 2.048 MPa and 2.462 MPa, respectively.
(2) When the SGP ternary binder content was 16% and the initial water content of SC was 35%, the 28 d USC of the SGP-solidified soil reached up to 3.29 MPa, which met the requirements of the CJJ/T286-2018 “Technical Standards for Application of Soil Curing Agents” for the UCS of tertiary cured soil.
(3) The incorporation of TEA and PVC improved the UCS of the SGP-solidified soil, while the incorporation of Na2SO4 significantly deteriorated the UCS of the SGP-solidified soil. This was because the Na+ concentration in the SGP-solidified soil increased after Na2SO4 was incorporated, and after a large amount of Na+ was adsorbed on the surface of the negatively charged soil particles, the thickness of the soil particle bilayer became thicker, and the adhesion between the soil particles was weakened, which led to the structure of the SGP-solidified soil becoming loose. In contrast, the TEA and PVC were able to play the roles of electrical neutralization and adsorption bridging, with adsorption and complexation of ions in solution, which promoted the continuous dissolution and hydration of SiO2 and the improvement of UCS.
(4) From the results of XRD and SEM-EDS tests, it can be seen that the C-S-H gels and C(N)-A-S-H gels generated by the hydration of the SGP-solidified soil interpenetrate, intertwine and adhere to each other to form a network-like agglomeration structure, which was capable of filling the inter-pore spaces between the soil particles while cementing the soil particles, and the UCS of the SC was enhanced. The generation amount of hydration products, the degree of development, uniformity and the degree of soil densification together determined the size of the strength of cured soil.