1. Introduction
Most areas of Caofeidian (Tangshan Caofeidian New District in Hebei Province, China) were formed by land reclamation. The reclamation area has reached 380 km2. The common method of shallow foundation treatment in Caofeidian area is the squeezing of silt with broken rocks. This can straightforwardly damage underground pipelines. Furthermore, a large amount of broken rocks is required, and these need to be transported from places that are 200 km away. Both the breaking of mountains to extract stones or rocks and the long-distance transport are hazardous to the environment and uneconomical. In addition, the dredger soil in this area was extracted from the offshore sand, which has a large salt content. The groundwater level in this area is high, and the seawater erosion is significant, which results in severe soil salinization. As a result, the engineering characteristics of dredger soil, such as short consolidation time, high water content, and salinization, have significantly hindered the construction and development of the Caofeidian area project.
Recently, soil solidifying agents have been developed and widely used in the construction of infrastructure such as ports, roads, and water conservancy facilities because of their remarkable properties, low prices, and good treatment effects [
1].
Lime and cement were the earliest solidifying agents used by humans and are widely applied worldwide. These agents have been applied and studied by many researchers. De Brito Galvao et al., Elhagwa et al., and Nafi Abdel et al. used lime to solidify clay [
2,
3,
4]. Rajasekaran et al., Rajasekaran, and McCarthy et al. used lime to strengthen sulphated coastal clay [
5,
6,
7,
8,
9]. Miller et al. conducted experimental research (on-site and laboratory) on cement kiln dust (CKD) as a soil stabilizer [
10]. Chan et al. studied dredged marine clay solidified with cement and fly ash and other binders [
11]. In addition, certain new solidifying materials have been examined by a few researchers: Bell investigated the use of lime-combined PFA (pulverised fuel ash) and cement-combined PFA to solidify clay soil [
12]. Attom et al. proposed the use of burned olive waste as a new soil solidifying agent [
13].
However, it has been demonstrated that conventional solidifying agents have a low solidifying effect on coastal dredger soils with high water content and salinization. Sabry et al. observed that the strength of cement-solidified soil increases slowly in a saline environment over the long term, and that durability is reduced significantly owing to the corroding effect of salt [
14]. Omar Saeed et al. demonstrated that the use of lime as solidifying agent cannot effectively improve the strength and durability of dry saline soil [
15]. Therefore, a few researchers improved conventional solidifying agents with additives for saline environments. For example, Wild et al. used ground granulated blast-furnace slag (GGBS) to replace part of the lime to solidify sulphate-containing Kimmeridge clay [
16]. The results revealed that the strength of solidified clay soil was improved significantly. Hossain et al. proposed that industrial wastes such as lime, volcanic ash, and their mixtures are good solidifying materials for saline clay soil [
17]. Kamon et al. proposed that industrial waste slag mixed with aluminium slime can enhance the early strength of saline soil [
18].
Simultaneously, X-ray diffraction (XRD), scanning electron microscopy (SEM), and other microscopic test methods have been applied in different research studies to clarify the mechanism of saline soil solidification and to select good solidifying agents. For example, based on these microscopic analyses, Suryavanshi et al. discussed the formation mechanism of Friedel’s salt (3CaO·Al
2O
3·CaCl
2·10H
2O) and the analogues in C
3A-rich (3CaO·Al₂O₃) cement soil [
19]. Huang et al. considered that the expansion of AFt (3CaO·Al₂O₃·3CaSO
4·32H
2O) plays a dual role: it can simultaneously fill pores to solidify soil and destroy the solidified soil structure formed by calcium silicate hydrate (CSH) [
20].
According to previous researches, soil-solidifying agents are used widely. However, the conventional method of using a single solidifying agent has a low solidifying effect on saline soil. Composite solidifying agents containing industrial waste and other cementing materials have a better solidifying effect on saline soil. Nevertheless, the solidifying mechanism has been studied inadequately, and there is an urgent need to investigate the components and their proportions for composite solidifying agents for saline dredger fill. The verification of certain hydration products (such as CSH, AFt, and Friedel’s salt) during the solidification of this soil is also highly worthwhile.
Therefore, in the present study, a composite solidifying agent (slag is used as the main solidifying agent and sodium silicate, quicklime, and gypsum powder are used as additives) is proposed to improve the strength of the Caofeidian saline dredger fill. The optimal mixture ratio of the composite solidifying agent was determined by unconfined compression tests. Finally, the hydration products and solidifying mechanism of the composite slag solidifying agent were investigated based on SEM scanning and XRD diffraction tests.
5. Mechanism Analysis of Composite Slag Solidifying Agent
Based on the results of the microstructure investigation, it can be inferred that the analysis of the mechanism of a composite slag solidifying agent is essentially the clarification of the process of formation of CSH, AFt, and calcium aluminate chloride sulphate hydrate.
In the solidified soil, the source of CSH is the hydration of composite slag solidifying agent and the reaction of SiO2.
Based on the XRD results of unsolidified soil, it can be stated that SiO
2 was a main component in the dredger fill. In addition, the slag was essentially composed of SiO
2, Al
2O
3, CaO, and other oxides. NaOH and Ca(OH)
2 from sodium silicate and quicklime hydrolysis increased the pH value of the sample. Next, SiO
2 and other oxides were activated. Finally, CSH was obtained. The reactions are expressed as:
- 2.
AFt
In the solidified soil, AFt (3CaO·Al
2O
3·3CaSO
4·32H
2O) was formed by the transformation of CAH. As introduced in
Section 4.2.4, CAH was obtained in the solidified soil with slag and quicklime. The formation of the CAH was similar to that of CSH: Al
2O
3 was activated in an alkaline environment and combined with Ca(OH)
2 to form CAH colloids. The reactions are expressed as follows:
Next, AFt is formed by the combination of CAH and sulphate ion, which is obtained from gypsum powder:
- 3.
Calcium aluminate chloride sulphate hydrate
Calcium aluminate chloride sulphate hydrate formed from the interaction between the composite slag solidifying agent and the soluble salt in the saline dredger fill. As presented above, a certain amount of CAH combined with sulphate ions in the gypsum powder to form AFt. Part of the remaining CAH reacted with Cl
− in the saline soil to form Friedel’s salt (Fs: 3CaO·Al
2O
3·CaCl
2·10H
2O). Next, the sulphate ions from the gypsum powder continuously replaced part of the chloride ions in Friedel’s salt in the sulphate ion-enriched environment to eventually yield calcium aluminate chloride sulphate hydrate. The reactions are expressed as follows:
The NaOH generated during the reaction, which provided an alkaline environment, further promoted the hydration of the slag powder and maintained the reactions. Therefore, soluble salts (such as Cl−) in saline dredger fill can be reduced effectively and, in turn, the solidified soil can exhibit good road performance.
To summarize, the mechanism of composite slag solidifying agent for saline dredger fill is essentially the formation of CSH, AFt, and calcium aluminate chloride sulphate hydrate. CSH is a colloid, which has a good gelling property. It can fill voids in soil, thereby increasing the bonding force between grains, and cover the surface of grains to produce an overall structure. AFt is a needle-bar crystal, which can fill voids to improve the compactness of soil. The intersecting needle-bar crystal structures combine with CSH to obtain a spatial network structure, which plays a good supporting role in soil. It causes voids to be thinner or even disappear. The effect of AFt formation can compensate for the shortage of CSH and further improve the strength of solidified soil. In addition, CAH can combine with SO42− and Cl− in the soil to form calcium aluminate chloride sulphate hydrate, which can effectively reduce the content of soluble salts in saline dredger fill and substantially improve the engineering characteristics of solidified soil.