**1. Introduction**

Cemented soil has the advantages of high compressive strength [1], low permeability, and low price. It has been widely used in projects such as soft soil foundation reinforcement, high-grade highway cushions, and seepage prevention in small reservoirs, among other projects, and has achieved good engineering benefits [2]. However, in practical engineering, it was found that coastal cemented soil (CS) cannot meet the mechanical requirements of some large-scale and special coastal engineering projects. For example, Liu et al. studied the heterogeneity in strength and Young's modulus of cement-admixed clay slabs using random finite-element analyses, considering three sources of variation: namely, a deterministic radial trend in strength and Young's modulus; a stochastic fluctuation component due to non-uniform mixing; and positioning errors arising from off-verticality of the mixing shafts [3]. Li et al. studied the influence of the number of freeze–thaw cycles and curing ages on the mechanical properties of ordinary cemented clay and polypropylene fiber-cemented clay [4]. Liu et al. examined the interaction between the spatial variations in binder concentration and in situ water content, in cement-mixed soil, using field and model data as well as statistical analysis and random field simulation [5]. The results of these studies suggest that it is necessary to add additives to modify the coastal cemented soil. On the other hand, as an industrial by-product, the annual output of iron tailings in China is huge. The accumulation of iron tailings not only takes up farmland and pollutes

**Citation:** Song, X.; Xu, H.; Zhou, D.; Yao, K.; Tao, F.; Jiang, P.; Wang, W. Mechanical Performance and Microscopic Mechanism of Coastal Cemented Soil Modified by Iron Tailings and Nano Silica. *Crystals* **2021**, *11*, 1331. https://doi.org/ 10.3390/cryst11111331

Academic Editors: Shujun Zhang, Cesare Signorini, Antonella Sola, Sumit Chakraborty and Valentina Volpini

Received: 6 October 2021 Accepted: 28 October 2021 Published: 31 October 2021

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the environment, but also tends to cause dangerous accidents such as collapses with the increasing of the accumulation height. The use of iron tailings to make composite cemented soil, and at the same time adulterating new admixtures to meet the requirements of engineering mechanics, can not only reduce environmental pollution [6], but also save building materials so as to achieve the dual benefits of economic efficiency and environmental protection, which is a very effective way to achieve sustainable development [7].

Extensive research studies on TCS have been conducted by scholars at home and abroad. Lucas et al. compared the effects of cement, lime, and slag composite iron tailings as roadbed fillers, and found that cement was the most effective stabilizer of iron tailings through unconfined compressive tests [8]. Bastos et al. found through CBR tests that cement composite iron tailings had satisfactory physical and mechanical properties, and thus, were suitable for road bases under most traffic loads with good durability [9]. Through compressive and flexural tests, Hou Rui et al. found through compressive and flexural tests that the compressive and flexural strengths of TCS increased gradually with age, but increased slowly in the later stage. When the iron tailing content was less than 25%, the mechanical properties of TCS were slightly increased compared with that of CS [10].

In recent years, many scholars have tried to develop new engineering materials by compounding various curing agents with iron tailings. Generally, cement can be used to improve the mechanical properties of iron tailings. Tanko et al. studied the performance of cement concrete mixed with fine aggregate of iron tailings [11]. Bao et al. studied the toughening properties of sand cement-based composites of high-performance environmentally friendly tailings [12]. Simonsen et al. studied mine tailings' potential as supplementary cementitious materials based on their chemical, mineralogical and physical characteristics [13]. Long et al. carried out research on the mechanical properties and durability of coal gangue-reinforced cement–soil mixture for foundation treatments [14]. On this basis, lime, slag, fiber, etc. can be added to further improve the mechanical properties of cement-modified iron tailings. Consoli et al. studied fiber-reinforced sand-coal fly ash-lime-NaCl blends under severe environmental conditions [15]. Feng et al. studied lime-and cement-treated sandy lean clay for highway subgrade in China [16]. Tebogo et al. studied mechanical chemically treated and lime-stabilized gold mine tailings using unconfined compressive strength [17]. Kumar et al. studied the use of iron ore tailings, slag sand, ground granular blast furnace slag, and fly ash to produce geopolymer bricks [18]. Falayi et al. conducted a comparative study on the mechanical properties of geological polymers of gold mine tailings modified by fly ash and alkaline oxygen slag [19]. Festugato et al. studied the cyclic shear behavior of fiber-reinforced mine tailings [20]. Chen et al. studied the compressive behavior and microstructural properties of tailings polypropylene fiber-reinforced cemented paste backfill [21]. Cristelo et al. studied the effect of fiber on the cracking behavior of cement-stabilized sandy clay under indirect tensile stress [22]. Lirer et al. studied the strength of fiber-reinforced soils [23]. Among them, cement is the traditional building material with the best modification effect, but it still cannot fully meet the engineering needs. Therefore, it is urgent to find a composite admixture to improve the engineering characteristics of TCS. Nanomaterials have the characteristics of large specific surface area, small particles, and high activity. Therefore, they have been introduced into the engineering exploration of TCS. For example, Ghasabkolaei et al. summarized the geotechnical properties of soil modified by nanomaterials [24]. Wang et al. studied characterization of nano magnesia–cement-reinforced seashore soft soil by direct-shear test [25]. Yao et al. studied effect of nano-MgO on the mechanical performance of cement-stabilized silty clay [26]. It has been found through various studies that nanomaterials can fully exert their own activity in cement-based materials, promote the process of cement hydration reaction, and fully fill the internal pores of cement-based materials at the microscopic scale, thus achieving the purpose of improving the engineering properties of cement-based materials. For example, Zheng et al. studied strength and hydration products of cemented paste backfill from sulfide-rich tailings using reactive MgO-activated slag as a binder [27]. Liu et al. studied the effect of graphite tailings as a

substitute for sand on the mechanical properties of concrete [28]. Sarkkinen et al. studied the efficiency of MgO-activated GGBFS and OPC in the stabilization of highly sulfidic mine tailings [29]. Through an indoor unconfined compressive test combined with a microscopic test, Li et al. found that the use of nano clay instead of cement in iron tailings can effectively improve its compressive strength. The addition of nano clay makes the microstructure surface of TCS change from flake to block; the granular structure becomes tightly cemented, and the whole structure tends to be stable [30]. Nano clay can improve the mechanical properties of TCS and can guide the resource application of iron tailings to a certain extent. It is feasible to apply nanomaterials to engineering practice. Due to its unique physical properties, nano silica has also recently been widely used in geotechnical engineering, for example, in modified calcareous sand in the South China Sea [31] and in stabilized coastal silty clay [32].

Therefore, the research on STCS has important theoretical significance and engineering application value, and microstructure has a certain influence on the mechanical properties of materials [33]. In this paper, the optimal iron tailing content was explored through unconfined compressive tests, the modification effect of nano silica on TCS was studied, and the strength growth mechanism of STCS was analyzed in combination with microscopic tests.

### **2. Test Overview**

#### *2.1. Test Materials*

The soil used in this experiment was collected from the coastal soft subgrade soil excavated in a certain expressway section in Shaoxing city, Zhejiang. The physical and mechanical indexes are shown in Table 1, and the microscopic characteristics are shown in Figure 1. The main chemical elements of the soil are Si, O, Mg, and Al. The natural state of the soil is a soft plastic state, with high porosity and organic matter content. The iron tailings were taken from Lizhu iron tailings pond in Zhejiang Province. The physical and mechanical indexes are shown in Table 2, and the microscopic characteristics are shown in Figure 2. The main chemical elements of iron tailings are Si, O, Mg, and C, and their particles are small and single. PO 32.5 Conch brand Portland cement, produced in Shangyu, Shaoxing, was used in this test. The physical and mechanical indexes are shown in Table 3, and the microscopic characteristics are shown in Figure 3. The cement's performance meets requirements and it can be used to modify soft soil. Ordinary industrial nano silica was used in this test. The physical and mechanical indexes are shown in Table 4, and the microscopic characteristics are shown in Figure 4. The nano silica used in the test is pure, its microstructure is dense, and it has a large specific surface area.

**Table 1.** Physical and mechanical indexes of coastal soft soil.


**Table 2.** Physical and mechanical indexes of iron tailings.


**Figure 1.** (**a**) SEM, (**b**) EDS. Microscopic characteristics of coastal soft soil.

**Figure 2.** (**a**) SEM, (**b**) EDS. Microscopic characteristics of iron tailings.

**Figure 3.** (**a**) SEM, (**b**) EDS. Micro characteristics of Portland cement.


**Table 3.** Physical and mechanical indexes of cement.

**Table 4.** Physical and mechanical indexes of nano silica.


**Figure 4.** (**a**) SEM, (**b**) EDS. Microscopic characteristics of nano silica.

#### *2.2. Test Scheme*

According to the application scenarios of cemented soil in actual projects, the moisture content of soil was set at 80%. The test was divided into two parts. First, the mechanical properties of TCS were studied, and the optimal iron tailing content was obtained. The cement content was set at 30% and the iron tailing content was set at 0, 10%, 20%, 30%, and 40%, respectively, to produce the TCS samples. Next, on the basis of the optimal iron tailing content, nano silica at contents of 0.5%, 1.5%, and 2.5% was added continuously to investigate the modification effect of nano silica on TCS. The content of cement, iron tailings and nano silica refers to the mass ratio in relation to dry soil. The test ages of all samples were 7d. The material composition and specific symbols of the samples are shown in Figure 5, and the specific test scheme is shown in Table 5.

**Figure 5.** Material composition and specific symbols of the sample.


**Table 5.** Test scheme for STCS.

Note: In STCS-Y-Z, Y represents the content of nano silica, Z represents the content of iron tailings, and the X represents the optimal content.

### *2.3. Sample Preparation and Maintenance*

According to the requirements of the Chinese "GBT 50123-2019" standard, the main process of the sample preparation is shown in Figure 6.

(**a**) (**b**)

(1) An appropriate amount of wet soil was placed into the mixer, and mixed well; the cement slurry was prepared according to the test mix ratio, then poured into the mixer and mixed evenly, as shown in Figure 6a.

(2) Iron tailings and nano silica were added into the mixer in turn, and were stirred twice, for 4 minutes each time, to ensure that the test materials were fully and evenly mixed. The mixed materials that had been stirred were put into a grouting bag, and were then added to the standard mold three times to ensure the same height each time. The diameter of the mold was 39.1mm, and the height was 80mm. After each feeding, manual vibration was required for about 40 times until it was dense, as shown in Figure 6b.

(3) The sample was left to stand for about 2h. After it was initially set, both ends of the molds were removed and smoothed with a spatula, as shown in Figure 6c. The metal clip outside the test mold was mainly used to apply tightening force to the test mold to ensure the forming of the sample. When demolding the sample, the metal clip was first removed.

(4) Filter paper and rubber bands were used to bind both ends of the sample, and then put in water for curing, as shown in Figure 6d.

(5) After reaching 7d, the sample was removed from the mold using the special demolding tool, as shown in Figure 6e. Then, the sample was placed in a safe place and prepared for subsequent tests, as shown in Figure 6f.

#### **3. Unconfined Compressive Test Analysis of TCS**

#### *3.1. Stress–Strain Curve Analysis*

Five repeated tests were carried out for each group of samples, and five stress–strain curves were obtained. The stress–strain curves of TCS with different iron tailing contents are summarized in Figure 7, which are all softening curves.
