**1. Introduction**

Nowadays, with the rapid development of infrastructure construction, there are fewer land resources available. In the process of building construction, soft soil foundations with lower strength are often encountered. Cement and lime [1–4] are often used to improve the bearing capacity of soft soil foundations to make them meet the needs of different engineering fields, such as highways, slopes and residential construction [5,6]. However, soft soil has many undesirable engineering properties, such as high compressibility and high porosity and obvious thixotropy and creep properties [7], which require a large amount of cement to solidify the soft soil foundation. Moreover, CO2 produced by cement production accounts for 5–7% of the total amount of human-generated CO2 [8]. Therefore, in order to reduce the dosage of cement and CO2 emissions, many scholars have found suitable materials and methods to further improve cement soil on the basis of adding new engineering materials as a curing agen<sup>t</sup> [9–12].

Nanomaterials are widely used in the field of construction engineering due to their characteristics of small particle size, large surface area and good stability. Commonly used nanomaterials are nano-SiO2 [13–15], nano-clay [16–18] and nano-MgO. Among them, nano-MgO is mainly made by sintering magnesite at 700 ◦C, while the temperature of cement in the production process is above 1450 ◦C [19]. Therefore, using nano-MgO to replace part of cement can also reduce energy consumption [20]. Gao et al. [21,22] explored through tests that 6% nano-MgO content could significantly increase the compressive strength and stability of clay, and better improve the shear strength and cohesion of the modified cement soil. However, it had little influence on the internal friction angle.

**Citation:** Wang, W.; Zhou, H.; Li, J.; Tao, F.; Li, C.; Qian, B.; Jiang, P. Influence of Carbonization Process on the Mechanical Properties of Nano-MgO Modified Cement Soil. *Sustainability* **2021**, *13*, 3558. https://doi.org/10.3390/su13063558

Academic Editor: Yeou-Fong Li

Received: 10 February 2021 Accepted: 21 March 2021 Published: 23 March 2021

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Moradpour et al. [23] found that the compressive strength and flexural strength of cementbased materials was significantly improved with 1.0% nano-MgO content. In addition, nano-MgO had expansibility, and the microstructure of nano-MgO-modified cement-based materials was denser than the ordinary cement-based materials. Wang et al. [24] found that under the erosion of sulfuric acid solution, the shear strength of modified cement soil reached the maximum with 0.5% nano-MgO content. Yao et al. [25,26] investigated through unconfined tests that the addition of nano-MgO could better improve the strength of cement soil. Moreover, the cohesion of modified cement soil reached the maximum, with the 1.0% nano MgO content, and the internal microstructure was more compact. Yuan et al. [27] and Hou et al. [28] found that the addition of nano-MgO would form a large amount of fluffy and acicular aluminate hydration products inside the cement paste, which could effectively fill the larger pores inside the cement paste and improve the stability of the structure.

As CO2 will lead to the corrosion of steel bars, research mainly focuses on the prevention and control measures of the carbonation of concrete [29–31]. However, the corrosion of steel bars is not considered in solidified soft soil, so CO2 can be injected into solidified soft soil to improve its engineering performance. Moreover, CO2 could react with hydration products to consume CO2 and reduce greenhouse gas emission. Yi et al. [32] found that after carbonization, the unconfined compressive strength of solidified soil with 5% MgO content was about 200% of that of ordinary cement solidified soil. However, with 10% MgO content, the increase in strength decreased. Cai et al. [33] found that with the increase in carbonization time, the number of cracks on the surface of MgO solidified sand gradually increased. Vandeperre et al. [34] found that MgO generated rankinite after carbonization, which had a larger volume expansion rate and could fill the pores of the mixture, so as to improve the strength and toughness of the mixture. Mo et al. [35] found that under the action of carbonization, the pore size and porosity of MgO-modified cement paste decreased, while the apparent density and microscopic density increased.

In sum, the research on the mechanical properties of nano-MgO-modified cement soil has been gradually carried out. However, there are few studies on the mechanical properties of the modified cement soil by carbonization, which requires further exploration. In this paper, on the basis of cement soil with 20% mass fraction of cement, the unconfined compressive strength tests were carried out on the modified cement soil with different nano-MgO contents and carbonized times. From the angle of strength and ductility, the influence of carbonization time on the mechanical properties of the nano-MgO-modified cement soil was investigated.

#### **2. Test Materials and Preparation**
