materials Article Laboratory Study on Improvement of Expansive Soil by Chemically Induced Calcium Carbonate Precipitation Shaoyang Han 1,2 , Baotian Wang 1 , Marte Gutierrez 2, *, Yibo Shan 3 and Yijiang Zhang 3   Citation: Han, S.; Wang, B.; Gutierrez,M.; Shan, Y.; Zhang, Y. Laboratory Study on Improvement of Expansive Soil by Chemically Induced Calcium Carbonate Precipitation. Materials 2021, 14, 3372. https://doi.org/10.3390/ma14123372 Academic Editors: Angelo Marcello Tarantino and Jacek Tejchman Received: 28 April 2021 Accepted: 15 June 2021 Published: 18 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, 1 Xikang Rd., Nanjing 210098, China; hanshaoyang0419@163.com (S.H.); btwang@hhu.edu.cn (B.W.) 2 Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, USA 3 Geotechnical Engineering Department, Nanjing Hydraulic Research Institute, 34 Hujuguan Rd., Nanjing 210024, China; ybshan@nhri.cn (Y.S.); zhangyijiang1025@163.com (Y.Z.) * Correspondence: mgutierr@mines.edu Abstract: This paper proposes the use of calcium carbonate (CaCO 3 ) precipitation induced by the addition of calcium chloride (CaCl 2 ) and sodium carbonate (Na 2 CO 3 ) solutions as a procedure to stabilize and improve expansive soil. A set of laboratory tests, including the free swell test, unloaded swelling ratio test, unconfined compression test, direct shear test, scanning electron microscopy (SEM) test, cyclic wetting–drying test and laboratory-scale precipitation model test, were performed under various curing periods to evaluate the performance of the CaCO 3 stabilization. It is concluded from the free swell tests and unloaded swelling ratio tests that the addition of CaCl 2 and Na 2 CO 3 can profoundly decrease soil expansion potential. The reduction in expansion parameters is primarily attributed to the strong short-term reactions between clay and stabilizers. In addition, the formed cementation precipitation can decrease the water adsorption capacity of the clay surface and then consequently reduce the expansion potential. The results of unconfined compression tests and direct shear strength tests indicated that the addition of CaCl 2 and Na 2 CO 3 has a major effect on geotechnical behavior of expansive soils. Based on the SEM analyses, new cementing crystalline phases formatted by sequentially mixing CaCl 2 and Na 2 CO 3 solutions into expansive soil were found to appear in the pore space, which results in a much denser microstructure. A laboratory-scale model test was conducted, and results demonstrate the effectiveness of the CaCO 3 precipitation technique in stabilizing the expansive soil procedure. The test results indicated that the concentration of CaCl 2 higher than 22.0% and Na 2 CO 3 higher than 21.2% are needed to satisfactorily stabilize expansive soil. It is proposed to implement the precipitation technique in the field by the sequential permeation of CaCl 2 and Na 2 CO 3 solutions into soils in situ. Keywords: expansive soil; precipitation technique; calcium carbonate; soil improvement 1. Introduction Expansive soils are known to exhibit large volume changes due to moisture fluctua- tions [1,2]. These soils expand due to an increase in water content and shrink due to drying. The swelling and shrinkage of expansive soils often induce large swelling pressures that can cause severe damage to shallow engineering structures such as runways, building foundations, roads and embankments [3–5]. In the US alone, it is estimated the expansive soils cause amounting to about US$27 billion in pavement maintenance in 2013 [6]. One of the most effective solutions to mitigate the negative impacts of expansive soils is soil stabi- lization by treating and improving expansive soils to reduce their swelling and shrinkage potential. Several methods have been introduced for the treatment of expansive soil [7–9]. Chem- ical stabilization is one of the effective techniques to overcome the undesirable behavior of expansive soils. Among the chemical stabilization methods, traditional treatments such Materials 2021, 14, 3372. https://doi.org/10.3390/ma14123372 https://www.mdpi.com/journal/materials