Stabilization of Highly Expansive Black Cotton Soils by Means of Geopolymerization

Abstract

This article presents a method of geopolymerizing black cotton soil (BCS), a type of highly expansive soil, to ascertain its potential use in subgrades. The BCS is stabilized by alkalis of calcium hydroxide [Ca(OH)2] or potassium hydroxide (KOH), and cementitious geopolymers are produced. Parameters of Atterberg limits, maximum dry density (MDD), optimum moisture content (OMC), unconfined compressive strength (UCS), and swelling percentages are measured. The combination of volcanic ash and alkalis reduces the plasticity index greatly (34.8%→14.2%). Potassium hydroxide is found to be more efficient in solidifying the BCS than Ca(OH)2. The swelling percentage of BCS can be decreased from 15.7% to 2.3–4.2%. The mechanical strength has an increasing trend, and the UCS reaches 16.55 MPa after 90 days. Characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscope, and Fourier-transform infrared spectroscopy are used to reveal the mechanism. In the first stage the soil is fully swelled in the presence of alkaline solutions. Polycondensation occurs in the subsequent stage, and the volume shrinkage is irreversible after geopolymerizaiton. The solidification is found to be related not only to the physical interactions but to chemical bonds as Si-O-Si(Al) or Al-O-Al(Si) constructed by the free species of Si- and Al-, which are verified to be leached from clays. The phase transformation is confirmed by the formation of zeolitic frameworks and the K-illitization observed in the KOH activated specimens.