Investigation on performance and mechanism of CO2 carbonated slag/fly ash solidified soils

Under the situation of increasingly serious global climate issue and increasingly urgent treatment of industrial solid wastes in China, how to effectively store CO2 and to use industrial wastes to prepare novel cementitious materials attracts significant attention from all around the worldwide. The combination of CO2 carbonation and industrial waste residues is an environmentally friendly and sustainable solidification technology, which cannot only sequester permanently CO2 emissions but make effective use of industrial residues and solidify soils. In order to explore the effect of many factors such as industrial residue content, carbonation mode and carbonation time on the carbonation effect, two kinds of bulk industrial wastes, i.e. slag and fly ash, were selected as binding materials to mix with soils and prepare samples for CO2 carbonation tests. The unconfined compressive strength(UCS) and pH detection tests were carried out to evaluate the mechanical properties of carbonated samples, and the X-ray diffraction(XRD), mercury intrusion porosimetry(MIP) and scanning electron microscopy (SEM) tests were performed to reveal the microstructure evolution and carbonation mechanism. The test results show that the compressive strength of samples subjected to CO2 carbonation of 3 h raises by 5%~15%, and increases initially with waste residue content followed by a slight reduction or an almost constant. The carbonation effect varies greatly under different carbonation modes, and the optimal carbonation mode is proved to be confining pressure of 300 kPa and carbonation pressure of 150 kPa. The compressive strength of carbonated samples increases to the maximum at 3-6 h and then decreases as the carbonation time extends. The microscopic results reveal that CaCO3 crystals are formed and identified in the forms of aragonite and calcite within carbonated residue-solidified samples. These crystals can fill in the pore spaces and bond fine particles together, which without doubt contributes to the promotion of the compressive strength of carbonated samples. A reasonable prolongation in carbonation time produces more CaCO3 crystals and reduces the cumulative pore volume and quantity of large pores. However, the carbonate crystals are loosely arranged and fail to be firmly interconnected to form a reticulated skeleton skeletal structure, which leads to a limited improvement in the compressive strength of carbonated residue-solidified samples. This study can provide a preliminary basis for further research on the performance improvement of novel solidification technique combined industrial residue and CO2 carbonation. © 2020, Science Press. All right reserved.