Performance and mechanism of CO₂ carbonated slag-CaO- MgO-solidified soils

The proposed technology of CO₂ carbonation combined with industrial solid wastes is a novel attempt to replace the traditional Portland cement for soil stabilization. A series of CO₂ carbonation tests are performed on the soils solidified with slag-CaO-MgO blend, which contains the main material, industrial waste slag, and the supplementary materials, reactive CaO and MgO. The effect of binder dosage, MgO/CaO mass ratio, carbonation time and initial moisture content on carbonated-solidified soils are investigated by the unconfined compressive strength (UCS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results show that the CO₂ carbonation treatment has an obvious improving effect on the mechanical properties of soils, and the compressive strength of solidified samples can be maximally increased by 25.77 times after carbonation of 24 h. The strength of carbonated soils is proved to be positively correlated with the binder content (except 6S4L0M) and proportion of reactive MgO. The prolongation of carbonation time induces an initial increase in strength until the peak, followed by an almost constant or slight decrease, and the optimal carbonation time corresponding to the peak strength is around 6 h. The compressive strength of carbonated samples increases first to the maximum and then decreases as the initial moisture content rises, and the optimal water content at peak strength is about 16%. The carbonation efficiency of reactive MgO is significantly better than that of reactive CaO, while the low reactivity CaO contained in slag cannot largely enhance the mechanical strength of solidified soils by accelerated carbonation. CO₂ carbonation promotes the formation of carbonate crystals (CaCO₃ and MgCO₃), and the growth of these crystals is closely related to the carbonation time, binder dosage and reactivity of components. The carbonate crystals can effectively fill the interparticle pores within samples and bind firmly soil particles together, forming an overall skeleton structure and improving greatly the strength behavior.