Development of magnesium slag with high carbonation efficiency under microwave action: Phase evolution, microstructure and carbonation mechanism

Carbonation of calcium silicate-containing minerals for the preparation of building materials has received widespread attention, but this technology still faces the challenge of low carbonation efficiency. In this study, the carbonation efficiency of magnesium slag (MS) was greatly improved by utilizing microwave, a green and efficient method, at a certain CO2 pressure and temperature. The effects of reaction time, temperature, and CO2 pressure on the carbonation efficiency and the product characteristics were investigated. The kinetics of the carbonation reaction and the formation process of the CaCO3 were revealed. Results show that the reaction temperature and CO2 pressure are the most important factors affecting the carbonation efficiency of the MS and the types of products formed. When the temperature is lower than 80 degrees C, the carbonation efficiency is less than 50 % under atmospheric pressure, and a large number of unreacted gamma-C2S remains in the MS. The gamma-C2S and beta-C2S in the MS are completely transformed to CaCO3 under the pressurized carbonation when the temperature is 140 degrees C or above. The needle-like aragonite with good crystallinity is obtained at 140 degrees C under pressurized conditions. An increase in temperature facilitates the transformation from needle-like aragonite to block-like calcite. The block-like calcite dominates in the products when the temperature reaches 160 degrees C or above. The formation process of CaCO3 under pressurized carbonation can be described well by the surface coverage model, and the activation energy (Ea) for this reaction is approximately 34.36 kJ/mol. These indicate that the rate-controlling step for the carbonation process is the CO2 diffusion. The optimal carbonation efficiency of the MS reaches approximately 89.8 %, and the maximum CO2 fixation amount reaches 459.4 kg/t. In the carbonation process, the SiO44- in the C2S reassembles into an amorphous SiO2 with a three-dimensional network structure dominated by Q4 and Q3 structure. The high CO2 fixation capacity during carbonation offers a practical approach to reducing carbon emissions in the construction industry. This advancement fosters the production of sustainable building materials and promotes the eco-friendly utilization of the MS, thereby aligning with broad goals of sustainability and environmental protection.