Performance degradation of CO₂ cured cement-coal gangue pastes with low-temperature sulfate solution immersion

The effect of CO₂ curing and coal gangue on sulfate attack of cement paste at low temperature was investigated in this paper. The strength change, mass change and volume deformation were analyzed. In addition, pore structure, phase composition and microscopic morphology were tested by using MIP (mercury intrusion porosimeter), TG-DTA (thermogravimetry differential thermal analysis), QXRD/Rietveld (X-ray diffraction), SEM (scanning electron microscope) measurements to analyze the microscopic mechanism. Results showed that CO₂ curing could reduce the compressive strength reduction caused by low-temperature sulfate attack. Compared with the control specimens added with 0%, 10% coal gangue before low-temperature sulfate immersion, the 6 months immersion compressive strength of the corresponding carbonated specimens were still 8.02%, 4.69% higher. The compressive strength of carbonated samples after 6 months of low-temperature sulfate immersion decreased by 7.43~33.16%, which was much lower than that (11.62%~45.17%) of the uncarbonated specimens. In addition, CO₂ curing inhibited the volume expansion of specimens during the low-temperature sulfate immersion because of the decrease of porosity and the improvement of pore structure. No typical thaumasite was observed in the CO₂ curing specimens after low-temperature sulfate immersion. The strength reduction and the total porosity of specimens after low-temperature sulfate immersion were all lower than those after room temperature sulfate immersion, indicating that sulfate erosion of cement at low-temperature was less serious than that at room temperature. Thus, the combination of CO₂ curing and coal gangue recycling offers potential for the production of cementitious materials with better long-term durability and environmental sustainability.