Effect of High Temperature on Compressive Strength and Microstructure of Cement Paste Modified by Micro- and Nano-calcium Carbonate Particles.

Micro-calcium carbonate (MC) and nano-calcium carbonate (NC) have been employed to improve the microstructure and mechanical properties of cement and concrete at ambient temperature. This study focuses on investigating the effect of high temperatures (200°C to 1000°C) on the compressive strength and microstructure of cement paste modified by micro- and nano-calcium carbonate particles. Microstructural analysis was performed using differential thermogravimetric–thermal gravimetric analysis (TG/DTG), mercury intrusion porosimetry (MIP), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). The results showed that NC and MC have a filling effect in the cement matrix. Furthermore, NC and MC act as "nuclei" in promoting the further hydration of unhydrated binder grains under hydrothermal conditions at high temperatures. Below 600°C, the compressive strengths of MC-modified cement paste (MCP) and NC-modified cement paste (NCP) increase with increasing temperature. NCP significantly changes with increasing temperature, and its microstructure remains relatively dense in the temperature range of 200°C to 600°C. Above 600°C, the decomposition of C-S-H (tobermorite) and CaCO3 cause a remarkable deterioration in the microstructure and compressive strength. The residual compressive strength and mass loss are related to the particle size of CaCO3 and the high temperature, and the NCP matrix displays better behavior at high temperatures. [ABSTRACT FROM AUTHOR]