Effect of water content and particle size on the thermal decomposition of ground cement paste

ABSTRACT A possible method for producing eco-efficient concrete would be the partial replacement of Portland cement by pastes and mortars residues subjected to thermal-mechanic treatment. In this context, this study evaluates the influence of water/cement (w/c) ratio and average particle size on the thermal decomposition of hydrated cement. Cement pastes with three w/c ratios (0.40, 0.50 and 0.60) and cured by 28 days were ground to obtain two particle distributions with average sizes of 0.60 mm and 0.15 mm. The thermal decomposition of pastes was analysed by thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). TGA analysis indicates that chemically bounded water content is improved by the increasing of w/c ratio between 0.40 and 0.50, but it is constant between w/c of 0.50 and 0.60. Particle size reduction almost does not interfere with TGA curves. According to XRD e TGA analysis, most hydrates are decomposed at 500-550 °C, while calcium silicates and calcium aluminates similar to the anhydrous phases of Portland cement were detected on the heated paste at 750 °C. Thus, higher temperatures would be required for the complete recovery of the anhydrous cement in hydrated pastes.