Strength-porosity correlation and environmental analysis of recycled Portland cement

Recycled cement is obtained by heat treating hydrated cementitious materials. At 550 degrees C, decarbonation does not occur owing to the low CO2 emissions during production. A partial regain of the binder ability is observed during rehydration. However, recycled cement has limited strength owing to its physicochemical characteristics, which require elevated water demand. Herein, the porosity and compressive strength of rehydrated cement paste were optimized through water/binder ratio reduction and dispersant use. The cement pastes were fully dispersed based on rotational rheometer analysis. The experimental data were complemented by a literature overview. Consequently, a single strength-porosity relationship was found for (re)hydrated pastes. Decreasing the water demand is key to reducing porosity and improving strength. The recycled cement paste produced in this study achieved the best binder efficiency index (cwf of 0.27 g/g) compared with those in the literature, 48% total porosity and 15 MPa at 28 days. Cement recycled at 550 degrees C indicated a 78% reduction compared to OPC emissions. Despite the low environmental impact of recycled cement, its low strength when used alone makes its environmental efficiency similar to that of Portland cement.