Quantification of Residual Unhydrated Cement Content in Cement Pastes as a Potential for Recovery

All types of concrete contain residual unhydrated cement. For example, unhydrated cement is present in high-strength concrete due to low water/cement ratios, as well as in old concrete due to coarser cement used in the past, and in fresh concrete waste due to the lack of curing. These residues of unhydrated cement are a waste of resources with potential for recovery and reuse. In this work, X-ray diffraction, thermogravimetric analysis, and analytical modeling were used to quantify the residual cement and the hydration degree of various cement pastes to explore their recovery potential. The study included cement pastes with water/cement ratios of 0.2–0.6 and residual unhydrated cement was found to be in the range 6–36%, indicating great potential for recovery and further use in the manufacture of new concrete.