Investigation of waste clay brick (chamotte) addition and activator modulus in the properties of alkaline activation cements based on construction and demolition waste

The use of construction and demolition waste (CDW) as a raw material for the manufacture of alkali-activated cements (AACs) is a promising and sustainable way to recover construction waste. CDW mainly consists of mineral residues such as concrete and ceramic waste. In this study, CDW with a high fraction of concrete waste is used, so the enrichment effect of the waste in the ceramic fraction is studied. For this purpose, different proportions of chamotte (CHM) have been incorporated (0-40 wt%). The CDW precursor or CDW-CHM precursors have been activated using 8 M sodium hydroxide and sodium silicate solution, using an activator modulus (mol SiO2/mol Na2O = Ms = 1.0) and the specimens were cured at room temperature. The liquid/binder ratio was 0.45 for all the pastes manufactured. The results indicate that the progressive enrichment of the ceramic fraction or the incorporation of CHM results in alkaline activated cements (AACs) with similar physical properties such as bulk density, water absorption and total porosity, but with improved mechanical properties. The incorporation of 40 wt% CHM results in optimum compressive strengths with values of 29.7 and 41.2 MPa after 28 and 56 days of curing respectively, higher than those obtained for the control specimens containing only CDW 15.2 MPa and 30.65 MPa respectively. Subsequently, the impact of the silicate modulus of the alkaline activator (Ms = 0.5; 1.0 and 2.0) on the performance of the AACs was evaluated in the control cement and in the optimum cement incorporating 40 wt% CHM. The mechanical performance of the AACs improves substantially with increasing activator modulus up to a certain threshold Ms = 1.0, but finally decreases slightly with Ms = 2.0. Therefore, this study demonstrates the possibility of valorising wastes from the construction sector through their use as precursors in the manufacture of environmentally friendly alkaline activated cements with macroscopic performances that improve with the content of the ceramic fraction (CHM).