Effect of the incorporation of an excavated earth fine grained fraction in cementitious mixes

In the current context of global warming, valorisation of waste materials is becoming a key issue in the cement and concrete industries. This research focuses on the effect of the incorporation of an excavated earth fine-grained fraction (< 63 μm) in cement mixes and its contribution to the cement hydration mechanisms which are mainly coming from the Paris region of France. This fine-grained fraction results from a washing and mechanical sorting of grains process for excavated earth. The omnipresence of gypsum in the subsoil of the Paris region, due to its geological context, is reflected in the fine-grained fraction with the presence of sulphates. Moreover, the fine-grained fraction is also composed of quartz, calcium carbonates and clay phases. The tests have been performed on cement pastes composed of a blast furnace slag cement and the fine-grained fraction as a sludge with its inherent humidity. Pastes with different sludge contents leading to different water-cement ratios (0.5, 0.7 and 0.8) have been studied. The impact of the incorporation of the fine particles and the water contained in the sludge on the rheology and the hydration of the cement pastes has been evaluated using water demand tests, thermogravimetric analysis and isothermal calorimetry. Results indicate that the incorporation of sludge in cementitious mixes increases the water demand, accelerates the hydration reactions and contributes to the formation of additional hydrate phases probably associated with the presence of gypsum in the studied sludge.