Calcined clayey soils as a potential replacement for cement in developing countries

Concrete is the building material with the greatest potential for meeting the increasing housing demand in developing countries. Locally available materials should be considered as cement replacement in order to lower the costs and the environmental impact associated with the production of cement. Hence, this study investigated the potential use of calcined clayey soils to be used as reactive mineral admixtures in concrete, with special emphasis on a Cuban clayey soil. The study of the thermal behaviour of common clay types such as kaolinite, illite and montmorillonite was of key importance for the understanding of more complex locally available soils generally composed of a mixture of clays with companion minerals and impurities. Clays were calcined at temperatures ranging from 500°C to 1000°C. The characterization of the raw and calcined clays included XRD, TGA, DTA, NMR, PSD, BET and SEM. The study of the pozzolanic activity was done in cement pastes by replacing 30% of cement by calcined clays at a water/binder ratio of 0.4. Curing was done in water at 30°C to simulate the Cuban climate. CH depletion was monitored using XRD and TGA up to 90 days. The degree of hydration of the clinker component was assessed by BSE-image analysis. The identification of the hydrated phases in pastes was done by XRD, NMR and SEM. Long-term Isothermal Calorimetry as well as chemical shrinkage allowed the chemical contribution of the pozzolans to be identified independently of the filler effect and the hydration of cement.. To see how did the different reactivities of calcined clays translate into mechanical properties, standard mortar bars (w/b 0.5) were prepared and cured under the same conditions as the pastes for testing in compressive strength at 1, 7, 28 and 90 days. Sorptivity measurements were also performed to assess the durability properties. Kaolinite was found to have the highest potential for activation when calcined in the range of 600°C to 800°C. Locally available clays with only around 40% of kaolinite showed good pozzolanic properties in pastes. The time at which the pozzolanic activity starts to be significant varied between 1 and 7 days. Calcined clays in cement tend to favour the formation of AFm phases such as hemicarboaluminate or monocarboaluminate. Strätlingite was also detected by 27Al NMR. The Compressive strength of mortars incorporating kaolinite-based calcined clays was systematically similar or higher than the 100% OPC mortar from around 7 days and the sorptivity of the blended systems was reduced. These studies indicate that locally available calcined clayey soils have significant potential for replacing cement in concrete.