Assessment of Hydration, Strength, and Microstructure of Three Different Grades of Calcined Marls Derived from Phosphate By-Products

One of the challenges that cement industry faces today is the need to reduce the production of clinker using sustainable supplementary cementitious materials (SCMs). SCMs are in high demand and their production is very limited (fly ashes, slags, silica fume, etc.), unlike calcined clays, which are widely available. By-products of phosphate mine exploitation can be an alternative resource to feed the cement industry. In this paper, several types of marl clays from phosphate extraction ores were calcined at three temperatures: 750 degrees C, 800 degrees C, and 850 degrees C. The optimum temperature was selected based on the mineralogical composition after calcination. The reactivity of these calcined marls was confirmed through solubility of Al and Si ions in alkaline solution. Microstructural analyses were performed to assess the phases formed in cement paste specimens after 28 days of hydration. Mortar specimens were evaluated for their mechanical performance (compressive and flexural strengths). The results showed that replacement of cement by up to 30 % calcined marls at 850 degrees C could achieve an optimum value of 43 MPa at 28 days. The improvement in the mechanical performance seemed to be related to the new phases (CASH) formed during hydration. Owing to the reaction between the aluminate phase and calcium carbonate, calcium carboaluminate hydrates also formed. This work shows that, with certain marls, calcium-aluminosilicate-hydrate and calcium carboaluminates can be formed without limestone addition.