Formulation of blended cement by the combination of two pozzolans: Calcined clay and finely ground sand

• Use of a kaolinitic clay as a pozzolanic material for cements.
• Use of ground sand as a pozzolanic material for cements.
• Optimization of a blended cement formula by using a cross mixture design.

The aim of this study was to investigate and optimize the properties of mortars in which calcined clay and finely ground sand are employed as a partial substitution of Portland cement. For this, a cross mixture design containing 28 runs was set up to study the influence of the mixture proportions of Portland cement, calcined clay and finely ground sand in the blended cement and also two process variables namely: fineness of ground sand and calcination temperature of clay. It was demonstrated that finely ground sand, despite its crystalline nature, presents a pozzolanic reactivity. It was also proven that the compressive strength of hardened mortars can be improved by increasing clay calcination temperature in the explored domain. Finally, analysis of isoresponse contour plots of the combined mixture design allowed the choice of the optimum formula giving the high values of compressive strength at 28 and 90 days which is 15% of ground sand using a sieve size of 40 μm fineness.