Strength and Durability Performance of Mortars Incorporating Calcined Clay as Pozzolan in Comparison with silica fume

The use of low-grade calcined clays is the only potential material available in large quantities to meet the requirements of eco-efficient supplementary cementitious materials by decreasing the clinker content in blended types of cement or cement content in concrete. In the present paper, six mortar mixtures with a constant w/b ratio were used to investigate the mechanical and durability properties of mixtures substituted with low-grade calcined clays with/without limestone compared to PC and silica fume mixtures. The results obtained show that using calcined clays with low or intermediate purity (i.e., kaolinite content) leads to enhanced durability properties besides the comparable strength development characteristics compared to PC mortar. In this regard, the addition of 30% calcined clay with/without limestone led to a marked decrease (about 80 and 40%) in chloride ion diffusion coefficient (Da) and an increase in surface chloride content (Cs) compared to the reference mixture. In addition, the utilization of calcined clay with a kaolinite content of 56.7% led to a reduction of 52% in Da and an increase of 140% in electrical resistivity compared to silica fume binder. The primary reason for the better performance is attributed to the refined pore structure and dense microstructure of the cement paste with the calcined clay pozzolan compared to PC and silica fume.