High replacements of reactive pozzolan in blended cements: Microstructure and mechanical properties

A study comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials was carried out. 10, 20 or 30-wt.% SF or DK were substituted for Portland cement. The kinetics of hydration were studied at water/solid ratios of 0.30 and 0.50. The hydration reactions were monitored by determining the compressive strengths, the free lime contents and the amounts of combined water. The hydration products were investigated using XRD and SEM/EDX. Both pozzolans increase the total porosities, when used without water reducers or superplasticizers. Both also decreased the compressive strengths at early ages. However, the strengths recovered with time exceed that of the reference Portland cement paste. Strength gain was more rapid in DK mixes than in SF mixes. The optimum content of each pozzolan varied with the water/solid ratios of the blends. Unreacted silica fume, in the highly-substituted mix (30%), caused a reduction in the mechanical properties by providing weak points for the crack propagation.