The effects of silica fume and diatomaceous earth on the mechanochemical activation and pozzolanic activity of kaolin

In this study, the mechanochemical activation of kaolin was combined with the addition of two different pozzolanic substances (silica fume or diatomaceous earth) to modify the structure and pozzolanic activity of kaolinite. The mixtures of kaolin and pozzolanic substances, in mass ratios of 25:75, 50:50, 75:25, and 100:0, were co-ground for the activation. It was performed by high-energy dry grinding to obtain completely (100%) and incompletely (~90%) amorphized kaolinite phases, which were examined by X-ray diffraction, Fourier transform infrared spectroscopy, thermal analysis, scanning electron microscopy, as well as by the determination of the specific surface area and particle-size distribution. To characterize the pozzolanic reactivity, the compressive strength of mortar specimens were studied, in which 10% of ordinary Portland cement was replaced by mechanochemically activated materials. The addition of silica fume or diatomaceous earth reduced significantly the required grinding time and energy of the complete and partial amorphization of kaolinite. The accelerated amorphization of kaolinite can be attributed to the action of amorphous silica or hydrous silica grains as grinding bodies. The effect of these amorphous silica grains on the mechanochemical amorphization of kaolinite was found practically the same as that of the crystalline quartz grains. The partial substitution of ordinary Portland cement by the mechanochemically activated materials increased the compressive strength of the mortar specimens due to the higher pozzolanic reactivity caused by the enlarged reactive silica and alumina content. The resulting strength data also demonstrated that the incomplete (~90%) amorphization of co-ground kaolinite, as compared to the complete amorphization, is more favorable to efficiently produce pozzolanic cement substitute materials with a high reactivity due to the high amount of amorphous kaolinite and the relatively weak interaction of particles.