Effect of microfines from manufactured sand on yield stress of cement paste

Abstract

With the depletion of natural sand, the manufactured sand is frequently used in cement-based materials, which inevitably affects the flowability. It is mainly due to that the yield stress is affected by the microfines (≤75 μm) in manufactured sand. However, the mechanism is still not fully understood. This paper investigated the effects of the microfines of limestone, granite, and two tuffs on the yield stress of cement pastes with superplasticizer. The average surface coverage, packing density, and particle size distribution (PSD) of the suspending particles were measured to reveal the affecting mechanism of microfines on the internal particle network of the pastes. Results show that the incorporation of the microfines increases the yield stress of cement pastes. Limestone with the highest specific surface area (SSA) and granite with the lowest SSA caused the largest and smallest increases of the yield stress, respectively. Two different tuffs with similar SSA led to equivalent increases of yield stress. It is ascribed to the reduction of the average surface coverage, which reduces the average surface-to-surface separation distance and enhances the colloidal interactions with the incorporation of microfines. Due to the high fraction of fine particles in microfines, although the packing density increases slightly, the PSD effect of the suspending particles is significantly enhanced, which increases the number and effective volume fraction of the contacting particles and therefore the particle contact interactions. The internal particle network of the cement pastes is jointly affected by colloidal interactions and particle contact interactions, which thereby leads to the different influences of the microfines on the yield stress.