The flowability of cement mortar partially replaced by microsilica and graphite

Abstract

This study aims to investigate the effects of partially replacing Portland cement with microsilica (MS) and graphite powder (GP) on the flowability of cement mortar in the fresh state. The water-to-binder ratio (w/b) was fixed at 0.30, and the binder-to-sand ratio (b/s) was maintained at 0.5. A total of 16 mix proportions were designed, consisting of a control mixture (plain cement) and hybrid mixtures in which cement was replaced at levels of 5%, 10%, and 15% by the weight of cement. The flowability test was conducted in accordance with ASTM C230/C230M and the results were expressed as the percentage of slump flow (%SF). The experimental results demonstrate that the incorporation of 5% GP alone yielded a higher slump flow than the control mixture, enhancing the flowability by approximately 10% compared with conventional mortar. In contrast, mixtures containing MS alone exhibited a significant reduction in slump flow with the decrease being directly proportional to the replacement level. This behavior is attributed to the high specific surface area of MS which increases water demand and reduces the amount of free water available within the system. However, when the GP content was increased to 10–15%, a significant decrease in the percentage of slump flow was also observed. This reduction can be explained by increased water adsorption on particle surfaces and the rise in cement paste viscosity due to the presence of high micron-scale fineness. The findings highlight the particle-level mechanisms governing the rheological behavior of cement mortar incorporating fine powder materials and provide essential fundamental data for designing concrete mixtures requiring controlled fresh-state flow performance.