Investigating the properties of bentonite and kaolin modified concrete as a partial substitute to cement

Using pozzolanic materials in concrete manufacturing is intended as an optimal solution to lower the rate of greenhouse gas emission, and diminish energy resources and cement consumption. This study investigates the effect of using Semnan bentonite and kaolin as partial replacement for cement in low-strength concretes. A total of 18 mix designs along with a control specimen are prepared and compared. The main parameter is considered to be the changes in the ratio of bentonite, kaolin, and natural materials (furnace slag and gum vine-resin) to the cement weight. For better evaluation the water to cementitious materials ratio (W/C) and the fine aggregate percentage considered constant. To study properties of hardened concrete, compressive strength and splitting tensile strength tests were performed. According to the results of compressive strength test, using bentonite, kaolin, and other substances (furnace slag and natural resin) at 2/7% ratio by weight of cement, for a 400 kg/m3 cement content, could improve the compressive strength by nearly 6%. However, at a lower cement content, the additives reduced the compressive strength and, consequently, the tensile strength. According to the results in mixtures containing bentonite, kaolin and natural materials (furnace slag and gum vine-resin) if the amount of bentonite, kaolin and natural materials is more than 2/7% by weight of cement, the tensile strength is reduced by 75% compared to CM. Accordingly, without cement, both compressive and tensile strength decreased drastically. It was decided that pozzolanic materials cannot be used alone, and their role must be limited to that of a partial substitute. Specimens with 400 kg/m3 cement offered an acceptable compressive strength for low-rise structures.