CHEMICAL AND PHYSICO-MECHANICAL PROPERTIES OF COMPOSITE CEMENTS CONTAINING MICRO- AND NANO-SILICA

Portland cement is one of the most used materials in the world. Due the environmental problems related to its use, such as CO2 emission and use of non-renewable raw materials, new materials are being researched. In the recent years, there is a great interest in replacing a long time used materials in concrete structure by nanomaterials (NMs) to produce concrete with novel function and better performance at unprecedented levels. NMs are used either to replace part of cement, producing ecological profile concrete or as admixtures in cement pastes. The great reactivity of NMs is attributed to their high purity and specific surface area. A number of NMs been explored and among of them nanosilica has been used most extensively. This work aims to study, the chemical and physico-mechanical properties of composite cements containing silica fume (SF) and nanosilica (NS). Different cement blends were made from OPC, SF and NS. OPC was substituted with SF up to 15.0 mass, %, then the SF portion was partially replaced by NS (2.0, 4.0 and 6.0 mass, %). The hydration behavior was followed by determination of free lime (FL) and combined water (Wn) contents at different curing ages. The required water for standard consistency (W/C), setting times (IST&FST), bulk density (BD) and compressive strength were also estimated. The hydration products were analyzed using XRD, DTA and SEM techniques. The results showed that, both of SF and NS improve the hydration behavior and physico-mechanical properties of composite cements. But, NS possesses higher improvement level than SF. This is due to that, both of them behave not only as filler to improve the microstructure, but also as activator to promote pozzolanic reaction, which enhances the formation of excessive hydration products. The higher beneficial role of NS is mainly due to its higher surface area, filling effect and pozzolanic activity in comparison with SF. The composite cement containing 85.0 % OPC, 11.0 % SF and 4.0 % NS gave the optimum mechanical properties at all ages of hydration.