Effect of Synthesized Calcium Carbonate Nanoparticles on Fresh and Mechanical Properties of High Volume Natural Pozzolan Mortars

摘要

Concrete with high levels of Pozzolan suffers from poor early age strength development and an extended setting time and may therefore lead to construction delays thus limiting its use in the concrete industry. The main objective of this study has been to evaluate the effectiveness of calcium carbonate nano-particles (CCNPs) on improving the fresh and mechanical properties of high volume natural Pozzolan (HVNP) cement mortars. At the beginning, CCNPs have been synthesized in a simple and inexpensive way, and then the optimum content of nano-CaCO_3 has been determined based on the highest compressive strength achieved by ordinary Portland cement mortar with different proportions of CaCO_3 nanoparticles. In the end, the determined optimum content has been used in order to evaluate the effect of nano-CaCO_3 on the properties of HVNP mortars containing 40% and 60% natural Pozzolan as partial replacement of cement. Scanning electron microscopy (SEM) and X-ray diffractometery (XRD) techniques have been used in order to investigate the microstructure, the properties, and the compositions of the synthesized nano-particles and the cement-Pozzolan mortars. CCNPs have been synthesized efficiently via the simple precipitation method and suitable sonication process has been used to disperse CaCO_3 nanoparticles. The results have indicated that nano-CaCO_3 has increased the compressive strength of the Pozzolan-cement mortars, and the best result has been obtained at an optimum content of 1% nano-CaCO_3. The addition of nano-CaCO3 to the HVNP mortar improved its setting behavior; on the other hand, it has decreased its workability. However, this reduction in the workability has been offset by an increase due to partial replacement of cement by HVNP. Blending nano-CaCO_3 with HVNP (40 and 60% replacement levels) has compensated for the low compressive and flexural strength at early ages of HVNP mortars. Thus, it has proved to be an effective way for improving the mechanical properties of high volume natural Pozzolan-cement mortars. The XRD and SEM results have confirmed that nano-CaCO_3 has improved the early strength development and the microstructure of HVNP mortars by making it denser with less pores.