Synergistic effect of nanosilica and high-volume granulated blast furnace slag on pore structure and mechanical properties of cementitious materials

Abstract

High-volume ground granulated blast furnace slag (GGBS) is often used in cementitious materials to produce environmentally friendly building materials. However, high-volume GGBS (HVS) can lead to numerous problems including low strength and loose structure of concrete at the early age. To overcome these drawbacks, nanosilica (NS) is used to compensate the early strength insufficiency of high-volume granulated blast furnace slag (HVS) cementitious material by improving the pore structure. Currently, it is still a challenge to perform accurate testing of the pore structure. Although mercury intrusion porosimetry is widely used to test the pore structure of cementitious materials, there is still some controversy. In this study, the 1H NMR tests are used to measure the pore structure of NS and HVS blended materials and compared with standard MIP tests to comprehensive evaluate the effect of NS on the pore structure of HVS cementitious materials. Results indicate that NS can significantly improve the early strength of HVS cementitious materials and the greater the volume of GGBS, the greater the degree of enhancement exhibited by NS. The compressive strength increases of 23.5% and 30.2% were presented by 3-day S6N2 and S8N2 samples while flexural strength increases of 25.3% and 68.7% was resulted. NS significantly reduces the amount of harmful pores and increases the amount of harmless pores (smaller than 100 nm) in the HVS cementitious materials, it was also found that NS had no significant effect on the reduction of the total pore volume. Results showed that the strength of samples with high porosity was not necessarily low, the pore distribution and pore composition have a greater effect on the strength than the total porosity. The pore volume (>3 nm) obtained from the MIP test was larger than the pore volume measured by 1H NMR for early age samples. The difference in the porosity and pore size distribution obtained from the MIP and 1H NMR test was large at early age. The effect of NS on the pore structure of HVS system was investigated in more detail by 1H NMR and MIP techniques to provide a further theoretical basis for the application of NS in HVS cementitious materials.