Analysis of engineered cement paste using silica nanoparticles and metakaolin using ²⁹Si NMR, water adsorption and synchrotron X-ray Diffraction

This work investigates the synergetic effect of nano-silica, NS, (up to 2% by weight) and metakaolin, MK, (up to 9.75% by weight) on the pore- and micro-structure of hardened Ordinary Portland Cement (OPC) pastes after 28 days of curing time. By addition of NS, it was observed that ultra-fine porosity of the cementitious matrix was formed due to the plombierite phase observed via synchrotron XRD analysis. The inclusion of MK along with NS facilitated formation of Calcium-Aluminum-Silicate-Hydrate (C-A-S-H) based crystal structure of wairakite. Mean chain length (MCL) values showed that addition of NS to MK helped in increasing the MCL values up to 7.8, while decreasing the Ca/Si ratio contributed to a higher degree of silicate polymerization that facilitated Calcium-Silicate-Hydrate (C-S-H)/C-A-S-H formation. Water adsorption analysis detected that the inclusion of NS to OPC increased the specific surface area up to 163.5 m²/g suggesting probable densification of the cementitious matrix.