Properties evolution of cement-metakaolin system with C-S-H/PCE nanocomposites

In this work, the effects of the synthetic calcium silicate hydrates(C-S-H)/polycarboxylate(PCE) nanocomposites (CPNs) on the hydration behaviour of Portland cement (PC) containing 20 wt% of metakaolin (MK) were evaluated. Six mixtures with different CPNs replacement levels ranging from 0 wt% to 4 wt% were designed to cast CPNs-modified pastes and mortars, respectively. The fluidity and compressive strength were tested and micro-analysis including hydration heat, hydration products, pore size distributions and microstructures for CPNs-modified pastes was also conducted to disclose the potential mechanism for modification of mortar properties due to CPNs. The current results showed that an increased CPNs content contributes to a continuously improved workability for the PC-MK binary mortars. The corresponding compressive strength development for the mortars also benefits from CPNs addition. The optimum CPNs content was determined to be 3 wt%, which contributed to the highest compressive strength obtained for all the mortars at each curing age: 36.41%, 21.37%, 14.17% and 5.66% higher compared with the control PC-MK binary mortar at 1 day, 3 days, 7 days and 28 days, respectively. It is also worth noting that mortars incorporated with 2.0 ~ 4.0 wt% of CPNs also presented higher compressive strength values than 100 wt% cement mortar at each curing age. Based on the micro-analysis results, it was speculated that CPNs not only accelerate cement hydration at the early age, but they also promote the pozzolanic reaction of MK with calcium hydraxide (CH). This synergistic effect between CPNs and MK effectively results in a much denser microstructure with a refined pore structure for the PC-MK paste, which is, thus, helpful to further improve the final compressive strength. The findings of this study provide theoretical guidelines for modifying the properties of blended cements by using nanoparticles.