Polycarboxylate/nanosilica-modified quaternary cement formulations – enhancements and limitations

The effect of polycarboxylate/nanosilica (nS) particles in quaternary cement formulations comprising Portland cement (PC), limestone powder (LS) and fly ash (FA) was investigated for the first time. The reference formulation contained 60% PC, 20% LS and 20% FA by mass of binder in an effort to minimise clinker and maximise the other constituents. nS particles were characterised by way of transmission and X-ray scanning electron microscopy (SEM). The nS was added at 0·3 or 0·6% by mass as a partial replacement for PC and different water-to-binder (w/b) ratios were explored. Compressive strength tests and thermal gravimetric analyses (TGA) performed at day 7, 28 and 56 testified to pozzolanic behaviour. Results suggest a mechanism of ‘de-activation’ of some FA particles with age. A new ratio: (compressive strength in MPa)/(calcium hydroxide content detected by TGA) was introduced, correlating microscale characteristics (hydration products) and macroscale performance (delivered compressive strengths). Back-scattered SEM images confirmed the calcium–silicate–hydrate (C–S–H) network formation, the presence of reacted/unreacted FA particles and the availability of calcium hydroxide for delayed hydration reactions. Tests on mortars also confirmed the enhancement offered by nS addition. The lower bound nS addition was determined to be 0·6% by mass of binder for pastes and 0·5% for mortars.