Yielding mechanisms and structuration of sodium silicate-activated fly ash and slag pastes: Origins of critical strains

This paper investigated the rheological behaviour of alkali-activated fly ash (FA) and ground granulated blast furnace slag (GGBFS) pastes. The pastes, activated with various concentrations of sodium silicate, were studied using rheological protocols to characterise critical strain values and zeta potential test to correlate surface charge with yielding behaviour. Two critical strains were identified in the pastes: the smaller one associated with the breakage of reaction product linkages, and the larger one related to colloidal interactions. Higher sodium silicate content, with its high viscosity, led to delayed flocculation due to viscous effects and repulsive forces. The time needed to form a well-percolated network in FA pastes was longer than GGBFS. The increase in storage modulus in pastes with high sodium silicate content is primarily due to particle interactions through reaction products rather than colloidal forces. Reversible flocculation enhances early-age viscosity recovery, while irreversible breakage of inter-particle linkages reduces viscosity recovery at later ages. • Critical strain origins for yielding in fly ash and GGBFS pastes identified. • Higher sodium silicate reduces colloidal interactions and delays structuration. • Fly ash pastes form interparticle networks slower than slag pastes. • Surface charge and paste reactivity influence yielding and flow behaviour.