Effect of alkali content in cement on the fluidity and structural build-up of plasticized cement pastes

Abstract

This paper focuses on the fluidity and structural build-up process of plasticized cement pastes (PCPs) with different alkali contents during the first 2 h of age. An optical microrheology analyser was used to monitor the evolution of the viscoelasticity of the PCPs. The storage modulus (G′) and the ratio of the loss modulus to the storage modulus (G″/G′) were obtained to characterize the structural build-up process of the PCPs. The agglomeration of particles, absorption behaviour of the superplasticizer and zeta potential of the cement particles in solution were measured in addition to the ionic constitution of the pore solution. The kinds of hydrates at 10 min of age were determined by using X-ray diffraction (XRD) analysis. The results show that an alkali addition decreases the fluidity of the PCPs and promotes the agglomeration of particles. The zeta potential of particles in the solution monotonically increases, resulting from the immediate change in the ionic constitution of the pore solution. The alkali addition promotes the structural build-up process of PCPs. When the alkali content in cement reaches a high level, some bridge-acting crystals, such as syngenite and Ca(OH)2, are formed at a very early age, resulting in an extreme increase in the storage modulus of pastes. The fluidity and structural build-up process of cement pastes with low C3A content are very sensitive to the variation in alkali content in the cement.