Performance of Superplasticizers in Blended Cement Systems

Abstract

This paper describes how the combination of different type of blending materials with portland cement such as natural pozzolans, fly ashes, and slags provides high flexibility in designing concrete mixtures with several technical advantages and low cost. However, their addition influences the properties of fresh concrete and causes problems of compatibility in blended cement systems containing superplasticizers that are usually used either for reduction of water demand or for increase of workability. In this study, two of the widely used type of superplasticizers, one based on sulphonated naphthalene formaldehyde condensate (SNF) and another based on polycarboxylate polymers (PC), were tested with ten blended cement mixtures containing 20, 30, 50, or 80% portland cement replacement with one or more of the above-mentioned supplementary cementitious materials. The action of superplasticizer on cement pastes was monitored by making zeta potential, pH, and temperature measurements as well as by using DTA-TG method for determining the hydration and hardening process. Porosity and strength development were also measured at different ages. The aim of the research work was to find which type of superplasticizer was more suitable for each type of binder system. The highest reduction of water demand was achieved with polycarboxylate polymer superplasticizer. Results show that compared to the plain portland cement system, the decrease of water/binder ratio with superplasticizer addition was lower in blended cements.