Influence of superplasticizers on the workability and mechanical development of binary and ternary blended cement and alkali-activated cement

A comparative study of the effect of superplasticizers on different formulations of binary and ternary cement and alkali-activated cement (AAC) is presented. The need to find sustainable alternatives to the manufacture of Ordinary Portland Cement (OPC), together with the increasing demand for superplasticizer admixtures for a range of applications, motivate this study. Superplasticizer admixtures are designed for OPC so, in presence of supplementary cementitious materials, such as metallurgical slags and combustion ashes, or in alkaline environments, their performance may differ. To assess the influence of the admixtures on these formulations in presence of additions and alkaline media with high and low pH, the fresh and hardened properties of the cementitious matrixes are evaluated. Experimental results show that plasticizing effect in composite cement does not depend on the addition nature (slag or fly ash) and it is well maintained for at least the first two hours after mixing but, in the case of AAC, this effect drops rapidly probably due to lack of chemical stability of the ad-mixtures in alkaline media. Regarding the properties in the hardened state, polycarboxylate admixtures in composite cement do not imply significant variations in mechanical strengths at an early age, but they do in AAC, which presents a detriment of around 17%. Mercury intrusion porosimetry reveals that the incorporation of polycarboxylate refines the pore structure of the cementitious matrixes.