Interactions Among MWCNTs, an Air-Entraining Agent, and Superplasticizers in Lightweight Cementitious Materials

This study examined the combined effects on highly foamed cementitious materials of varying concentrations of multi-walled carbon nanotubes (MWCNTs), an air-entraining agent (AEA), and three superplasticizers (SPs): a lignosulfonate-based superplasticizer (SP-LS), a polyacrylate-based superplasticizer (SP-PA), and a polycarboxylate ether-based superplasticizer (SP-PCE). Setting time, semi-adiabatic exothermic-temperature (EXO) profile tests, zeta potential analysis, pH and electrical conductivity (EC) measurements, and foam stability were used to assess the suspensions and pastes. Adding up to 1.5% MWCNTs to an alkaline air-entraining agent (AEA) and SP-LS mostly shifts the zeta potential toward a negative value and stabilizes the suspension and foam. When MWCNTs and SP-LS were added to the foamed paste, the initial viscosity dropped by 12.1% to 7%, whereas with SP-PA and SP-PCE it dropped by 20–48% and 18–50%, respectively. Adding MWCNTs and SP-LS to the foamed paste mostly decreases the density. Using SP-LS is very helpful for interactions between AEA and MWCNTs, thereby stabilizing the air bubble walls. This study underscores the importance of pH and EC of the AEA and SP in affecting the cement paste’s hydration process.