Study on the effectiveness of PNS and LS superplasticizers in air lime-based mortars

Abstract

Polynaphtalenesulfonate (PNS) and lignosulfonate (LS) were tested as superplasticizers in pastes and mortars of pure air lime and air lime with a pozzolanic additive, nanosilica (NS). LS showed a better plasticizing effect than PNS: the flowability of the samples with LS as well as the slump retention over time was larger. LS strongly hindered carbonation due to its ability to form Ca2+ complexes. In mortars with NS, PNS was seen to interfere with the C-S-H formation, resulting in lower mechanical strengths. Adsorption isotherms and zeta potential showed that PNS was more adsorbed than LS onto lime and C-S-H particles. PNS acted mainly through electrostatic repulsion owing to its high anionic charge density and flat adsorption. However, LS was more efficient as superplasticizer in air lime media, steric hindrance being the main mechanism responsible for avoiding flocculation. Furthermore, LS yielded high compressive strengths in mortars with NS.