Participation and disturbance of superplasticisers in early-stage reaction of class F fly ash-based geopolymer

Abstract

This work explores how various superplasticisers (SPs), including a lignosulfonate-based (LS), a polynaphthalene-based (PNS) and two polycarboxylate-based (PCE-1 and PCE-2) ones, affected the reaction of a class F fly ash-based geopolymer (FABG) at 60℃ within five days. Setting time, reaction heat, pore solution elemental concentration, and product evolution were investigated. By linking to steps of dissolution, speciation equilibrium, gelation, reorganisation, and polymerisation, it is found that the SPs accelerated the dissolution, advancing the subsequent speciation equilibrium, gelation, and reorganisation, but hindered the polymerisation. More dissolved species can raise paste viscosity, potentially limiting SPs’ dispersing effect and reducing initial setting time. PCE-1 and PCE-2 led to a faster dissolution than LS and PNS, while due to the quicker pace of Ca hydration than geopolymer formation, the Ca-based LS and PNS further shortened the initial setting. Hydrogen bonds and cation-bridge attractions between SPs and Si/Al species reduce intermediate products’ reactivity, retarding polymerisation and post-hardening stage reaction, disadvantaging FABGs’ strength development. Comb-like PCE-2 induced more steric hindrance, causing the most significant retardation. Semi-crystalline products were detected when a linear SP (PNS or decomposed PCE-1) was used, with an unclear impact on FABGs’ fresh workability and strength, suggesting further exploration in relevant areas.