Effect of retarders on setting and strength development of geopolymer activated by Na2CO3+Ca(OH)2: Properties and mechanisms

Abstract

Controlling the setting/thickening behavior of geopolymer remains a significant challenge, which hinders its broader application. Na2CO3+Ca(OH)2 is used as an activator instead of NaOH, while retarders are employed to control the reaction rate between Na2CO3 and Ca(OH)2, which is an effective method to realize the adjustable setting/thickening behavior of geopolymer. However, the type of retarder has significant influence on the performance of Na2CO3 and Ca(OH)2-activated geopolymer, and the retarding mechanisms of various retarders in this system remains unclear. In this work, the effects of three retarders, namely ethylene diamine tetra methylene phosphonic acid sodium (EDTMPS), D-gluconic acid sodium (D-GA) and sodium tartrate (ST), on the setting time and strength development of Na2CO3 and Ca(OH)2-activated geopolymer were studied. The results indicate that, in comparison with the reference NaOH-activated geopolymer, Na2CO3 and Ca(OH)2-activated geopolymer exhibits much longer setting time. At 70℃, when NaOH is used as an activator, the setting time is the longest with the addition of EDTMPS. The initial setting time with 2 % EDTMPS is only 55 min. However, when Na2CO3 and Ca(OH)2 are used as activators, the initial setting time with 2 % EDTMPS can reach 130 min. At the same dosage of retarders, EDTMPS shows the best retarding effect, while ST exhibits the lowest retarding efficiency. Under the condition of similar final setting time, the strength of the geopolymer with ST is the lowest, only 5.9 MPa after curing at 70℃ for 1d. The strength of geopolymer with D-GA is the highest, and the strength can reach 19.3 MPa after curing at 70℃ for 1d. XRD, SEM, MIP, 29Si NMR, FTIR and hydration heat analysis revealed that the extent of the reaction between Na2CO3 and Ca(OH)2 under the influence of different retarders, as well as the geopolymer structure significantly affect the strength development of geopolymer.