Investigation of activation kinetics in geopolymer paste using quasielastic neutron scattering

Abstract

h i g h l i g h t sQuasielastic neutron scattering analysis in geopolymer paste. Activation kinetics is affected by temperature and molarity of the activator. QENS is effective in studying the alkali-activation process in geopolymer cements. a b s t r a c tQuasielastic neutron scattering (QENS) has been used to investigate the binding process of water molecules in pastes of calcium geopolymer prepared with low and high calcium fly ash contents, and at two different NaOH molarities, 10 and 14 M. The in situ measurements were carried at ambient and elevated curing temperatures (60°C). By carefully monitoring the time evolution of the elastic peak intensity, we infer a gelation process, followed by polymerization and a hardening in the high calcium geopolymer paste at 60°C, in agreement with previously proposed geopolymerization model. This behavior was neither observed at ambient temperature in both low and high calcium geopolymer cement paste, within the precision of the neutron instrument. Our study clearly shows that a minimal amount of heat is necessary to form gelation and polymerization during the activation process. The activation of geopolymer paste with high NaOH molarity involves more chemically bound water molecules than that at lower activator concentration. This work shows that the QENS technique can be effectively used to characterize the alkali-activation kinetics in certain geopolymer pastes, and that temperature and molarity of the activator play a vital role in controlling the gel mechanism.

A comprehensive review of synthesis kinetics and formation mechanism of geopolymers

“…The concentration of NaOH and temperature played main roles in the initial gel formation and polymerization at later stages. 105 The mechanism of geopolymerization of phosphoric acid based geopolymer consisted of three steps such as removal of aluminum from metakaolinite, the reaction between tetrahedral PO 4 and –Si–O– layer to form amorphous (–Si–O–P–) structure and the reaction of aluminum with PO 4 tetrahedra to form crystalline AlPO 4 , and the condensation of the amorphous phase to form a geopolymer. 106 The addition of graphene (rGO) in metakaolin geopolymer decreased the voids with the increase of geopolymerization time.…”

In situ investigation of phosphonate retarder interaction in oil well cements at elevated temperature and pressure conditions

“…QENS facilitates quantification of various H/H 2 O environments by evaluating free, constrained, and chemically bound (CB) H-atoms. QENS has widely been used to study the hydration process of C-S-H, [13][14][15][16][17][18][19] portland cement paste with natural 20 and nano-additives, 21 geopolymer cements, 22 and alkali-activated binders. 23 The QENS data have been used to model the nucleation and growth process by providing insight into the induction period of C 3 S hydration.…”

In situ quasi-elastic neutron scattering study on the water dynamics and reaction mechanisms in alkali-activated slags

“…22 However, for formation processes that possess multiple reaction peaks in the heat flow data, such as an AAM based on silicate activation of blast furnace slag, 9,13,15 ICC cannot determine the reaction type (i.e., dissolution versus precipitation) or reveal which reactants are involved. Other experimental characterization techniques that are capable of providing valuable insight on the reactions mechanisms in cementbased systems include neutron and X-ray scattering, 12,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] pair distribution function (PDF) analysis, 17,18,34,39,40 X-ray nanotomography, 41 nuclear magnetic resonance (NMR), 42,43 differential scanning calorimetry, 44 and Fourier transform infrared spectroscopy (FTIR). 21,45 One key neutron scattering technique that has been widely used to study reaction kinetics and associated mechanisms in OPC-based materials is quasi-elastic neutron scattering (QENS), 23-27, 29, 37, 44 which traces the evolution of different H/H2O components (e.g., bound and free water) during the hydration process.…”

“…To date, there is only one QENS study in the AAMs literature where in situ QENS was used to probe the formation process of a low-Ca fly ash-based AAM. 33 In this study, in situ QENS and INS are employed for the first time to investigate the reaction kinetics and formation mechanisms of alkali-activated slags (AASs), a type of AAMs. We have collected in situ QENS and INS data for two AAS samples activated with different activator solutions (i.e., NaOH and Na2SiO3) during their initial ~8-12 hours of reaction.…”