Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation

Abstract

20Binders formed through alkali-activation of slags and fly ashes, including 'fly ash 21 geopolymers', provide appealing properties as binders for low-emissions concrete production. 22However, the changes in pH and pore solution chemistry induced during accelerated 23 carbonation testing provide unrealistically low predictions of in-service carbonation resistance. 24The aluminosilicate gel remaining in an alkali-activated slag system after accelerated

Microstructure and durability of alkali-activated materials as key parameters for standardization

“…Reflections of hematite (Fe2O3; PDF# 00-001-1053), mullite (Al6Si2O13; PDF# 00-015-0776) and quartz (SiO2; PDF# 00-033-1161) are also identified. These phases are present in the unreacted fly ash, and it has been identified that they do not participate in the alkaliactivation reaction [12,14].…”

“…In alkaliactivated blended concretes these properties are strongly influenced by the content of fly ash incorporated [12]. Recent studies [13][14][15][16] have demonstrated that the durability of an AAC based on GGBFS/FA blends is directly linked to the microstructure of the reaction products forming in these binders, as a function of the nature and concentration of the activator, and also the maturity of the material. This is associated with the fact that the microstructure controls the transport properties of the binder and the stability of the matrix when exposed to aggressive agents.…”

A review on alkaline activation: new analytical perspectives

“…Nonetheless, gel structure may vary significantly depending on the degree of reaction, curing temperature and particularly the presence of soluble silica in the activator. Duxson et al (93) analysed the nanostructure of N-A-S-H gel generated during metakaolin alkaline Studies conducted by a number of authors (37,73,(85)(86)(87) showed that both curing temperature and the nature of the activator play a significant role in reaction kinetics. While the role of the alkalis in the alkaline aluminosilicate hydrate structure is assumed to be to neutralise the excess negative charge generated by the replacement of silica tetrahedra with AlO 4 species during gel formation, scant information is available on this question.…”

“…This gel was also observed to contain a small amount of alkalis (normally Na), that neutralised the charge imbalance created when a Si is replaced by an Al tetrahedron. Actually, then, these are calcium-sodium aluminosilicate hydrate [C-(N)-A-S-H] gels (70)(71)(72)(73) proposed a model to describe this gel as a mix of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM).…”

Synthesis of stoichiometrically controlled reactive aluminosilicate and calcium-aluminosilicate powders

“…This can be particularly challenging due to the wide variation of chemical and physical characteristics exhibited by commercial geopolymer precursors where a range of chemical reactivity is observed, Postprint of a paper published in Powder Technology, 29(2016): [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Version of record is available at http://dx.doi.org/10.1016/j.powtec.2016.04.006 3 meaning it is often difficult to distinguish reactive from inert species.…”