High‐Resolution X‐ray Diffraction and Fluorescence Microscopy Characterization of Alkali‐Activated Slag‐Metakaolin Binders

Abstract

The effect of the activator concentration on the structure of alkali silicate‐activated slag/metakaolin pastes is assessed through synchrotron radiation‐based X‐ray techniques. As main reaction products, both calcium aluminosilicate hydrate (C–A–S–H) and sodium/calcium aluminosilicate hydrate [(C,N)–A–S–H] type gels are formed in activated binders solely based on slag, along with the zeolitic products gismondine and garronite. In activated blended pastes, the inclusion of metakaolin in the binder hinders the formation of zeolite products, instead favoring the formation of a (C,N)–A–S–H type gel consistent with the activation of metakaolin in the presence of high concentrations of Ca. The formation of the two distinct binding products is confirmed by high‐resolution X‐ray fluorescence microscopy, where the “inner” products and the “outer” products have compositions consistent with (C,N)–A–S–H and C–A–S–H type gels, respectively. These results provide important new insights into the gel chemistry and micro/nanostructure of blended alkali‐activated binder systems.

Microstructural Changes Induced by CO2 Exposure in Alkali-Activated Slag/Metakaolin Pastes

“…The fact that more weight is lost by samples with GBFS/(GBFS + MK) = 0.8 might indicate that water is more loosely bound in the reaction products forming in this paste. The small peak observed at 107°C in the data for samples with GBFS/ (GBFS + MK) = 1.0 and 0.9 is attributed to the dehydration of the zeolite gismondine (Bernal et al, 2011a), identified as one of the main crystalline reaction products in activated GBFS/MK under the activation conditions used in this study (Bernal et al, 2013b). Gismondine was not observed as a reaction product in pastes with 20 wt.% MK (Bernal et al, 2013b), consistent with the thermogravimetry results presented here.…”

“…High-resolution X-ray diffractograms of non-carbonated pastes of the compositions studied here, as a function of the curing time, were previously reported by Bernal et al (2013b). As the main partially crystalline reaction products, tobermorite-type phases with three different basal spacings (9, 11, and 14 Å) were observed in these cements, most likely with some Al substitution (Myers et al, 2013).…”

“…A sodium calcium silicate hydrate (Na2Ca2Si2O7·H2O; Powder Diffraction File (PDF) # 00-022-0891), and two aluminosilicate zeolite products, gismondine (CaAl2Si2O8·4H2O; PDF # 00-020-0452), and garronite (NaCa2.5(Si10Al6)O32·13H2O; PDF # 99-000-1300), the last of which was solely identified in specimens containing MK. Variations in the intensities of the reflections of some of these phases were observed as a function of the MK content, which is discussed in detail by Bernal et al (2013b). Other crystalline phases, including gehlenite (Ca2Al2SiO7, PDF # 00-035-0755), quartz (SiO2) (PDF #01-078-2315), and calcite (CaCO3) (PDF# 01-072-1214), attributed to the unreacted slag, were also observed.…”

“…The addition of up to 20 wt.% MK to alkali-activated slag cements does not have a notable impact in their mechanical performance, but in the case of highly reactive slags, it contributes to the ability to control the setting time (Bernal et al, 2010). When using high doses of activator (>5% Na2O by mass of GBFS + MK), a C-A-S-H type gel is formed as the main binding phase (Bernal et al, 2013b). Concretes and mortars produced with these high activator dose slag/MK blends developed high compressive strengths when compared to reference alkali-activated slag systems (Bernal et al, 2012a;Bernal, 2015).…”

Evolution of geopolymer binders: a review

“…For the 29 Si spectra both metakaolin and fly ash geopolymers showed a broad resonance in the range 80-100 ppm, however, the sharper peaks were noted for the fly ash geopolymers which may be related to the presence of the crystalline originally provided by the unreacted materials like quartz [10,23]. Figure 4.…”