Microstructural comparison of the AH₃ phase in the hydration of three structural modifications of ye'elimite

Abstract

This paper investigated the hydration and formation of the AH3 phase in three structural modifications of ye'elimite (namely, ss‐ye'elimite, Sr‐ye'elimite, and Ba‐ye'elimite). Three structural modifications of ye'elimite were synthesized, and their reactions with water were studied. The similarities and differences of the microstructure of the final AH3 phase formed during the hydration of different structural modifications of ye'elimite were investigated using a multitechnique approach. The crystal structure of three structural modifications of ye'elimite had an important influence on the kinetics of the reaction. The AH3 phases formed from ss‐ and Sr‐ye'elimite pastes were nanocrystalline, whereas the AH3 phases formed in Ba‐ye'elimite pastes were microcrystalline. The distinct microstructures of the AH3 phase in three structural modifications of ye'elimite had been investigated by the amount, composition, and crystal size of the AH3 phase.

Microstructure Control of AH3 Gel Formed in Various Calcium Sulfoaluminate Cements as a Function of pH

“…Recent work has reported that Ba-ye'elimite had the fastest hydration rate among the three ye'elimite in the deionized water environment. 18 This work further confirmed this result in various alkaline environments. The quantitative phase analysis for each hydrated paste was carried out using Rietveld refinement with the internal method.…”

“…15 Additionally, a recent work, exploring the structure of the AH 3 phase formed by the three ye'elimite hydrated with H 2 O, suggested that the AH 3 formed by ss-ye'elimite and Srye'elimite had a larger crystal size; however, AH 3 formed by Ba-ye'elimite presented a smaller crystal size according to the saturation index. 18 It is worth noting that the AH 3 might be initially formed with an amorphous nature. Le Saout et al, 17 for instance, studying the AH 3 in CAC cements using a low L/ S ratio, found that no signals of the AH 3 phase can be detected in XRD or TG measurements at the early age (before several hours), which indicated that the amorphous AH 3 may be present in this stage.…”

“…18,24 Also, the microstructure of the AH 3 phase formed by different ye'elimite varied significantly, e.g., (a) the amount of AH 3 formed by Ba-bearing ye'elimite was low, but that by stye'elimite was high; (b) a small amount of Ca, Fe, and Sr existed in the corresponding AH 3 structure; and (c) the AH 3 phase formed by different ye'elimite exhibited different crystal sizes. 18 However, the influence of alkali on the AH 3 gel formed by ye'elimite with different crystal structures has not been investigated to date. As mentioned in previous studies regarding Portland cement, Ca(OH) 2 crystals with a large, bulky, and hexagonal plate-like morphology were formed and overlapped on adjacent bulky crystals, easily forming large pores; and C−S−H gel with a small size and an amorphous structure was formed with a large amount, making the paste dense and largely contributing to the strength.…”

“…32,33 Also, AH 3 with different sizes, contents, and compositions existed in various CSA-based cements. 18 pH largely affected the structure of the AH 3 phase. 19,21,22 Thus, owing to the favorability of the AH 3 phase, it is interesting to use pH to control the AH 3 phase formed by various CSA-based cements, which would be beneficial to design the CSA cement performances according to the requirements of civil engineering.…”

“…Entropy, free-energy changes in thermodynamics, and an irreversible and oriented aggregation mechanism may be the main reasons for the existence of this special AH 3 phase. The term named this special state of the AH 3 phase has been chaotic in the past. − Generally, a phase can be amorphous with the local order below 0.5–1.0 nm, nanocrystalline with the local order of 1–5 nm (very broad diffraction peaks), and crystalline with the long-range order larger than about 5–10 nm, which may have different crystallinity degrees. Therefore, the special state of the AH 3 phase would be named based on this guideline.…”