Nanostructural characterization of Al(OH)₃ formed during the hydration of calcium sulfoaluminate cement

Abstract

Ye'elimite (Ca4Al6SO16) is a main mineral in calcium sulfoaluminate cements. Aluminum hydroxide is one of the products formed by hydration of ye'elimite. To characterize this phase, various aluminum hydroxides were synthesized from their chemical constituents using sol‐gel processing and compared with the aluminum hydroxide formed during the hydration of ye'elimite. The nanostructure of aluminum hydroxide formed during the hydration of ye'elimite was investigated in detail using X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. No evidence was observed that indicated the existence of amorphous aluminum hydroxide in the hydration of ye'elimite. The pH values, thermogravimetric analysis, particle morphology, and Al‐O coordination indicated that the aluminum hydroxide from the hydration of ye'elimite had a crystal‐like structure. The X‐ray diffraction analysis, particle sizes, and ion activity product showed that the aluminum hydroxide from the hydration of ye'elimite had a microcrystalline structure.

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

“…Similar XRD results can also be found in calcium aluminate cements . A very recent research has reported that the AH 3 phase formed during the hydration of C 4 A 3 has a microcrystalline nature due to (a) small particle size, (b) broad XRD peaks, (c) same Al‐O coordination and weight loss in the 200 to 280°C interval with respect to the well‐crystalline AH 3 phase, and (d) same solubility product with respect to microcrystalline AH 3 phase . Some studies found that a small amount of Ca might be present in the structure of the AH 3 phase due to the nucleation of the AH 3 phase on the surface of hydrogarnet nanocrystals .…”

“…Taking into consideration the principle of XRD techniques (ie, Ewald diagram and interference function (| G 2 |)), smaller crystal size leads to the broadening of diffraction peaks. As discussed in the previous study, there is no any periodicity or lattice structure in the truly amorphous phases, and no characteristic diffraction peak of XRD data is visible in the amorphous AH 3 phase. Thus, the AH 3 phase formed during the hydration of three structural modifications of ye'elimite had a small crystal size presented in a microcrystalline or nanocrystalline structure.…”

“…The Rietveld plot for the ss‐ye'elimite pastes at 28 days, as an example, is listed in . The measured amount of AFt and AFm in the ss‐ye'elimite pastes at 28 days were 19.5 wt.% and 20.6 wt.%, respectively, and that in the Sr‐ye'elimite pastes at 28 days were 21.8 wt.% and 15.5 wt.%, respectively (), in contrast to the content distribution of the st‐ye'elimite pastes . The types of final hydration products regarding Ba‐ye'elimite pastes were far more different than that of other pastes (Figure A).…”

“…The presence of the AH 3 phase can be inferred from some broad, multiple but weak X‐ray diffraction (XRD) diffraction peaks corresponding to the same reflection position of gibbsite . Similar XRD results can also be found in calcium aluminate cements .…”

“…Secondly, the final hydration products formed during the hydration of three structural modifications of ye'elimite varied significantly. Compared to the final hydration products of st‐ye'elimite, ettringite was detected in the ss‐ye'elimite pastes at 28 days. CAH 10 and strätlingite were also the types of characteristic hydration products for ss‐ye'elimite (Figure A).…”