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Abstract
In this study, we investigated the impact of aluminium ion (Al3+) incorporation on the microstructure and the phase transformation of the magnesium silicate hydrate system. The magnesium silicate hydrate system with aluminium was prepared by mixing magnesium oxide and silica fume with different aluminium ion contents (the Al/Si molar ratios of 0.01, 0.02, 0.05, 0.1, 0.2) at room temperature. The high degree of polymerization of the magnesium silicate hydrate phases resulted in the limited incorporation of aluminium in the structure of magnesium silicate hydrate. The silicon-oxygen tetrahedra sites of magnesium silicate hydrate layers, however, were unable to substitute for silicon sites through inverted silicon-oxygen linkages. The increase in aluminium ion content raised the degree of polymerization of the magnesium silicate hydrate phases from 0.84 to 0.92. A solid solution was formed from residual aluminum-amorphous phases such as hydroxyl-aluminum and magnesium silicate hydrate phases. X-ray diffraction (XRD), field emission scanning electron microscope (F-SEM), and 29Si and 27Al MAS NMR data showed that the addition of Al3+ promotes the hydration process of MgO and has an obvious effect on the appearance of M-S-H gel. The gel with low aluminum content is fluffy, while the gel with high aluminum content has irregular flakes. The amount of Al3+ that enters the M-S-H gel increased with the increase of Al3+ content, but there was a threshold: the highest Al/Si molar ratio of M-S-H gel can be maintained at about 0.006.
Highlights
As we all know, the hydration product of CaO-SiO2-H2O system is calcium silicate hydrate (C-S-H) gel, like C-S-H, at normal temperatures and pressures, and the main hydration product of MgOSiO2-H2O system is magnesium silicate hydrate (M-S-H)gel(Li et al, 2014; Jia et al, 2017a)
This study aims to understand the influence of Al3+ content on the phase transformation and the structural change of M-S-H
The cementitious system is all amorphous phase; and the characteristic peak of M-S-H gel at the broad diffraction at 22–30° becomes weaker with the increase of the dosage of Al3+, which may be the result of the change of molecular structure
Summary
The hydration product of CaO-SiO2-H2O system is calcium silicate hydrate (C-S-H) gel, like C-S-H, at normal temperatures and pressures, and the main hydration product of MgOSiO2-H2O system is magnesium silicate hydrate (M-S-H)gel(Li et al, 2014; Jia et al, 2017a). The molecular structures of magnesium silicate hydrate (M-S-H) and calcium silicate hydrate (C-S-H) phases are significantly different. The former is an amorphous precursor of talc or sepiolite with layered silica structure (Lothenbach et al, 2015; Bernard et al, 2019), while the latter is a silicadeficient solid solution and is a precursor of two crystalline phases, tobernorite and jennite SiO2 MgO Al2O3 SF CaO Fe2O3 0.2 < 0.01 K2O 1.6 < 0.01 Na2O
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