Abstract
The effects of Ca/Si ratio, aluminum and magnesium on the carbonation behavior of calcium silicate hydrate (C-S-H) were investigated by using X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR) and thermogravimetric analyzer (TGA). The results showed that the Ca/Si ratio, Al/Si ratio and Mg/Si ratio had a significant influence on the structure, carbonation products and carbonation resistance of C-(M)-(A)-S-H. The mean chain length of silicate chains in C-S-H increased as the Ca/Si ratio decreased. Aluminum uptake in C-S-H increased the content of bridging silicate tetrahedron (Q2). A cross-linked structure (Q3) appeared when magnesium uptake in C-S-H. The carbonation product of C-S-H was vaterite if the Ca/Si ratio was lower than 0.87. The carbonation products of C-S-H were vaterite and calcite if the Ca/Si ratio was higher than 1.02. C-M-S-H had more polymerized units, stronger bond strength and better carbonation resistance than C-S-H.
Highlights
Calcium silicate hydrate (C-S-H) is the main reaction product of alkali-activated slag (AAS) [1].The carbonation of C-S-H is a process of decalcification when CO2 reacts with calcareous phases, resulting in the decomposition of C-S-H, the formation of calcium carbonate with different crystalline forms and silica gel, and damage to AAS [2,3,4]
The carbonation behavior of C-S-H mainly depends on its structure, which is affected by the Ca/Si ratio, uptake of aluminum and magnesium in C-S-H [7,8,9,10]
The effects of Ca/Si ratio, Al/Si ratio and Mg/Si ratio on the structure and carbonation behavior of C-(M)-(A)-S-H were investigated by means of X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR) and thermogravimetric analyzer (TGA)
Summary
Calcium silicate hydrate (C-S-H) is the main reaction product of alkali-activated slag (AAS) [1].The carbonation of C-S-H is a process of decalcification when CO2 reacts with calcareous phases, resulting in the decomposition of C-S-H, the formation of calcium carbonate with different crystalline forms (i.e., calcite, aragonite and vaterite) and silica gel, and damage to AAS [2,3,4]. The carbonation behavior of C-S-H mainly depends on its structure, which is affected by the Ca/Si ratio, uptake of aluminum and magnesium in C-S-H [7,8,9,10]. There is a model named the “Cross-linked Substituted Tobermorite Model” (CSTM) that can explain the structure evolution when aluminum uptake in C-S-H [18]. Al (V) represents the aluminum enters the position of interlayer. Al (VI-1) represents the aluminum that enters the position of interlayer or in the C-S-H structure by replacing calcium [24]. Reference [26] suggested that the aluminum mainly taken up as Al (VI) in TAH when the Ca/Si ratio of C-S-H was high, otherwise, Materials 2019, 12, 1268; doi:10.3390/ma12081268 www.mdpi.com/journal/materials
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