Carbonation of C-S-H and C-A-S-H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy

Synthesized calcium silicate hydrate (C-S-H) samples with Ca/Si ratios of 0.66, 1.0, and 1.5 have been exposed to atmospheric CO2 at room temperature and high relative humidity and studied after one to 12 weeks. Si-29 NMR reveals that the decomposition of C-S-H caused by carbonation involves two steps and that the decomposition rate decreases with increasing Ca/Si ratio. The first step is a gradual decalcification of the C-S-H where calcium is removed from the interlayer and defect sites in the silicate chains until Ca/Si = 0.67 is reached, ideally corresponding to infinite silicate chains. In the seconds step, calcium from the principal layers is consumed, resulting in the final decomposition of the C-S-H and the formation of an amorphous silica phase composed of Q(3) and Q(4) silicate tetrahedra The amount of solid carbonates and of carbonate ions in a hydrous environment increases with increasing Ca/Si ratio for the C-S-H, as shown by C-13 NMR. For C-A-S-H samples with Ca/Si = 1.0 and 1.5, Al-27 NMR demonstrates that all aluminium sites associated with the C-S-H are consumed during the carbonation reactions and incorporated mainly as tetrahedral Al(-OSi)(4) units in the amorphous silica phase. A small amount of penta-coordinated Al sites has also been identified in the silica phase. (C) 2015 Elsevier Ltd. All rights reserved.