Carbonation of C–S–H and C–A–S–H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy. (May 2015)

• Record Type: Journal Article
• Authors: Sevelsted, Tine F.; Skibsted, Jørgen
• Is Part Of: Cement and concrete research. Volume 71(2015)
• Page Start: 56
• Page End: 65
• Publication Date: 2015-05
• Subjects: Carbonation (C) -- Calcium–silicate–hydrate (C–S–H) (B) -- Spectroscopy (B) -- CaCO3 (D) -- Microstructure (B); Cement -- Periodicals; Cement -- Research -- Periodicals; Concrete -- Periodicals; Concrete -- Research -- Periodicals; Ciment -- Périodiques; Béton -- Périodiques; Cement; Concrete; Periodicals; 620.135
• DOI: 10.1016/j.cemconres.2015.01.019
• Languages: English
• ISSNs: 0008-8846

Abstract

Abstract: 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. 29 Si 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 13 C NMR. For C– A– S– H samples with Ca/Si = 1.0 and 1.5, 27 Al 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.