Chemical changes and phase analysis of OPC pastes carbonated at different CO2 concentrations

Abstract

Chemical changes and phase analysis of OPC pastes exposed to accelerated carbonation using different concentrations of CO2 (3%, 10% and 100%) have been undertaken and compared with those of natural carbonation (≅0.03%). 29Si Magic Angle Spinning-Nuclear Magnetic Resonance (29Si M.A.S-N.M.R), Thermogravimetric analyses (TG) and X-Ray Diffraction (XRD) have been used for characterisation. The carbonation of the samples has resulted in a progressive polymerisation of CSH that leads to formation of a Ca-modified silica gel and calcium carbonate. The carbonation of CSH and portlandite occurs simultaneously and the polymerisation of the CSH after carbonation increases with the increase in concentration of CO2. When ≅0.03% and 3% CO2 are used, CSH gel with a lower Ca/Si than that of the uncarbonated sample, and quite similar for both samples remained. When carbonating at 10% and 100% of CO2, the CSH gel completely disappears. For every condition, a polymerised Ca-modified silica gel is formed, as a result of the decalcification of the CSH. From these results it can be deduced that among the different concentrations of CO2 tested, carbonation up to a 3% of CO2, (that is to say, by a factor of 100) results in a microstructure much more similar to those corresponding to natural carbonation at ≅0.03% CO2 than those at the 10% and 100% concentrations.