EFFECT OF CALCIUM HYDROXIDE CONTENT ON THE FORM, EXTENT, AND SIGNIFICANCE OF CARBONATION

摘要

The literature is incomplete on describing the significance of carbonation as a deleterious phenomenon in concrete, except for its effect on corrosion of reinforcement. The authors present recent observations that indicate that, under certain conditions, carbonation can have a severe negative impact on strength development. The results also indicate that addition of pozzolans to concrete with high water/cement ratio increases the likelihood that carbonation will reduce the strength of concrete. These results are based on microstructural observations using scanning electron microscopy techniques correlated with microhardness data. It is proposed that, as carbonation proceeds, the reduction in the pH shifts the form of the carbonation from a microscopic precipitation of carbonate (which densifies the paste) to a form which is dominated by growth of discrete calcite crystals in a matrix of silica gel. The strength of the resulting microstructure is extremely dependent on the moisture content of the concrete. This effect is further aggravated by the presence of competing demands for calcium hydroxide, either from pozzolonic activity, leaching, or sulfate attack.