Carbonation, moisture and empty pores

Carbonation was measured on cement pastes preconditioned and maintained at selected relative humidities. Specimens with a range of water/cement ratios were tested, and the effect of replacing Portland cement with pulverized fuel ash or ground granulated blastfurnace slag was investigated. Carbonation was characterized using a variety of techniques in parallel. The various techniques provided descriptions of carbonation that were generally, but not always, self-consistent. Carbonation under normal environmental conditions of carbon dioxide concentration and relative humidity was largely controlled by gas diffusion through the empty pores in the exposed surface layer. The relative carbonation performance for mixes with different pore structures was strongly dependent upon moisture conditions during carbonation; the more porous mixes carbonated rapidly at the higher relative humidities where reinforcing steel in carbonated concrete would be likely to corrode.