Rate and mechanism of the reaction of silicates with cement pore fluids

Abstract

The reaction mechanisms and rates of reaction of a number of the common rock-forming silicates with synthetic cement pore fluids have been evaluated in a series of laboratory experiments at 70°C. Mass transfer is dominated by the dissolution of the primary silicate and the precipitation of a range of Na-K-Al substituted calcium silicate hydrates, and a possible zeolite. Calcium was lost from, and silicon gained by, the fluid phase as a result of the reactions. Secondary solids formed thick layers on primary silicates, but dissolution of the silicates was not diffusion-limited. The rate of dissolution of the silicates was determined to be 2–3 orders of magnitude greater at pH 12–13 than at neutral pH, and confirm measurements by other authors. The rate of growth of calcium silicate hydrates was limited by the rate of supply of silicon from the primary silicates. Although the results of the laboratory experiments were dominated by the loss of calcium from the fluid and the precipitation of calcium silicate hydrates, thermodynamic modelling suggests that these may be replaced by zeolites and/or feldspars when groundwater residence times are considered.