Dissolution kinetics of crushed concrete waste: Effect of pH on leaching behaviour

Large volumes of concrete wastes are produced during building demolition that potentially can be reused as a secondary material. Reuse of crushed cementitious construction wastes as a fill material can result in leaching of reactive cement phases. The short-term dissolution kinetics of crushed concrete waste (CCW) from a UK nuclear site were studied in leaching tests using deionised water, acidic and alkaline leachants. The pseudo-steady state leaching rates for major and trace element constituents were determined as a function of leachate pH. At alkaline pH (>10), Ca and Si leaching rates were similar (2-6.5 x 10^-11 mol m^-2 s^-1) producing leachates with Ca/Si ratios (1.1 ± 0.4) suggesting congruent leaching of the calcium silicate hydrate phases in the CCW (pre-leached Ca/Si = 0.9 ± 0.3). Below pH 10, the Ca/Si ratio in the leachate increased with decreasing pH because Ca leaching rates increased without much variation in the Si leaching rate. In this pH range leaching processes were dominated by initial rapid calcium carbonate dissolution and incongruent dissolution of calcium silicate hydrate phases, which produced Ca-depleted solids with overall higher rates of mass loss than in alkali-leached solids. The leaching rates of Mg, Fe and Mn were pH-dependent (with higher leaching rates at low pH) and solubility-controlled whilst the leaching rates of Cr, V, Pb, As, K and Zn were mostly pH independent. Trace element leaching rates were generally low relative to Ca or below detection between pH 3-13. This information can be used to help predict CCW leaching behaviour when disposed in scenarios where meteoric or groundwater flow through the waste is expected.