2. Cementitious Materials and Their Sorption Properties

The cementitious materials considered here include hydrated cement grout, concrete, and mortar. These materials may be used for waste conditioning, as waste container, and for backfilling. Dry cement (clinker) is a hydraulic binder; concrete is a mixture of hydrated cement, water, and coarse and fine aggregates; mortar and grout are a mixture of water, hydrated cement, and fine aggregates. Clinker is a mixture of several anhydrous minerals, mainly calcium silicates. Ordinary Portland cement (OPC) consists of finely ground clinker plus a small amount of gypsum. Hydration results in the formation of portlandite Ca(OH)₂, largely amorphous calcium silicate hydrates (CSHs), and minor crystalline phases containing aluminium, iron, and sulphate (e.g. ettringite, monosulphate, hydrogarnet, and hydrotalcite). Solutions in equilibrium with fresh hydrated cement are hyperalkaline (pH ≥ 13.2) with correspondingly high Na- and K-concentrations. It follows that hydrated cement systems are not in equilibrium with infiltrating water, which leads to dissolution and re-precipitation reactions. It is well established that the degradation of hydrated cement follows a pattern of several different, more or less distinct states, characterised by progressively lower pore water pH and Ca/Si ratio. Hydrated cement solid phases provide a variety of potential sites for different uptake reactions for dissolved elements, ranging from surface adsorption to incorporation and solid-solution formation (these processes are termed sorption henceforth). At low sorbate concentration (below the respective solubility limit), use of a single distribution coefficient for quantifying uptake under a defined set of conditions (e.g. a specific degradation state) is a defensible approach.