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Calcium hydroxide in cement matrices: physico-mechanical and physico-chemical contributions

Calcium hydroxide (CH) represents a significant volume of the products formed from the cement-water reaction. The extent to which the CH phase contributes to the engineering integrity and volume stability of cement-based binders is however moot. Evidence in support of the view that calcium hydroxide has a significant role in determining the mechanical performance and volume stability of cementitious materials is presented. The elastic properties and fracture behavior of CH and CH – C-S-H mixtures are described. Volume stability of CH is described with reference to water sorption isotherms (CH) and length change measurements in various solvents and salt solutions. A dissolution-based expansion mechanism in which CH plays a major role is described for the behavior of cement binders in aggressive media. INTRODUCTION Hydrated Portland cement paste contains two major constituents – calcium silicate hydrate and calcium hydroxide. The latter can be present in amounts up to 26% by volume. The principal binding phase, calcium silicate hydrate (C-S-H) has been extensively studied. The properties and behavior of calcium hydroxide (either as a pure phase or in mixtures of cementitious solids) have been investigated to a much lesser extent. The importance of the role of calcium hydroxide (CH) in the development of the mechanical, physical and chemical behaviors of cementitious systems has often been understated and can be considered moot. This has been a result of difficulties associated with the separation of C-S-H and CH (in hydrated paste) without affecting the integrity of the composite system. The role of CH (in typical cement-based materials) with respect to descriptors of mechanical performance and durability is treated in this paper. The experimental evidence (obtained at the National Research Council, Canada) describing the physico-mechanical and physico-chemical contributions of CH in cement binders is collated and discussed. The contribution of CH is elucidated using the following approach. Reference to sorption phenomena (with water as adsorbate) and extension mechanisms are made to validate the use of compacted specimens as representative solid bodies. Compacted systems comprised of CH and CH/C-S-H mixtures are used as model systems to determine the potential contribution of CH to mechanical behavior and volume change stability in various chemical environments. This is a precursor to discussion of solvent exchange phenomena of CH compacts in methanol and isopropanol. This is followed by the presentation of length change (volume instability) behavior of CH in aggressive salt solutions. The microstructure and properties of paste systems including undisturbed CH-depleted pastes are also described to provide evidence of a significant role for CH in the systems investigated. SORPTION PHENOMENA Sorption isotherms (eg. length change) in addition to providing surfacechemical data contain information pertaining to the elastic response of the sample. The isotherm for CH compacts exhibits large primary and secondary hysteresis (Figure 1). Length change on second adsorption is reduced and the desorption curve is co-incident with the adsorption curve. There is large irreversible shrinkage on desorption. The length change – mass change curve (not shown) is linear up to p/po = 0.35. This is similar to that observed for hydrated portland cement paste. The length change on wetting is often referred to as the Bangham effect. It is due to the reduction of the solid surface energy resulting from the physical interaction of the surfaces with water molecules. The length change is directly proportional to the free energy change.