Hydrated cement paste - thermodynamics of length change

The length change behaviour of microporous cement hydrates exposed to both methanol and water vapour is described in thermodynamic terms and is correlated with the elastic and adsorptive properties of the adsorbent. The approach assumes that both the adsorbate and adsorbent can each exist separately in equilibrium with externally applied forces in states that are thermodynamically identical with their states in the combined adsorbate-adsorbent system. Estimates of the compressibility of the solid hydrates using scanning curves obtained from methanol adsorption isotherms are similar to those obtained from reconstructed ‘reversible’ water adsorption isotherms satisfying the requirement for thermodynamic reversibility. Calculations of the modulus of elasticity as a function of the equilibrium partial pressure of both methanol and water adsorbates are in agreement with measured values. Differences in the role of water and methanol in the elastic response of the hydrated cement matrix are discussed.