Influence of metakaolin on pore structure-related properties and thermodynamic stability of hydrate phases of concrete in seawater environment

In this study, the pore structure of concrete incorporating metakaolin (MK) was characterized when exposed to two types of curing conditions, seawater and fresh water, respectively. The traditional properties such as compressive strength and carbonation resistance of concrete were experimentally evaluated in relation to their pore characteristics. Meanwhile, the influence of MK, carbonation and curing conditions on thermodynamic stability of hydrate phases were further investigated. The experimental results show that samples with MK have a lower content of porosity than those without MK. The seawater curing condition further causes a corresponding low porosity at early days and a corresponding high porosity at later days. The change is more remarkable and the chemical corrosion due to SO 4 2 - , Cl− and Mg2+ becomes worse in seawater at later curing days. The development of the compressive strength and carbonation resistance is related to the evolution of the pore structure. The analyses of thermodynamic stability of hydrated phases indicate that MK, carbonation and seawater curing condition influence the formation of hydration productions. In the MK-free system CaO–Al2O3–CaSO4–H2O, monosulfoaluminate is more stable than the phase assemblage of ettringite and C3AH6 above 5 °C. If MK is added to the system, the phase assemblages of ettringite and C3AS0.8H4.4 have a lower Gibbs free energy of reaction (ΔrG) and are therefore thermodynamically more stable than monosulfoaluminate at temperatures from 1 to 99 °C. Additionally, influence of carbonation and seawater environment on thermodynamic stability of hydrate phases are also analyzed according to changes of Gibbs free energy.