Investigation of pore structure and ITZ of concrete blended with mineral admixtures in a seawater environment

Abstract

The pore structure and interfacial transition zone (ITZ) in seawater concrete as indicators of marine concrete durability are explored and analysed. Mercury intrusion porosimetry, microhardness tester and scanning electron microscopy are employed to characterise the evolution of the pore structure and ITZ with curing time. Effects of mineral admixtures including slag, silica fume and metakaolin on pore structure and ITZ are also investigated. In addition, the influence of slag, silica fume and metakaolin on the thermodynamic stability of hydrate phases is investigated. The experimental results show that, compared to normal concrete, concrete subjected to simulated seawater conditions at early ages has a higher ratio of fine porosity as well as higher ITZ microhardness, and the ITZ is narrower. However, the microstructure of concrete in the seawater environment degrades up to longer ages. With the addition of mineral admixtures, the pore characteristics of marine concrete are greatly improved. The effect of the mineral admixtures on microstructure is in the sequence: metakaolin > silica fume > slag. Thermodynamic analysis indicates that stable hydration products ettringite (C6AsH32:6CaO·Al2O3·3CaSO4·32H2O) and siliceous hydrogarnet (C3As0·8H4·4:3CaO·Al2O3·0.8SiO2·4.4H2O) form due to pozzolanic reactions to optimise the microstructure and enhance the compressive strength of concrete.