Evaluating the effect of autoclave curing on the microstructure and compressive strength evaluation of a high strength concrete

ABSTRACT

High strength concrete is usually cast with low water-to-binder ratio (w/b) in order to produce a compact microstructure with high strength and durability. Autoclave curing can accelerate cement hydration and pozzolanic reactions and further modify microstructure of cement pastes (such as decreasing the Ca/Si ratio and forming crystallized calcium silicate hydrate). The present paper evaluates the effect of w/b ratio, in the range 0.20 – 0.40, on compressive strength, porosity and microstructure of a high strength concrete. Some samples were subjected to autoclave curing in order to evaluate the effect of this process on the parameters evaluated. The microstructure was evaluated using X-ray diffraction and scanning electron microscopy. The results show the porosity decreased when w/b ratio decreased from 0.40 to 0.30 and thereafter the total porosity remained virtually constant while the water permeable porosity increased. An inverse relationship is observed between the porosity and the compressive strength. Autoclave curing affects significantly the phases developed in hardened concrete. It accelerates hydration and pozzolanic reactions, affects the interfacial transition zone and triggers the formation of crystalline C-S-H. These changes reflect in improved compressive strength at a low w/b but its effect is negligible at moderate w/b ratios.

Keywords
Calcium silicate hydrate; Autoclave curing; UHPC; Quartz flour; Compressive strength and porosity