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.

ABSTRACT Portland cement pastes are highly heterogeneous material and exhibits heterogeneous features over a wide range of length scales. Mechanical properties of microstructure can be determined using depth-sensing indentation. Coupled indentation/SEM technique can be used to location the indents and provides a way to determine the mechanical properties of a specific phase. Thus, the present paper aims to determine the hardness of different phases of cement pastes prepared with different mineral admixtures including sugarcane bagasse ash. The microstmcture of cement pastes prepared with different mineral admixtures is analyzed by X ray diffraction, scanning electron microscopy and dynamic hardness tests on polished sections. The different backscatter coefficient allows to differentiate anhydrous phases from C-S-H, calcium hydroxide, silica fume and quartz. A grid of indentation is used to determine the hardness of the different phases and a complete phase segmentation of the different samples is achieved. The results show that the hardness of the different phases follow the sequence (from higher to lower hardness) quartz, anhydrous particles, calcium hydroxide, C-S-H and agglomerated silica fume. The presence of agglomerated silica fume is clearly observed in scanning electron microscopy images and the poor mechanical properties of these areas might compromise the cement pastes. The microstructure of cement pastes prepared with sugarcane bagasse ashes is similar to the observed in samples with crashed quartz.