Microstructure and hardness of cement pastes with mineral admixture

Résumé

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 microstructure 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 crushed quartz.