Abstract

The microstructures of cement systems such as pastes, mortars and concrete control other physical properties such as strength, transport properties and durability. The microstructure of cement changes with time and is made up of hydrated and anhydrous phases, including pores containing pore water. This study investigated changes in the microstructures of plain and slag-blended cement pastes from 7 to 28 days. These were achieved using scanning electron microscopes (SEM) – backscattered electron (BSE) images. Two slags (S1&S2) were blended with CEM I 52.5R at 30% and 70% replacement. These were compared with plain CEM I 42.5R for microstructural developments. X-ray diffraction (XRD) was used to follow patterns of hydration products. The results for plain cement pastes show more coarser microstructure at 7 days than 28 days, indicating refinement as hydration progressed. In the slag blends, unhydrated Portland cement clinker grains were scarcely present after 28 days of hydration. Less presence of calcium hydroxide and more outer products calcium silicate hydrate (C-S-H) were observed in the slag blends. Hydration of plain cement at 20 ºC increased between 7 and 28 days, from 74% to 79% , while the weighted hydration of the slag blends at 20 ºC increased from 50% for 70% slag-blend, to 85% for 30% slag blend. Increasing curing temperature from 20 ºC to 38 ºC had a positive impact on the slag-blend.