Ultrasonic and calorimetric measurements on fresh concrete with blast-furnace slag

Blast-furnace slag (BFS), a by-product of the steel production, is commonly used in the production of blast-furnace slag cements but can also be added to the concrete mixture separately. The effect of BFS on the hydration process and on the setting of the fresh concrete has not been investigated thoroughly, certainly when high percentages of the cement are replaced by the slag. Therefore, calorimetric and ultrasonic measurements were performed on fresh mixtures made with increasing Portland cement replacement by BFS. With increasing BFS content, the second peak in the heat production rate under isothermal conditions appears earlier and a third peak attributed to the hydration of BFS becomes clearer. This slag reaction can also be observed as a second steep increase in the ultrasonic velocity curves. Thus, BFS clearly influences the early hydration and setting of the concrete mixtures