Dehydration and Rehydration of Blast Furnace Slag Cement

Hydrated cementitious fines can be recycled by thermal treatment (<500 degrees C) as an alternative binder with low energy consumption and carbon dioxide (CO2) emissions during production. This paper analyzed the transformations of the phases during the dehydration and rehydration of slag-portland cement pastes using laser diffraction, surface area, X-ray fluorescence, X-ray diffraction, thermogravimetry, isothermal calorimetry, and compressive strength tests. Dehydrated slag cement fines can be rehydrated, reforming phases similar to that formed during pure hydration of clinker cement (calcium silicate hydrates, portlandite, and so on), but also other complex compounds, such as hydrotalcite (carboaluminates containing Mg). Dehydrated slag cement fines have high surface area. The heat released by rehydrated paste in the first hours is 10 times higher than that released by the hydrated paste due to the water readsorption and rebinding in the dehydrated cement phases (most of them amorphous). For similar water/binder ratios, the compressive strength of rehydrated cement paste at 28 days was about 66% that of the hydrated paste, using only approximately one-third of the thermal energy of the clinker of the cement manufacturing process.