Microstructure of recycled concrete

Abstract

Interfacial Transition Zone (ITZ) between old and new cement paste in recycled mortars and concretes is characterized. The microstructure is investigated during the first phase of hardening (2 and 28 days) for mortars and up to 1 year for concretes. Mortars and concretes made with over-saturated fine and coarse recycled aggregates develop more porous ITZ with lower anhydrous profiles. Transport of water from the over-saturated recycled aggregates to the cement paste, and water lens formation by micro-bleeding effect, are expected to explain this phenomenon. On the other hand, mortars made with dried fine recycled aggregates develop “denser” ITZ with anhydrous profile similar to that observed on mortars made with dried natural fine aggregates with the same targeted average W/C ratio. Transport of portlandite from the new cement paste to the old cement paste is observed during the first 28 days of hardening. This phenomenon is stronger for mortars made with over-saturated fine aggregates. The reduction of W/C ratio to obtain C25, C35 and C45 recycled concretes improves slightly the porosity profiles. But concretes based on 100% of recycled coarse aggregates have higher porous ITZ than concretes based on 30% of recycled fine and coarse aggregates. The low porosity of the ITZ of the C45 concrete based on 30% of fine and coarse recycled aggregates could be the consequence of a positive curing effect, with a contribution of the recycled aggregate water to cement hydration because of the partial desaturation during the hardening of the cement paste with a low initial water cement ratio (W/C close to 0.41). A treshold value for paste porosity which separates porous networks wealkly interconnected (low increase of permability with porosity) and porous network very interconnected (high increase of permeability with porosity) has been identified. The value is closed to 17%–18% of porosity.