Influence of very fine fraction of mixed recycled aggregates on the mechanical properties and durability of mortars and concretes

Recycled aggregates from mixed recycled wastes have a high content of particles with size under 63 μm. This material is generally considered undesirable, since it increases water demand. However, these fines could fill out gaps between cement grains and increase packing density. The present work assesses the influence of fines under 63 μm on mortar and concrete made with mixed recycled aggregates. Pozzolanic activity of the powder, measured by isothermal calorimetry proves to be very low, so the material acts basically as a filler. Mortars with 6% of this fine material have a higher water demand, and thus a reduced compressive strength. However, the fine material contributes to decrease the effective porosity and increases the density of the matrix. This effect could help improving durability of the cementitious matrix. Alkalis tests carried out on mortars having 6% of fines under 63 μm show that expansion is reduced, and the probable cause is the refinement of the pore structure through the increased packing density. Observations of polished section of concrete having 6% of fines under 63 μm made with at SEM, coupled with image processing, show a densification of the ITZ around the aggregates, possibly due to the contribution of the very fine fraction to packing density. The beneficial contribution of the very fine fraction could compensate problems with rheology and mechanical properties assessed in mortars and concrete.