The effect of highly dispersed amorphous silica on the properties of concrete containing concrete rubble as coarse aggregate

A comparative study was conducted to examine the effect of highly dispersed types of amorphous silica with different specific surface areas—microsilica, precipitated silica, and colloidal silica—on the properties of concrete containing crushed concrete (concrete rubble) as coarse aggregate. It was found that the direct addition of ultrafine additives to a concrete mix containing recycled aggregate does not contribute to an increase in concrete strength. However, pre-treatment of the recycled aggregate with suspensions of ultrafine additives before adding the remaining components significantly increases the early strength of concrete with recycled aggregate, to the level of concrete with crushed granite. This can be explained by the positive effect of the additives on the weakened surface of the recycled aggregate grains. Moreover, precipitated silica, which has pozzolanic activity comparable to colloidal SiO₂ but a smaller specific surface area, is the most effective—possibly due to its higher resistance to coagulation in the concrete mix. It's also possible that the particle size distribution plays a role in its effectiveness.