Improved storage and handling of fine recycled concrete aggregates by accelerated carbonation: towards a quantification method

The fines fraction generated by crushing of concrete waste for production of recycled concrete aggregates amounts to 40 - 60 mass-%. Technically, fine recycled concrete aggregates (0 - 2 mm, fRCA) may be employed as a sand substitute in new concrete products. This is, however, hampered by the materials tendency for bridging/caking during storage in open stockpiles or silos. Here we investigated whether the reactivity of fRCA can be decreased and, hence, its storage and handling be improved by accelerated carbonation. Carbonation experiments were conducted in a pilot-scale rotating drum reactor (void volume V0 ~ 75 L) at around unity CO2 partial pressure (re-dosing setpoints - 50 hPa and + 50 hPa in-vessel differential pressure) over reaction times of one hour. Re-dosing cycles and thermogravimetric analysis served to quantify the CO2 uptake by fRCA. Penetration resistance and sieve passage against control samples served to quantify the effect of carbonation on the bridging tendency of fRCA. Compared to control samples, carbonated fine RCA formed more brittle crusts upon exposure to ambient conditions. Thus bridging/caking of fRCA may be prevented by accelerated carbonation prior to storage. Consequences of the use of carbonated fRCA as a sand substitute in concrete are subject of ongoing investigation while test methods to assess the effects on handling and storage properties are under consideration.