Influence of sand drying and mixing sequence on the performance of mortars with fine recycled concrete aggregates

Fine recycled concrete aggregates (fRCA, 0–4 mm) are produced from demolished concrete structures and consist of natural aggregates and old cement mortar. The presence of old cement mortar has detrimental effect on the fresh properties and strength of new concrete. This study aims to investigate the working mechanism and effectiveness of different methods for the optimization of mortar mixtures with fRCA. Three streams of fRCA were considered in the approach. As reference material, river sand was used. The river sand was replaced at 0 wt%, 25 wt% and 50 wt% with fRCA. The use of tailor-made superplasticizers (SP's), drying of fRCA, modified content of 0–0.250 mm, modified mixing sequence, increase of cement content were investigated. Once the mortar mixtures were optimized, the reaction kinetics was investigated with isothermal calorimetry. SP was applied to prevent use of additional water and to maintain mix consistency. When the river sand was replaced at 25 wt% with fRCA, no extra cement was needed. The air content of mortars with fRCA was up to 18 %, due to some unforeseen effects. The results indicated that using as received, agglomerated and unwashed fRCA may have a negative effect on the working mechanism of SP leading to high air content in the fresh mortars. Using dried fRCA has substantially decreased air content in mortars. In addition to drying of fRCA, change of mixing sequence has equal or even superior importance to reduction of air content. As a result, the compressive strength was comparable to reference mix with river sand at 25 wt% replacement level, however, the strength of mortars with 50 wt% fRCA was reduced despite that the cement paste content was increased. The use of fRCA did not affect the kinetics and degree of cement hydration in mortars with 25 wt% fRCA. The positive side of this is that the fRCA can be considered as non-reactive.