Abstract
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.
Highlights
In the Netherlands, yearly 33 Mton new concrete is produced while 12 Mton concrete recycled aggregates becomes available
It can be seen that the flow of the mortars increased when fine recycled concrete aggregates (fRCA) was used as dried in the mixing while the air content decreased
The air content was considerably higher in mixes C25 and D25 compared to reference mix A100 and mix B25
Summary
In the Netherlands, yearly 33 Mton new concrete (roughly 15 million m3) is produced while 12 Mton concrete recycled aggregates becomes available. Little knowledge and no guidelines or regulations exist to make optimal use of these characteristics of fRCA in order to make high quality concrete [14,15]. Various subfractions with their own gradings are possible within the 0–4 mm fraction of fRCA. Each of these subfractions have their own physico-chemical characteristics depending on the parent concrete, recycling technology as well as the storage of recycled material [16,17,18]. These physico-chemical characteristics have an influence on the performance of the fresh concrete mix as well as the hardened concrete [9,19,20,21]
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