Philip Van Den Heede

Concrete manufacturing and recycling must evolve to meet sustainability and carbon reduction demands. While the focus is often on reusing coarse aggregates, fine fractions are also produced during recycling. This study explores using ground fine fractions (0/2) as a partial cement substitute. The fines were characterized for their mineralogical, chemical, and physical properties, and experiments were conducted on pastes and mortars with 0% to 30% cement substitution, including isothermal calorimetry and strength tests. Two concrete mixes—a reference mix with natural aggregates and CEM I, and a mix with 10% concrete fines replacing CEM I—using recycled sand and coarse aggregates were tested for compressive strength, carbonation, shrinkage, and freeze–thaw resistance. The results indicated that the recycled concrete had a comparable strength to the reference and a slightly reduced durability in freeze–thaw conditions. In terms of shrinkage, recycled concrete with 10% concrete fines had an increased drying shrinkage and a lower autogenous shrinkage due to the water retention capacity of the recycled aggregates.

In order to meet the new demand for sustainability and lower carbon emissions for concrete, the production and recycling process has to be adapted. Regarding concrete recycling, the main focus in literature and industry is on the reuse of coarse aggregates. However, the recycling of concrete also generates a fine fraction. In this research, the fine fraction with size 0/2 is ground and studied as a partial cement replacement. First of all, the fines were characterized and their physical, chemical and mineralogical properties were determined. Preliminary tests on pastes and mortars with 0%, 5%, 10%, 15%, 20%, 25% and 30% cement replacement, such as isothermal calorimetry and strength tests, were performed. Based on these results, two concrete mixes were made: one reference mix with CEM I as a binder and natural aggregates and a concrete mix with 10% of CEM I replaced by concrete fines and a combination of natural and recycled aggregates. In this second mix, 27.8% of the sand and 100% of the coarse aggregates were replaced by recycled sand and recycled coarse aggregates. The concrete samples were tested on compressive strength, shrinkage and freeze–thaw resistance. The compressive strength of the green concrete was comparable to the strength of the reference concrete after 28, 56 and 90 days of curing. Concrete specimens containing the recycled fines and recycled aggregates had a higher drying shrinkage; however, a lower autogenous shrinkage was measured in comparison to the reference. Regarding durability, the green concrete had a lower freeze–thaw resistance compared to the reference.

The chemical reaction between CO₂and a blended Portland cement concrete, referred to as carbonation, can lead to reduced performance, particularly when concrete is exposed to elevated levels of CO₂(i.e., accelerated carbonation conditions). When slight changes in concrete mix designs or testing conditions are adopted, conflicting carbonation results are often reported. The RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ has conducted a critical analysis of the standardised testing methodologies that are currently applied to determine carbonation resistance of concrete in different regions. There are at least 17 different standards or recommendations being actively used for this purpose, with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth after exposure. These differences strongly influence the carbonation depths recorded and the carbonation coefficient values calculated. Considering the importance of accurately determining carbonation potential of concrete, not just for predicting their durability performance, but also for determining the amount of CO₂that concrete can re-absorb during or after its service life, it is imperative to recognise the applicability and limitations of the results obtained from different tests. This will enable researchers and practitioners to adopt the most appropriate testing methodologies to evaluate carbonation resistance, depending on the purpose of the conclusions derived from such testing (e. g. materials selection, service life prediction, CO₂capture potential).

Fine recycled concrete aggregate (FRCA) possesses substantial limitations for its use due to its significant porosity. Extensive research on the effect of carbonation on the accessible porosity of coarse recycled concrete aggregate is available in the literature, but FRCA information is scarce. The present paper presents results of the effect of carbonation on different fractions of FRCA from <0.125 mm to 4 mm previously sprayed with wastewater from mortar production as an additional source of portlandite. The results of water absorption via the electrical conductivity method, TGA, XRD+Rietveld+PONKCS analysis, EDS mapping, and Life Cycle Assessment (LCA) demonstrate that the efficiency of the treatment depends on the particle size range of the FRCA. It was most effective for the extreme size fractions 1 mm of FRCA as a result of the relationship between the amount of precipitated calcium carbonate and the initially accessible porosity in each fraction. As a consequence, mortar produced with 30% replacement of natural sand with carbonated FRCA showed no impact on compressive strength.