The German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt) funded a research project (reference 35818/01‑23) that culminated in a December 2024 report by a team of engineers and professors from several German universities. The project’s goal was to reduce the environmental impact of concrete by combining a clinker‑efficient cement with recycled concrete aggregates that had been subjected to accelerated carbonation. The collaboration involved experts in materials science, civil engineering and environmental assessment, and the work was carried out over a period that concluded in late 2024.

Technically, the study focused on two types of recycled aggregates: coarse recycled concrete aggregate (cRCA) and fine recycled concrete aggregate (fRCA). These were used as partial replacements for natural stone and sand in concrete mixes designed for railway platforms and high‑durability architectural applications. To mitigate the higher water absorption, porosity and potential alkali‑silica reaction (ASR) associated with recycled aggregates, the researchers applied an accelerated carbonation process in a rotary reactor. Carbonation converted calcium hydroxide and calcium‑silicate‑hydrate in the aggregate into calcium carbonate, thereby reducing water uptake and improving the interfacial transition zone (ITZ) between aggregate and cement matrix.

The performance tests showed that both the non‑carbonated and carbonated aggregate mixes exhibited ASR expansion values below 1.00 mm/m, qualifying them for the lowest ASR class (E‑I‑S) under the European directive. The carbonated aggregates even achieved lower expansion, indicating a stronger suppression of ASR. Workability measurements revealed that a multi‑stage mixing procedure was necessary to accommodate the higher water absorption of the aggregates, but the final mixes maintained acceptable slump values for construction. Compressive strength development was monitored over 28 days; the carbonated mixes reached strengths comparable to or slightly higher than reference mixes containing conventional Portland cement, demonstrating that the reduced clinker content did not compromise structural performance. Frost‑salt resistance tests confirmed that the carbonated aggregates did not adversely affect freeze‑thaw durability, and packing density optimization further improved the mechanical properties by reducing voids.

In addition to the concrete performance, the project conducted an environmental life‑cycle assessment. The use of clinker‑efficient cement lowered CO₂ emissions by approximately 20 % compared with ordinary Portland cement. Accelerated carbonation of the recycled aggregates captured significant amounts of CO₂; the study quantified uptake for various concrete waste streams, showing that concrete sludge—a by‑product of concrete production—has a particularly high carbonation potential. These findings suggest that the combined approach of using recycled aggregates and clinker‑efficient cement can substantially reduce the carbon footprint of concrete while maintaining or improving key durability characteristics.

Overall, the project demonstrated that a hybrid strategy—replacing natural aggregates with carbonated recycled aggregates and substituting a portion of Portland cement with a clinker‑efficient binder—yields concrete suitable for demanding infrastructure and architectural uses. The collaboration, supported by the German Federal Environmental Foundation, produced a comprehensive set of technical data and environmental metrics that can inform future sustainable concrete design and policy.