Hardened cement paste activation as a pathway for concrete decarbonization

• Applicant: Professor Dr.-Ing. Frank Dehn
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541766792

Concrete stands as the world's most widely utilized building material, but its production heavily contributes to CO2-emissions, mainly due to the carbon-intensive process of cement clinker production. Consequently, a dual-pronged approach is imperative to mitigate these emissions: making production more eco-friendly and reducing the need for new cement clinker by repurposing hardened cement paste from demolished concrete. An innovative solution lies in leveraging these recycled concrete fines (RCF) to create new composite cements with partial clinker substitution through two distinct methods: thermal activation and carbon activation. Thermal activation involves subjecting hydrated RCFs to high temperatures. This process triggers a reverse reaction of cement hydration by disintegrating hydration products such as calcium hydroxide and calcium silicate hydrate (C-S-H). As a result, the materials used for concrete production after thermal activation closely resemble those of "new" cement. Conversely, in wet carbonation, CO2 is introduced to the RCFs. This interaction prompts C-S-H and CO2 to form an amorphous, pozzolanic (aluminum) silica gel. A notable advantage of this method is that it utilizes and stores CO2 in the activation process, potentially leading to negative CO2-emissions when producing new composite cements with lowered clinker content. Both methods exhibit the potential to reduce CO2-emissions and conserve resources. The question remains, which of processes prove to be most advantageous in terms of fresh and hardened concrete properties with regards to hydration heat release, setting times, flowability, strength development or CO2-emissions. While literature and preliminary experimental analyses have proven both methods to be effective, a direct comparison to highlight individual benefits and application limitations for individual parameters is lacking. The proposed strategy of this research proposal revolves around conducting parallel assessments of various composite cements and mortars using both activation methods, enabling a direct comparison of their performance. The process begins with the comprehensive examination and characterization of diverse precursors for RCFs. Subsequently, these materials undergo thermal and carbonation activation. In the next step, the activated RCFs are scrutinized and compared. These activated RCFs are then blended with traditional cements, gradually increasing the substitution rate for ultimately reaching a clinker content of up to 20 wt. %. The resulting mixtures are also characterized and analyzed. To evaluate strength development, all mixtures are produced as mortars and thoroughly assessed. Simultaneously, reference cements and mortars are consistently evaluated, serving as benchmarks for comprehensive assessment. Finally, the experimental results serve as input for extensive microstructural and multiscale modeling for predicting mechanical properties of composite cements containing recycled fines.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2436: Net-Zero Concrete