re-CEM: Applicability of Recycled Concrete Paste as SCM through CCU – elucidating the correlation between chemical composition, amorphous gel structure, and pozzolanic reactivity

The need of today’s society to reduce emission of greenhouse gasses and resource consumption is currently prevailing in discussions and media more than ever. The construction sector is a massive contributor to global CO2 emissions, with cement production alone accounting for approximately 7-8% of total anthropogenic CO2 emissions. The re-CEM project addresses this critical challenge by exploring the potential of carbon capture and utilization (CCU) technologies to convert recycled concrete paste (RCP) into a viable supplementary cementitious material (SCM). This research aims to clarify the complex relationships between RCP's chemical composition, the amorphous Si-Al-gel structure formed during carbonation, and the pozzolanic reactivity of the resulting carbonated RCP (cRCP). Cement production traditionally relies on high-energy processes and natural resources, leading to substantial CO2 emissions. The integration of SCMs into cement can significantly reduce these emissions. However, conventional SCMs like fly ash and slag are becoming scarce, necessitating the development of alternative materials. RCP, a by-product of concrete recycling, presents a promising solution due to its high availability and potential for CO2 sequestration when subjected to carbonation. Despite this potential, RCP is underutilized, with much of it ending up in landfills. The wet carbonation of RCP can enhance its reactivity by forming a reactive Si-Al-gel, transforming it into a highly pozzolanic SCM. The re-CEM project is organized into four interrelated work packages (WPs), each targeting key aspects of this transformation process: WP1: Preparation and characterization of lab-scale RCP and cRCP This work package focuses on preparing well-hydrated cement pastes that simulate the composition of RCP. These pastes will undergo wet carbonation, a process where gaseous CO2 is introduced into an alkaline solution containing the RCP, leading to the formation of carbonates and a Si-Al-gel. The resulting cRCP will be characterized by QXRD, TGA, FTIR, 29Si and 27Al NMR to determine its chemical and mineralogical composition, with particular emphasis on understanding how the initial composition of RCP affects the structure and properties of the carbonated material. WP2: Reactivity testing of cRCP and its impact on cement hydration WP2 is dedicated to assessing the pozzolanic reactivity of cRCP, which is crucial for its effectiveness as an SCM. This will be done using the R3 test, a standardized method that measures the heat of hydration and portlandite consumption to evaluate the reactivity of SCMs. In addition to reactivity tests, the impact of cRCP on the hydration process and microstructure of blended cements will be studied. This work package aims to identify the optimal replacement levels of ordinary Portland cement (OPC) with cRCP, which could range from 20% to as much as 50%. WP3: Correlation of composition, structure, and reactivity of cRCP The goal of WP3 is to establish a systematic understanding of how the composition and structure of the amorphous Si-Al-gel in cRCP correlate with its pozzolanic reactivity. By combining data from the characterization studies in WP1 and the reactivity assessments in WP2, this work package seeks to develop predictive models that can forecast the performance of cRCP based on its pre-carbonation composition. These models will be essential for determining the most suitable types of RCP for CCU applications and maximizing the efficiency of the carbonation process. WP4: Feasibility and upscaling of cRCP application WP4 explores the practical application and scalability of cRCP in the construction industry. This work package will test cRCP derived from real-world recycled concrete materials, evaluating its performance in mortar samples and comparing it to laboratoryprepared cRCP. The feasibility of integrating cRCP into industrial cement production processes will be assessed, with a focus on validating the laboratory findings from WP1-3. The findings from the re-CEM project are expected to provide a scientific basis for the broader application of CCU methods in the cement industry. By leveraging recycled materials like RCP, the project aims to contribute to developing sustainable construction practices that reduce CO2 emissions, promote resource efficiency, and advance the circular economy.      «

The need of today’s society to reduce emission of greenhouse gasses and resource consumption is currently prevailing in discussions and media more than ever. The construction sector is a massive contributor to global CO2 emissions, with cement production alone accounting for approximately 7-8% of total anthropogenic CO2 emissions. The re-CEM project addresses this critical challenge by exploring the potential of carbon capture and utilization (CCU) technologies to convert recycled concrete...     »