Recycling of demolition wastes for manufacturing CO2-reduced cement

The cement industry is one of the major contributors to global CO₂ emissions, primarily due to the limestone decarbonation and high energy consumption during clinker production. With increasing regulatory pressure to meet climate goals and the declining availability of traditional Supplementary Cementitious Materials (SCMs) like fly ash and blast furnace slag, there is a growing need to explore alternative, sustainable materials. This research addresses that need by investigating the recycling of fine construction and demolition waste (CDW) into viable SCMs for the production of CO₂-reduced cement. The thesis is structured as a cumulative work comprising four peer-reviewed articles, each contributing to the overarching goal of assessing the technical feasibility and durability performance of CDW-derived SCMs. The study specifically focuses on fine fractions of recycled concrete and ceramic waste, two abundantly available yet underutilized by-products in the construction sector. A key innovation in this research lies in the thermal treatment of hydrated cement paste within recycled concrete at moderate temperatures. This process successfully reactivates cementitious phases without decomposing carbonates, thus minimizing CO₂ emissions. Concrete fines were subjected to activation at two temperatures (400 and 500 °C), and their hydraulic reactivity, fresh and hardened properties, and microstructural characteristics were systematically evaluated. Similarly, the pozzolanic potential of finely ground ceramic waste, such as terracotta and porcelain, was studied in both ordinary blended cements and LC3 (limestone-calcined clay cement) formulations. Comprehensive durability assessments were conducted, including 28 freeze-thaw cycles, fire resistance at 200, 300, 500 and 900 °C, 1% sulphuric acid solution attack, shrinkage, and capillary absorption. The findings indicate that incorporation of recycled concrete-based powders does not significantly affect the durability performance of mortars, neither before nor after thermal activation. By contrast, ceramic roofing wastes significantly enhance the long-term performance of blended cement systems when properly processed, due to the formation of densified microstructures through pozzolanic reactions. Importantly, the study also explores the practicality of using unsorted mixed waste streams, providing insights into cost-effective and scalable industrial applications. The research outcomes support the viability of a circular economy model in the cement and construction industries by offering environmentally sound, technically robust alternatives to traditional SCMs. Through statistical analysis, the work establishes correlations between chemical composition, particle treatment, and performance indicators, offering a scientifically grounded framework for industrial implementation. This thesis contributes to the fields of materials science, environmental engineering, and construction technology by presenting a holistic approach to reducing the carbon footprint of cement through waste valorisation. The results not only pave the way for more sustainable building practices but also provide a strategic solution to the twin challenges of construction waste management and climate change mitigation.