Transforming Waste Materials into Supplementary Cementitious Materials: An Approach to Mitigate the Carbon Footprint of Cementitious Composites

Abstract

This doctoral research investigates the transformation of waste materials into cement alternative to reduce the carbon footprint of cementitious composites while maintaining structural and durability performance. The study focuses on recycled gypsum drywall (RGD), untreated and copper-treated biochar, and hydrochar derived from spent coffee grounds as partial replacements for ordinary Portland cement (OPC). A comprehensive experimental program was conducted at both mortar and concrete scales. More than 490 mortar specimens, with a dimension of 50×50×50 mm, and over 360 concrete cylinders, with a diameter of 100 mm and height of 200 mm, were prepared and tested to evaluate compressive strength, bulk density, ultrasonic pulse velocity (UPV), stress–strain behavior, dimensional stability, and microstructural evolution. Cement replacement levels reached up to 50% for fly ash, 30% for recycled gypsum drywall (RGD), and 15% for biochar or hydrochar. Carbonaceous materials were evaluated using both direct cement replacement and additive incorporation approaches. Engineered interface modification using copper-treated biochar was also investigated to enhance interfacial transition zone stability. Results indicate that optimized OPC-RGD–fly ash systems improved long-term compressive performance compared with OPC–RGD blends, while maintaining dimensional stability. Low biochar dosages (≤5%) exhibited filler and internal curing effects, whereas higher replacement levels introduced stiffness-sensitive behavior due to the porous and low-modulus nature of carbonaceous inclusions. Hydrochar performance was governed by interaction mechanisms with other cement alternatives, where synergistic cement alternative systems mitigated long-term deterioration. Structural-scale stress–strain analysis demonstrated that peak broadness, elastic modulus, and post-peak response were sensitive to carbonaceous inclusion content, highlighting the need for dosage optimization in structural applications. Durability was evaluated under drying, marine, and sustained 5% sodium sulfate exposure for up to 6000 hours. Measurements showed bulk density variations remained within approximately ±2.5% of baseline values, with observed transitions from pore-filling densification to stiffness-sensitive disturbance under prolonged sulfate exposure. Environmental assessment revealed that incorporating RGD and high-volume fly ash mixtures reduced carbon dioxide emissions by up to 55% relative to conventional OPC concrete. Overall, this thesis establishes a mechanistic framework for stabilizing sulfate-bearing and carbonaceous waste materials in low-carbon cementitious systems while satisfying structural performance requirements.