Making Cement from Demolished Concrete: A Potential Circular Economy Through Geopolymer Chemistry

Abstract

Concrete is the most used construction material, but it is generally considered impossible to truly recycle and it has high embodied CO₂ content. To make concrete more sustainable, we must lower the CO₂ emissions associated with cement (cement production currently emits at least 5% of anthropogenic CO₂), and develop a way to truly recycle waste concrete back into cementitious material in support of a circular economy. In an attempt to recycle waste concrete for re-use as cement, a process was developed to convert concrete into a glassy cementitious reagent. This paper presents some preliminary performance results using this reagent as a supplementary cementitious material (SCM) in ordinary Portland cement (OPC) mixes, and as a primary reagent in geopolymer cement. Setting times, compressive strength, acid corrosion, and heat resistance were tested. In conclusion, waste concrete can be converted into a glassy cementitious reagent with lower anticipated CO₂ impact than OPC; this can be used as a primary reagent in geopolymer cement and as SCM in blended OPC. More detailed work is needed to develop, evaluate, and standardize this promising technique before commercial application. Nonetheless, this proof-of-concept is an important step toward low-CO₂ cement and a circular economy for concrete.