CO2 uptake by accelerated carbonation of recycled concrete aggregates – Co2ncrete

Abstract
Concrete construction is responsible for around 8% of global greenhouse gas emissions. Reducing the carbon footprint of concrete is becoming crucial as its global production is expected to increase sharply in the coming years. One way to achieve this goal is to use the ability of concrete to fix CO2 by carbonation. Controlled by the diffusion of gaseous CO2 gas, carbonation is a reactive transfer which turn calcium oxides into calcium carbonates. At the end of their service-life, concrete structures are reduced by crushing into recycled concrete aggregates (RCA). Commonly used in road construction, recycled aggregates are also more and more reused in concrete mixtures as a substitute for natural aggregates. The CO2NCRETE project is focused on the sequestration of CO2 by accelerated carbonation of aggregates recycled with industrial gases. Recent literature reviews showed that this approach is the most promising among the strategies envisaged to reduce the carbon footprint of concrete construction. Most studies also show that carbonation improves the properties of RCA and thus their recyclability. The carbonation of crushed concrete aggregates is therefore a step towards a circular and carbon neutral economy, called the wishes of Europe.

Among CO2 mineralization methods for capture, utilization and carbon sequestration (CCUS), carbonation of RCA can be envisaged as an attractive one, since the raw material, i.e. demolition concrete, is available in large quantity. The most obvious source of CO2 for this process is gas from cement kiln, since cement manufacturing is the main cause of the high CO2 emissions from concrete construction. However, there are other potential sources of gas with high concentrations of CO2. These includes petrochemical plants or more local sources such as household waste incinerators.

Optimizing the carbonation process is the main scientific obstacle raised by the accelerated carbonation of RCA. If the atmospheric carbonation of concrete is an inevitable phenomenon, the degree of carbonation under natural conditions can remain quite low. Consequently, the ambition of the CO2NCRETE project is a fundamental understanding of the sequestration of CO2 by RCA for gases rich in CO2 at high temperatures and / or pressures. The expected result is the identification of the levers of acceleration and maximization of carbonation by means of the process conditions. These are in particular the CO2 concentration, the pressure and the temperature of the gas, the contact time between the gas and the material and the initial state of RCA. To that end, the CO2NCRETE project is organized into five tasks offering original approaches, from experiments on carbonation of RCA under severe conditions to advanced modelling with coupling of discrete models and reactive transfer models. A task is also devoted to assessing the effectiveness of the processes with respect to environmental and economic impacts. The project brings together laboratories recognized for their work in the fields of carbonation, recovery of deconstruction waste, multi-scale modelling and thermochemistry of complexes, namely: LaSIE, GeM, IFSTTAR, IRC and LGM. CERIB completes this team to make the link with the industrial sector of concrete.