Quantification and micro-mechanisms of CO2 sequestration in magnesia-lime-fly ash/slag solidified soils

•Two quantitative approaches are proposed to estimate CO2 uptake in carbonated soils.•CO2 uptake grows with carbonation period, binder amount and magnesia/lime mass ratio.•Carbonation induces finer pore size, decreased pore volume and denser microstructure.•Mg and Ca carbonates are mechanisms for permanent storage of CO2 and pore-filling.•CO2 emission of MgO-CaO-fly ash/slag mix is largely reduced in comparison to cement. The combined use of CO2 and industrial by-products offers a novel alternative to traditional Portland cement in soil stabilization, sequestering permanently CO2 emissions and producing low-carbon cementitious materials in an accelerated carbonation environment. This study attempts to propose two approaches to evaluate the CO2 uptake amount of reactive magnesia-lime-fly ash/slag solidified soils, rather than soil improvement proved by previous findings. The CO2 uptake efficiency, pore structure and micro-mechanisms are examined, and the carbon footprint of each designed mixture is evaluated based on life cycle assessment. The key outcomes from accelerated carbonation, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests reveal that: (i) CO2 uptake amount and uptake efficiency estimated by direct weight gain are largely greater than that defined by indirect thermogravimetric analysis, (ii) CO2 uptake amount and uptake efficiency tend to increase with carbonation duration, binder content and mass ratio of magnesia/lime, (iii) accelerated carbonation induces reduced total pore volume, smaller pore size and denser microstructure and reactive magnesia contributes more than lime in filling pore spaces, (iv) magnesium and calcium carbonates are formed in magnesia-lime-fly ash/slag solidified soils storing permanently CO2, and (v) carbon emissions for magnesia-lime-fly ash/slag blends are greatly reduced during their whole product life cycle in comparison to Portland cement.