Steel slag, a by-product of iron and steelmaking industries, is nowadays largely adopted as aggregates in road construction or as supplementary cementitious material blended with Portland cement. While a large part of the produced steel slag is reused, especially in advanced economies, large amounts are still landfilled with significant economic and environmental impact. A concern arises from the high content in free lime and magnesia, leading to volume instability and limiting its use in building applications.

Recently, an increasing number of research studies focused on the use of steel slag to sequester carbon dioxide. The formation of stable carbonates by promoting reactions between CO₂ and calcium/magnesium oxides, referred to as carbon mineralisation, emerged as a valuable technique for CO₂ permanent storage. End-products are typically in the form of powder or mortar/concrete elements, as the volume stabilization, due to the conversion of the basic oxides into the corresponding carbonates, boosts the application as building materials.

Direct aqueous carbonation is induced in wet conditions by flushing CO₂ in slag slurry for carbonates precipitation. The end-product consists in carbonated powder that can be adopted as filler or supplementary material in blended cements. Semi-dry carbonation occurs by placing in dry CO₂ chamber solid samples produced by compacting fresh mortar/concrete mixtures, where finely milled steel slag acts as the binder. The carbonation confers mechanical properties to the end-product that can be adopted as monolithic element in constructions.

The mineralisation efficiency can be defined by quantifying the CO₂ content within the treated material. Although many different techniques can provide an estimate, they are not suitable for all carbonated end-products, and they are characterised by different reliabilities. Moreover, the lack of regulations makes it difficult to select the quantification method suitable for the specific end-product. As a consequence, although many studies focused on steel slag mineralisation, results are often controversial due to the use of inappropriate CO₂ quantification techniques or to poor attention to mineralisation efficiency assessment in favour of insights on the end-product mechanical performance.
 
This study proposes an innovative versatile approach for CO₂ content quantification that can be adopted for the assessment of mineralisation efficiency on samples of different nature, i.e. powders, pastes and mortars. Specimens are produced by means of slurry and semi-dry carbonation processes.
Two thermal analysis, TG-DTA and decomposition in muffle furnace, are adopted for carbonates quantification. These techniques assume that carbonate decomposition occurs in the temperature range 550°C-850°C. Acid digestion method, in which the chemical decomposition of carbonates is provoked through their reaction with hydrochloric acid, is considered as well.

The study, showing a CO₂ uptake of the different samples ranging from 13.3% and 16.6%, highlights a considerable efficiency of the adopted mineralisation processes. Moreover, it proves that by means of semi-dry carbonation it is possible to produce clinker free elements with mechanical performance suitable for application as building material. Finally, it provides useful findings to guide during the identification of the most suitable CO₂ quantification technique with respect to the nature of the carbonated sample.