Chloride-induced corrosion has been one of the main causes of reinforced concrete deterioration. One of the most used methods in assessing the chloride penetration resistance of concrete is the rapid chloride migration test (RCMT). This is an expeditious and simple method but may not be representative of the chloride transport behaviour of concrete in real environment. Other methods, like immersion (IT) and wetting–drying tests (WDT), allow for a more accurate approach to reality, but are laborious and very time-consuming. This paper aims to analyse the capacity of RCMT in assessing the chloride penetration resistance of common concrete produced with different types of aggregate (normal and lightweight) and paste composition (variable type of binder and water/binder ratio). To this end, the RCMT results were compared with those obtained from the same concretes under long-term IT and WDT. A reasonable correlation between the RCMT and diffusion tests was found, when slow-reactive supplementary materials or porous lightweight aggregates surrounded by weak pastes were not considered. A poorer correlation was found when concrete was exposed under wetting–drying conditions. Nevertheless, the RCMT was able to sort concretes in different classes of chloride penetration resistance under distinct exposure conditions, regardless of the type of aggregate and water/binder ratio.

Cement paste powder (CPP) is a by-product of the recycling process of concrete with an elevated carbonation capability and potential to be recycled as a binding material in new concrete batches. The application of a carbonation treatment to CPP improves this potential even more, besides the evident gains in terms of CO₂ net balance. However, the long duration usually adopted in this treatment, from 3 to 28 days, hampers the industrial viability of the process. We studied the feasibility of a short-duration carbonation process, with a duration of two hours, carrying out a comprehensive characterization of the material throughout the process. The test was performed on CPP with an average initial water content of 16.9%, exposed to a CO₂ concentration of 80%. The results demonstrate two main carbonation rates: a rapid growth rate in the first 18 minutes of the process, involving all the calcium-bearing compounds in CPP, and a slow growth rate afterwards, where only C-S-H contributes to the carbonation reaction. During the 2 h carbonation process, the main CPP compounds, calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), reached different carbonation degrees, 31% and 94%, with, however, close CO₂ uptake values, 8% and 11%, respectively. Nevertheless, the total CO₂ uptake for this process (≈19%) attained values not distant from the values usually obtained in a carbonation of 12 days or more (19–25%). Hence, these findings highlight the blocking role of C-S-H in the carbonation process, indicating that longer carbonation periods are only going to be useful if an effective carbonation of this compound is accomplished. In the present scenario, where CH is the main contributor to the reaction, the reduction in the process duration is feasible.

Carbon capture storage and utilization is the main technology for reducing CO₂ emissions, accounting for 56% of the overall reduction required to achieve the carbon neutrality of concrete by 2050. Different strategies have been explored in cement-based materials towards this end, namely, in concrete. However, the impact on carbonated concrete differs depending on the moment at which cementitious material comes into contact with CO₂, either in terms of CO₂ uptake or in terms of its lifetime performance. This paper presents three leading strategies that rely on the direct carbonation of a cementitious binder to reduce the carbon footprint. For each strategy, the effect of the carbonation process on the kinetics and microstructure of cementitious paste, the estimation of its carbon capture capability and the application feasibility are discussed. Accelerated carbonation curing is one approach widely studied by academics. However, despite some CO₂ capture effectiveness, its industrial processing is still a long way off. A second strategy consists of incorporating CO₂ during the mixing process, which has been shown to speed up the hardening reactions of cement. However, this effect is of short term and may negatively affect its long-term performance. Finally, the carbonation of hydrated cement waste is shown to be a very promising strategy that enables the recycling of hydrated cement waste as a supplementary cementitious material which also has a potentially high CO₂ uptake. The integrated analysis of the three strategies highlights a wide variability in the reduction of CO₂ emissions from 1% to 37% in relation to current emissions, where the best result was achieved using carbonated waste (third strategy) in the production of a concrete subjected to carbonation curing (first strategy).

The effort towards a greener future will entail a shift to more environmentally friendly alternatives of many human activities. Within this context, the path towards a decarbonized society in general, and industrial decarbonization in particular, will require using low carbon solutions and/or capturing carbon emissions at the source. This flux of captured carbon will then require management and one option is to store it in concrete. The incorporation of the captured CO2 can be done during the mixing and/or curing. While the latter is more efficient and effective in terms of the amount of CO2 incorporated, it is limited to concrete in elements that are compatible with chamber curing. In practice, this would be restricted to the concrete pre-fabrication industry and, most probably, only to small size elements. Despite the lower performance, incorporation of CO2 into concrete during the mixing stage is a relatively universal alternative. The present research effort reveals that the latter solution is beneficial from an environmental point of view, with an estimated yearly carbon storage of 23 million tonnes worldwide against emissions of 2.5 million tonnes to do it.

The recycling process of concrete originates a byproduct, cement paste powder (CPP), which is a material composed mainly of hydrated cement. This cementitious material has demonstrated promising results when applied as a binder in new concrete batches, provided it has been subjected to a previous carbonation process. One of the obstacles to the industrial application of this strategy is the long duration of the typical carbonation process, which requires from 3 to 28 days. Recently, the authors have developed a short two-hour carbonation process and thoroughly analysed it over its entire extension. In this paper, a parametric analysis of the carbonation process is performed towards CO₂ uptake maximization, aiming to increase the feasibility of its short duration. CO₂ uptake is evaluated using the ignition by furnace method and thermogravimetric analysis. Among the parameters considered, the initial water content and the CPP thickness present the highest impact on CO₂ uptake. The investigation of different CO₂ concentrations inside the carbonation chamber showed that the maximum CO₂ uptake does not occur for the highest concentration value. Moreover, a minimum resident time for the forced carbonation of CPP in industrial contexts is presented, and is found to be highly dependent on the CO₂ concentration. The particle size and purity degree of CPP revealed a limited influence on the CO₂ uptake achieved. Additionally, this paper provides further insight into the mechanisms involved in the carbonation of mature cement paste while increasing the feasibility of our recently proposed short duration carbonation process.