Valorisation of Recycled Cement Paste: Feasibility of a Short-Duration Carbonation Process

Abstract

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.

Parametric Study towards Optimization of a Short Duration Carbonation Process of Recycled Cement Paste

“…All the curves present near-steady behaviour during the second hour; for this reason, this period was not included. C25% and C40% are included in Figure 4 a,b, respectively, along with the C80% curves [ 23 ] in both figures (grey lines) for easier comparison. The results indicate similar behaviour for each CO 2 concentration, with a much higher carbonation rate phase before 2SP than after this point, as previously observed for C80%.…”

“…However, by the time process reaches 2SP, the three CO 2 concentrations present similar CO 2 uptake values, between 22% and 24%, suggesting a carbonation threshold at this point. This result can be explained by the depletion of calcium-bearing compounds other than C-S-H in CPP, promoting deceleration of the carbonation reaction rate [ 23 ]. In fact, after 2SP, among these calcium bearing compounds only C-S-H remains active in the carbonation reaction.…”

“…Comparing the maximum heat peaks, Figure 4 indicates that this parameter is directly related to the CO 2 concentration (the higher the concentration, the higher the peak). Note that the heat burst is mainly associated with the carbonation reaction of CH, the second highest calcium-bearing compound in CPP (wt.%) [ 23 ]; thus, a higher heat peak is associated with a higher CH carbonation rate. Hence, the temperature results suggest that C80% has the highest CH carbonation degree at 1SP, while C25% presents the lowest.…”

“…For this purpose, the previous setup was adapted ( Figure 2 a) to enable the evaluation of the mass growth rate of CPP during the 2 h test. Hence, three stopping points were defined (1SP, 2SP, and 3SP) for three different rates of mass growth (rapid, moderate, and slow), as shown in Figure 2 b [ 23 ].…”

“…Despite this, the long duration of the carbonation process usually applied, ranging between 12 and 28 days, hampers the implementation of an industrially viable process. To tackle this problem, a short-term carbonation process with a duration of two hours and a CO 2 concentration of 80% was developed and thoroughly analysed in its entire extension in our previous paper [ 23 ]. The setup parameters defined there allowed for a significant carbonation degree (47.9%), especially considering the low duration of the process.…”