In this paper the tensile strength of Lightweight Aggregate Concrete (LWAC) produced with different types of expanded clay aggregates is characterized. A comprehensive experimental study was carried out on different concrete compositions with mean compressive strengths from 30 to 70 MPa and density classes from D1.6 to D2.0. The influence on the splitting tensile strength and modulus of rupture of the curing conditions and initial wetting of lightweight aggregate is studied. For concretes of equivalent compressive strength, the tensile strength is almost independent of the type of lightweight aggregate. The tensile strength of LWAC was about 0.8–0.85 of that of Normal Weight Concrete (NWC) of equal strength, decreasing to about 0.7 in LWAC with lightweight sand. Contrary to compressive strength, the LWAC with less porous aggregates has lower tensile structural efficiency than NWC. But the tensile structural efficiency of moist-cured concrete is little affected by the volume and wetting conditions of aggregate. There is a reduction in the splitting strength of air-cured concrete that is not predicted in the normalization and it is greater for NWC than for LWAC. However, the modulus of rupture of air-cured LWAC can only be about 0.5–0.8 of that of NWC of equal strength.

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.