Given the efficiency of reducing the carbon footprint of cement and concrete by using alternative raw materials for their production, attention should be paid to the largely unused source of raw materials obtained through the more efficient recycling of old concrete. This approach allows to produce clean recycled coarse aggregates, sand, and cement paste. In this case, recycled cement paste can be reused as a replacement for part of the cement or fine aggregates. Using carbonated recycled cement paste as a supplementary cementitious material in concrete seems to be an especially efficient solution, as carbonating the paste not only binds CO2, but also results in some pozzolanic reactivity. At the same time, the most noticeable negative effect of carbonated cement paste reactivity is a decrease in the workability retention of cement mortars and concretes, especially when superplasticizers are used. The deterioration of the workability of such systems has been explained by the reaction of the decomposed (due to carbonation) products of calcium aluminates with the cement portlandite, resulting in the uncontrolled precipitation of ettringite and monosulfate. One possible solution to this problem is to control such precipitation reactions using retarders.