Greener Concrete Paving Blocks with Hybrid Alkaline Cements and Recycled Aggregates

The construction sector must deliver methods of construction targeting the valorization of waste that shows potential to play value-added roles. This is a key step to reduce the heavy environmental impacts caused by a sector whose outcomes are decisive to the well-being of society. The present study is intended to address the production of concrete paving blocks using a cement with a reduced clinker content and recycled aggregates. For achieving such purpose, hybrid alkaline cements with fly ash and portland cement were studied. The research involved the development of pastes and mortars prior to concrete production. The study of mortars included compositions with two different types of recycled aggregates. After evaluating the behavior of those mortars, a recycled aggregate was selected to be used in concrete. Once the most proper composition was determined, concrete and, finally, paving blocks were developed. Besides monitoring the heat flow and total heat released by pastes through isothermal conduction calorimetry, experiments included the assessment of the compressive strength of pastes, mortars, and concrete. In addition, X-ray diffraction (XRD) and backscattered scanning electron microscopy/energy dispersive X-ray (BSEM/EDX) were used to characterize the developed materials. The results showed that the type of solid activator has impact on the early-age strengths and kinetics of hybrid alkaline cements. It is worth mentioning that it was possible to produce a concrete paving block meeting European standard mechanical demands having a more sustainable binder with high fly ash content activated by a solid chemical compound of moderate alkalinity and replacing coarse natural aggregates with recycled aggregates. The commercial-scale production of these greener paving blocks using waste as raw material would contribute to a more circular and sustainable construction sector.