Greener Concrete Paving Blocks with Hybrid Alkaline Cements and Recycled Aggregates

Abstract

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. The construction sector is called to undertake solid steps in order to avoid being one of the most polluting industries. One of the avenues that should be exploited is the use of waste as raw material. In this work, a more sustainable concrete paving block incorporating a cement with a significant amount of fly ash as replacement for portland cement and recycled aggregates as partial subsitute of natural aggregates is developed. It is worth mentioning that the studied paving block complied with mechanical strength requirements stipulated by the European standard in force. The simultaneously use of an alternative binder and recycled aggregates in a concrete composition targeting the development of a concrete plain product (paving block) can be considered an additional contribution to previous studies that addressed those two research topics mostly in isolation within the context of the design of more sustainable concrete mixtures. The potential widespread use of this greener paving block will contribute to reduce the overwhelming extraction of natural resources carried out by the construction sector and the volume of waste sent to landfills, thus setting a path to reduce the environmental impacts that are harmful to both current and upcoming generations.