**1. Introduction**

The construction industry is one of the most relevant sectors contributing to global warming, involving the extensive consumption of raw materials, depletion of non-renewable resources, extensive greenhouse gas (GHG) emissions and significant construction and demolition waste (CDW) disposal. Therefore, stringent environmental measures and relevant international agreements have been established to reduce the environmental impact of this industry. In this context, large companies worldwide have been investing in more sustainable practices, promoting the development of alternative more eco-efficient building materials towards a truly circular economy supported on recycling, resource efficiency and low carbon emissions [1, 2]. Two major goals of the European council are the reduction of GHG emissions to 60–80% by 2050, on a 1990 base year and the reuse of at least 70% of CDW, excluding backfilling operations [3, 4].

In particular, concrete, as the most used building material in the world and involving substantial extraction of raw materials, significant GHG emissions and extensive CDW generation, becomes a serious source of environmental concern [5, 6]. Cement is the main concrete constituent responsible for the significant carbon footprint of concrete, accounting with over 80% of the total CO2 emissions of concrete production [7]. In fact, over 5% of the world's anthropogenic CO2 emissions are attributed to the cement industry [8, 9]. This alarming value is expected

to further increase due to the ever-growing demand for this product, which is expected to rise by over 20% in 2050 [8, 9]. Therefore, the concrete industry and the scientific community have been focused on the urgent development of more sustainable eco-efficient concrete. The use of recycled aggregates in concrete production has been largely studied [10], but its effective acceptance in the construction sector is still a way off. On the one hand, the available technology is not efficient in providing high quality recycled aggregates with minor contamination with adhered paste that reduces their physical, mechanical and durability properties. On the other hand, the simple substitution of natural aggregates with recycled aggregates fails to significantly reduce the GHG emissions, which, as mentioned, are essentially related to cement production. Therefore, various strategies have been implemented concerning the efficient reduction of cement's environmental impact [8, 11, 12], namely the development of carbon capture solutions, new and more efficient production technologies, alternative fuels, alternative cements and the reduction of the clinker to cement ratio. Among these carbon reduction levers, the most promising and effective solution lies in carbon capture, but its viable implementation has yet to overcome some challenges [13]. The use of supplementary materials as partial clinker substitution has been considered for many years, but further GHG emissions reduction with currently available mineral additions is hard to explore. Moreover, the availability of some of these additions, namely those that are by-products of pollutant industries, such as fly ash, is becoming scarce.

Alternative low-carbon cements, such as calcium aluminate and alkali-activated cements have also been the object of intense research, but their implementation in the construction market is still far from being economically viable [13, 14]. More recently, a very promising approach relies on the production of low-carbon recycled cements (RC) from dehydrated waste hardened cement. The idea is to recover the binding properties of waste cement through its thermal activation at low temperatures, reducing the thermal energy of the clinker manufacture and avoiding the limestone decarbonation phase, which represents about 60% of the carbon emissions during the sintering process [13, 15].

The rehydration capacity of concrete subjected to high temperatures has long been shown from post-fire studies of concrete behavior [16–18]. This recovery was found to be related to the regeneration of new hydration products, despite the eventual existence of unreacted cement left in old concrete [19]. The possible reactivation of the hardened cement was a relevant finding and many authors started to explore this idea regarding the production of an innovative recycled binder. As mentioned, if thermoactivation temperatures are set to be under the decarbonation stage, the CO2 emissions may be significantly reduced, and a low-carbon binder is obtained [20]. Moreover, retrieving the waste concrete highly encourages the valorization reuse of CDW, the reduction of natural resources depletion and the relevant decrease of landfill disposal. However, recycled cement is still a very young domain of research and various aspects related to its production process and its behavior when incorporated in cement-based materials must be further explored before the implementation of this very promising eco-efficient solution in the construction industry.

The objective of this chapter is to review some of the most relevant research and main contributions achieved in the domain of thermoactivated recycled cements. First, a general overview of the recycled cement manufacture is presented. Then, Sections 3 to 5 are dedicated to the phase development in anhydrous RC and subsequent rehydration, Sections 6 to 8 discuss the main physical, microstructural and mechanical properties of cement-based materials. Section 8 also covers the use of RC as partial cement replacement, showing the higher potential of this new recycled binder compared to current mineral additions used by the concrete industry.
