# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 1 Introduction  
The growing concern with the impact of the construction industry on the environment has prompted investigation works on concrete recycling to reduce the consumption of natural resources and the emissions of greenhouse gas, as well as to repurpose construction and demolition waste [1, 2]. For various decades, concrete recycling studies have mainly addressed its use as coarse and fine aggregates as well as low-activity filler additions [3]. In recent years, the attention has shifted towards the production of recycled cement (RC) with rehydration capacity through thermal activation [1, 4]. In this still relatively unexplored domain, studies have mostly focused on the microstructure and rehydration behaviour of RC [5, 6, 7, 8, 9, 10], as well as the mechanical characterisation of cement pastes and mortars produced with this binder [11, 12, 13, 14, 15, 16, 17, 18]. RC pastes have been reported to achieve compressive strengths of up to 32 MPa at 28 days [7], though generally presenting high water demand and low setting times [4, 6]. [PERSON] et al. [9] recounted similar flexural and compressive strength up to 3 days of pastes with RC treated at 600-700 \({}^{\circ}\)C and reference ordinary Portland cement (OPC) paste of the same water/binder ratio (w/b), though, at 28 days, the compressive strength of the RC pastes was about 73% of the OPC paste. In mortars with fine thermoactivated recycled concrete aggregates, [PERSON] et al. [11] also reported no significant evolution in the compressive strength between 7 and 28 days. The early strength development of RC pastes has been attributed to the high surface area of the RC particles [4, 9, 19]. Moreover, [PERSON] et al. [10] explained that a portion of the mixing water is directed to the formation of hydration products within the porosity of the RC particles, reducing the amount of water available between anhydrous particles. As such, the w/b becomes lower in bulk RC paste, increasing the proximity between particles, which, in turn, results in the acceleration of particle cohesion, improving the early strength of RC pastes. [PERSON] et al. [15] investigated the influence of the treatment temperature and residence period on the mechanical strength of mortars with 25% RC obtained from the cement fraction of waste mortar. The authors found that the incorporation of RCtreated at 650 \({}^{\circ}\)C for 80 min led to the highest compressive strength. [PERSON] et al. [9] studied cement pastes with RC treated from 400 to 900 \({}^{\circ}\)C, having concluded that 650 to 700 \({}^{\circ}\)C led to the optimal rehydration capacity. [PERSON] et al. [14] reported higher performance for 650 \({}^{\circ}\)C.  
[PERSON] et al. [12] characterised mortars with 10-30% incorporation of RC treated at 500 and 800 \({}^{\circ}\)C, having found that the compressive strength decreased with the incorporation of RC. [PERSON] et al. [16] also evaluated the influence of different incorporation percentages of RC and of various thermal activation temperatures. The authors found that for up to 50% incorporation of RC treated at 700 \({}^{\circ}\)C, the water demand and setting time were not significantly affected. Moreover, the replacement of OPC with up to 20%RC led to minor variation of the workability and mechanical strength of the mortars. The authors also found that mortars with 100%RC achieved up to 27 MPa at 28 days, about 53% of that of OPC mortar with the same w/b. [PERSON] and [PERSON] [20] analysed the mechanical strength of pastes produced with RC from waste cement pastes treated at 650 \({}^{\circ}\)C. The authors found that the maximum compressive strength was obtained for an optimal amount of 5-10% RC incorporation. The highest improvement was attained when RC particles were previously dispersed with ethanol and sodium polyacrylate, underlining the relevance of adequate RC de-agglomeration on the mechanical properties. The incorporation of RC in concrete has barely been studied, with only a few published papers [21; 22]. [PERSON] et al.