Influence of the Treatment Temperature on the Microstructure and Hydration Behavior of Thermoactivated Recycled Cement

Abstract

This paper intends to contribute to a better knowledge of the production and rehydration of thermoactivated recycled cement and its incorporation in cement-based materials. To this end, the influence of the treatment temperature on the properties of recycled cements and recycled cement pastes was assessed by means of a wide array of tests. Anhydrous recycled cement as well as the resulting pastes were characterized through density and particle size, water demand and setting time, thermogravimetry, X-ray diffraction, field emission gun scanning electron microscopy, isothermal calorimetry, 29Si nuclear magnetic resonance spectroscopy, flowability, mechanical strength, mercury intrusion porosimetry and scanning electron microscopy. The treatment temperature had a significant influence on the dehydration and hydration of recycled cement, essentially resulting in the formation of C2S polymorphs of varying reactivity, which led to pastes of different fresh and hardened behaviors. The high water demand and the pre-hydration of recycled cement resulted in high setting times and low compressive strengths. The highest mechanical strength was obtained for a treatment temperature of 650 °C.

Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete

“…For the XRD analysis, the samples were scanned at a 5-60 • C 2θ range, with a step size of 0.03 • and a step time of 100 s, using a PANalytical X'Pert Pro diffractometer with CuKα radiation. The TG analysis was performed using a PC LUXX Thermobalance with a heating ratio of 10 • C/min, from ambient temperature to 950 • C. The degree of hydration and the calcium hydroxide (CH) content were determined according the procedure described in Real et al [9]. The obtained recycled cement paste (RCP), with rehydration capacity, was mainly composed of α' H -C 2 S and CaO, as well as traces of calcite ( Figure 1).…”

“…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].…”

“…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]. Real et al [9] recounted similar flexural and compressive strength up to 3 days of pastes with RC treated at 600-700 • 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, Shui et al [11] also reported no significant evolution in the compressive strength between 7 and 28 days.…”

“…In mortars with fine thermoactivated recycled concrete aggregates, Shui 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, Bogas 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.…”

“…The authors found that the incorporation of RC treated at 650 • C for 80 min led to the highest compressive strength. Real et al [9] studied cement pastes with RC treated from 400 to 900 • C, having concluded that 650 to 700 • C led to the optimal rehydration capacity. Bogas et al [14] reported higher performance for 650 • C.…”