Concrete has been used as a leading construction material for many decades. Its increasing demand is associated with the continuous increase of the earth’s population leading to the depletion of natural resources. On the other hand, construction modernization requires demolishing old buildings, roads, and bridges, producing a massive amount of Construction and Demolition Wastes (CDW). Much of it is composed of old concrete. The substantial reduction of natural resources and the dumping of these wastes constitute a significant source of environmental problems. Besides, the production of cement, which is the principal binder of concrete, is accountable for considerable CO2 emissions and is very energy-intensive. Therefore, recycling old concrete for partial replacement of Virgin Cement (VCe) in new concrete or mortar is essential for protecting the environment and reducing CO2 emissions.

Thermal treatment has been proven to recover the hydration ability of Hydrated Cement Powder (HCeP), which helps as a reference material for evaluating Recycled Concrete Powder (RcCoP). Several techniques separate the components of old concrete. This research considers the Smart Crushing (SC) and Electrodynamic Fragmentation (EF) methods to obtain the RcCoP with as much cementitious content as possible. These two types of materials and the methods used to produce the RcCoP are compared after thermal treatment at temperatures from 200 °C to 1000 °C.

The necessary chemical transformations happen at different temperatures during the thermal treatment process. Subsequently, the strength development phases, specifically the calcium silicate phases, can be reformed depending on the pre-treatment temperature. Thus, the treatment temperature defines the dehydration products that control the rehydration ability. Several laboratory techniques assess the chemical transformations and phase development resulting from the thermal treatment at different temperatures. These techniques include Thermogravimetry (TG), Differential Scanning Calorimetry (DSC), and X-Ray Diffraction (XRD). The microstructure, porosity and pore size distribution, and mechanical strength associated with pre-treatment temperatures were investigated by Scanning Electron Microscopy (SEM), Mercury Intrusion Porosimetry (MIP), and compressive and flexural strength testing, respectively.

The temperature treatment range of 400 °C – 800 °C produces the best content of strength development phases. 600 °C is the optimum thermal treatment temperature. It results in significant content of dicalcium silicate (C2S_α and C2S_β) and the highest content of the remaining previously unhydrated tricalcium silicate (C3S). Nevertheless, the amount of this C3S phase is little compared to the one in VCe. The C3S is the primary phase that controls strength development, especially at an early age (first week). It is highly reactive and cannot be recovered, indicating that the total hydration capacity cannot be regained. However, the C2S_α that forms through thermal treatment is more reactive than C2S_ β, which benefits strength reformation.

Using 100% thermally treated HCeP and RcCoP indicates that approximately 55% of strength development ability can be recovered for the HCeP, while only <10% can be retrieved for the RcCoP. This massive difference is due to the content of a high amount of quartz and coesite phases (SiO2), especially quartz, dominating the phase composition. The SiO2 is not a strength-forming phase. It is thermally stable and not decomposed during thermal treatment. Also, the crystals of the SiO2 phase dominate the microstructure of all thermally treated RcCoP. In contrast, the microstructure of thermally treated HCeP shows more condensed rehydration products and is much more compact, emphasizing an increased ability for strength development. The HCeP pre-treated at 600 °C can replace the VCe in mortar for up to 20% without affecting the mechanical strength. The replacement of VCe in mortar by thermally treated RcCoP was not conducted because a minimal strength development ability was recovered in 100% thermally treated RcCoP, regardless of the pre-treatment temperature.