**7. Microstructure of RC pastes**

Few works have been published concerning the microstructure characterization of RC cement-based materials. Most studies only involved simple qualitative scanning electron microscopy (SEM) analysis [55, 57, 68], where the phase morphology and global porosity were poorly assessed and only for a limited range of temperatures.

From SEM analysis, Shui et al. [49] documented the morphology of rehydrated phases of RC treated at 600°C, as fine bundles of C-S-H intermixed with honeycomb shape structures over the surface of dehydrated RC particles. In opposition, C-S-H in OPC presented a typical foil and fiber morphology. Overall, the RC pastes presented a looser structure than reference OPC pastes.

Zhang et al. [43], for RC treated at 600°C, also reported a highly porous hydrated paste with a weak bonding between phases. In a later study, Baldusco et al. [57] etermined the total porosity of pastes produced with RC treated at 500°C from blended OPC and GGBFS. Based on water saturation-vacuum tests it was found that RC pastes developed higher total porosity at 7 days than reference OPC pastes. This was essentially explained by the higher intrinsic porosity of dehydrated cement particles. Noteworthies are two recent studies developed by Bogas et al. [67] and Real et al. [2], where the microstructure of RC pastes was deeply characterized over time and for different dehydrated temperatures, respectively.

Bogas et al. [67] analyzed the microstructure and phase evolution of cement pastes with RC thermoactivated at 700°C, by means of TG, XRD, SEM, quantitative backscattering electron (BSE), mercury intrusion porosity (MIP) and nitrogen adsorption (NA) analysis. Tests were performed in pastes with 8, 14 and 24 hours, as well as after 3,7 and 28 days. The RC showed effective rehydration with the formation of C-S-H, AFm, AFt and other carbonation phases since early ages. Contrary to common OPC, AFm phases were developed, at least since 8 hours, associated with the formation of sulfoaluminates and carboaluminates. It was confirmed the progressive increase of CH and C-S-H over time, which shows that the hydration mechanism extends in time. The authors described the RC paste microstructure as a "dual structure", where the intraparticle porosity is surrounded by a bulk interparticle matrix, in which the available space is lower. Basically, part of the mixing water is retained by inner hydration products, reducing the amount of water and w/b ratio between RC particles. This leads to a greater proximity between RC particles, increasing the packing density and paste cohesion, and consequently the early mechanical strength. Compared to OPC pastes, RC pastes showed higher reactivity up to 3 days, higher volume of hydrated products and more refined porosity, especially at early age. However, at 28 days the compressive strength was 32% lower in RC pastes, with less development of long-term interparticle hydration products. The total porosity was only slightly lower in RC and OPC pastes of equal w/b. The higher final setting time of RC was attributed to the very early formation of interparticle hydration products that do not contribute to the cement paste cohesion.

Following this work, a comprehensive study was carried out by Real et al. [2], where the influence of the dehydration temperature, between 400°C and 900°C, on the hydration and microstructure of RC pastes was analyzed by means of TG, XRD, 29Si NMR, MIP and SEM analysis. Due to the above mentioned "dual microstructure" of RC pastes, a denser microstructure was found for RC pastes tested over 600°C than in Ref. OPC pastes. However, due to the insufficient repolymerization, pastes tested up to 500°C developed very loose microstructures associated with coarse porosity and low mechanical strength. On the other hand, RC treated above 800°C was less reactive, resulting in a less dense C-S-H network. It was thus concluded that the optimal treatment temperature was within the 600–650°C range. Similar to the previous work, the compressive strength was higher in RC pastes than in OPC pastes until 3 days and 27% lower at 28 days.
