Mechanical properties and microstructures of thermally activated ultrafine recycled fine powder cementitious materials

High-efficiency methods of thermal activation and ultrafine grinding coupled with the activation of the RFP were innovatively proposed, which solved the problems of poor activation efficiency, a low recovery rate, and an unclear influence mechanism of the RFP. In this study, the impacts of recycled fine powder (RFP), ultrafine recycled fine powder (URFP), and thermally activated ultrafine recycled fine powder (TAURFP) on the properties of RFP-cement-based mortar were investigated through mechanical property tests, X-ray diffraction (XRD), thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and pore structure analysis. The results indicated that the addition of URFP improved the mechanical properties of the mortar, whereas TAURFP further enhanced these properties compared with those of URFP. At a 45 % substitution rate of TAURFP, the 28-day compressive strength of the RFP-cement-based mortar reached 43.1 MPa, which was 96.5 % of the compressive strength of the reference cement group. Microscopic analysis revealed that URFP and TAURFP can accelerate the dissolution and hydration of materials, promote the formation of more calcium silicate hydrate (C-S-H) gel and dense calcium aluminum silicate hydrate (C-A-S-H) gel, refine the pore structure and improve the pore structure distribution. The main effects of TAURFP are the filling effect and nucleation effect. Moreover, TAURFP, with its coarse surface and high water absorption, accelerated hydration and transformed more pores into harmless pores and less harmful pores. Hence, TAURFP outperformed URFP and RFP in enhancing the mechanical properties and microstructure of the materials.