Nano particles prepared from hardened cement paste by wet grinding and its utilization as an accelerator in Portland cement

Utilization of recycled powder (RP) from construction and demolition wastes into construction projects would bring great economic and environmental benefits. In this research, RP was prepared from hardened cement paste by wet grinding (i.e. WGRP) in the laboratory, and an attempt to utilize these nano particles as an accelerator for facilitating Portland cement (PC) hydration was done. The basic properties of WGRP were characterized, and the effect of WGRP on the properties of PC was studied, including setting time, compressive strength, and permeability. Related mechanisms were discussed by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Nuclear Magnetic Resonance (NMR), and Mercury intrusion porosimetry (MIP). Results showed that WGRP with median particle size (D50) of 249 nm was prepared. The addition of WGRP accelerated the setting process of PC pastes, and significantly augmented the early strength and impermeability of PC mortars. 4.0% WGRP shortened the final setting time by 175 min and increased the 12 h compressive strength by 180%. Due to the brilliant nucleation seed effect of WGRP, the release of PC hydration heat was greatly facilitated, and the inducing period almost disappeared; the formation of hydrates was accelerated, resulting in a higher hydration degree at the early age. Moreover, WGRP reduced the porosity of PC pastes and increased the harmless pore volume fraction. Such results suggested that WGRP expected to be utilized as an accelerator in PC.