Effect of ultrafine recycled brick powder on the properties of blended cement: Hydration kinetics, microstructure evolution and properties development

Ultrafine grinding is expected to be an effective method to increase the pozzolanic activity of RBP, since RBP has superior grindability. The aim of this paper was to investigate the effects of 30 % ultrafine RBP with particle size of 8.6 & mu;m, 4.5 & mu;m, 3.6 & mu;m and 2.7 & mu;m on the fundamental properties, hydration kinetics and microstructure evolution of blended cement. The setting times, water requirement for normal consistency, rheological properties, hydration heat, hydration degree, mechanical strength and pore structure of cementitious material were evaluated. Results show that when 30 % RBP was added, the water requirement and viscosity of blended cement increased significantly with the decrease of RBP particle size, while the setting time was shortened due to the acceleration of early-age cement hydration. The parameters for the Boundary Nucleation and Growth (BNG) model indicate that the nucleation and growth rates of hydration products increased by 1.43 times and 1.27 times when RBP with D50 = 2.7 & mu;m was added, thus the cement hydration degree reached more than 50 % in the first 72 h. Based on the thermogravimetric analysis (TGA), the pozzolanic reaction degree of ultrafine RBPs was more than 15 %-25 % depending on its size, and the hydration degree of cement in RBP-PC pastes was more than 80 %. As a result, blended cement mortars with up to 30 % ultrafine RBPs has slight or even no strength loss after curing for 90 days, as RBP-PC paste has a denser microstructure.