Experimental studies and Microstructure Analysis for Ultra High-Performance Reactive Powder Concrete

In this paper, the strengthening mechanism of curing temperature, fine aggregate gradation, reactive materials, water reducer type, dosage and types of fibers on the microstructures and mechanical properties of Ultra-high-Performance Concrete (UHPC) was analyzed in detail by means of microstructure analysis using Scanning Electron Microscope (SEM) and mechanical tests. Based on the mechanical tests and analysis of the microstructure, a new optimal mix proportion of UHPC was also developed by considering the economic benefits. It is found that the gradation of UHPC fine aggregate can achieve the densest stacking state gradation after the optimization of mix proportion. Gradation Optimization promotes UHPC to be hydrated step by step. A large amount of hydrated calcium silicate (C-S-H) gel and a small amount of crystal produced in the early hydration phase together form the original structure of the concrete. The initial hydration products consume a large amount of Ca (OH)2 to produce C-S-H and other cementitious substances by so called Secondary Hydration Reaction, the C-S-H can further catalyst hydration. It is also found that the low water-cement ratio can reduce porosity and improve the compactness and compressive strength of UHPC microstructure. The fibers can effectively delay the appearance and development of micro-cracks in the concrete matrix, help to improve the toughness, ductility and flexural properties of UHPC, and avoid brittle failure. High temperature curing is beneficial to the formation of cementitious substances with lower calcium-silicate ratio (C/S) and catalyst the occurrence of secondary hydration reaction