The Mechanism of Compressive Strength Development for Cement Pastes Cured at High Temperature and the Possibility of Additives as to Improve Long-Trem Strength
Hiroaki MORI, Ryuichiro KUGA, Haruka TAKAHASHI and Masami UZAWA
Abstract:The purpose of this study is to discover a method for improving the long term strength gain of steam cured cement pastes. Pastes of ordinary portland cement (OPC) and blended cement (containing 40% ground granulated blast furnace slag GGBFS) were cured at 60 for 24 hours and subsequently stored in limewater at 20 . The development of the microstructure in the cement paste was analyzed and studied using backscattered electron images and the hydration products formed inside the cement grains were classified by energy dispersive spectroscopy. The two main findings in this study are; first, the microstructure shows the formation of coarser hollow shells and cement grains surrounded by inner C-S-H rims. The smaller cement grains hydrate completely and leave hollow shells. Precipitations of calcium monosulfoaluminate were observed within the structure. And second, the blended cement pastes see a reduction in the coarse hollow shells and thicker rims compared with the OPC pastes at the early stages of hydration. The Ca/Si ratio of the rim is lower than that of OPC pastes and its chemical composition changes during the curing time. Hydration products form as dispersed particles throughout the microstructure and then fill the capillary pores and hollow shells. It can be concluded that for blended cement pastes, calcium ions diffuse into the previously formed microstructures and precipitate as hydration products such as CH and C-S-H phases. Furthermore, the effects of using colloidal silica as an admixture to improve long-term strength were studied in order to determine if using a pozzolanic supplementary cementitious material can prevent the formation of thick rims around the cement grains. Key Words:Heat curing, Compressive strength development, Cement, Blast furnace slag, Microstructure, Backscattered electron imaging, Image analysis, Hydration products, Colloidal silica