Influence of mineral additions on the phase composition in autoclaved Ultra-High Performance Concrete (UHPC)

摘要

Curing under water vapor saturation pressure (autoclaving) has a significant influence on the micro structure and mechanical properties of UHPC. It generally leads to an enhanced cement hydration and accelerated reaction rate of supplementary cementitious materials (SCM) and/or mineral additions, forming crystalline calcium silicate hydrates (C-S-H). The increased reactivity affects the C-S-H phases by changing their elemental composition, depending on the type of SCM or addition. It is known that the silicon content of the C-S-H has a great influence on the mechanical properties. With custom designed mix proportions and curing conditions, it was possible to create a UHPC with a controlled elemental composition of hydrate phases by using common raw materials. The amount of SCMs has been increased in favor of cement clinker without much negative impact on chemical and mechanical performance. The influence of sulfate rich SCMs was investigated in particular. During autoclaving sulfur was bound in form of crystalline calcium silicate sulfates, but not affects the chemical composition of C-S-H in cement paste. It is assumed that this reaction process opens a possibility to utilize increased amounts of SCMs/mineral additions with higher sulfur content (e.g. some class-C fly ashes) without harming the properties of the cement paste.