ABSTRACT

During the first day, the hydration of the blended cement at 20°C proceeded similarly to OPC hydration. Ettringite formed during the first minutes; its quantity increased during the first day. The presence of portlandite was observed between 1-7 days and the presence of hemi-and/or monocarbonate up to 28 days. The relatively low amount of portlandite formed and its consumption with time is related to the reaction of the silica fume. During the first days, the reaction of the Portland cement is faster than the reaction of the silica fume. However, the silica fume continues to react during the first month (Fig. 1) and consumes

1 INTRODUCTION

The use of supplementary cementitious materials (SCMs) such as blast furnace slag or fly ash represents a viable alternative to Portland cements and utilizes by-products of industrial manufacturing processes. An industrial application of SCMs, however, is often hindered by the different and varying chemistry and that other hydrates are formed during hydration than in Portland cements. In Si-rich systems, portlandite can be absent and a calcium silicate hydrate phase (C-S-H) with a low Ca/Si ratio develops. A lower Ca/Si ratio leads also to an increase in alkali uptake by C-S-H and to a reduction of pH values in the pore solution. Most of the available studies on the properties of blended systems focus on mechanical or on durability aspects of a specific silica fume, fly ash or slag, while little is known about the fundamental connection between the composition and hydrates formed and its impact on the long-term development of such systems. In this paper, the hydration of a 50 wt% of a Portland cement and 50 wt% of silica fume (SF) was investigated at 7, 20 and 50°C experimentally. Thermodynamic modelling was used to predict the changes during hydration and the changes associated with the presence of different amounts of SiO2.