The Influence of Amorphous, Sub-Micrometer Silica Particles in Cement Pastes and Mortars with Very Low Water-to-Cement Ratios (Ultra-High Performance Concrete)

The focus of this work is to describe the influence of amorphous, sub-micrometer silica particles in UHPCs. The present literature is equivocal about whether silica enhances clinker hydration due to its dissolution and subsequent pozzolanic reaction to calcium–silicate–hydrate phases (C–S–H phases) and/or due to a heterogeneous nucleation of C–S–H phases from alite hydration on silica surfaces (seeding effect). Herein, the influence of the reactivity of silica has been rarely considered. Furthermore, the incorporation of discrete, not agglomerated particles becomes increasingly important with respect to the particle packing density (filler effect) which is a central aspect in the formulation of UHPCs. However, current investigations provide little information on the effect of sub-micrometer silica with an almost monomodal particle size distribution on the compressive strength of mortars. The following scientific approach was used in this work to address the research objectives. First, the different types of silica are characterized with respect to further reactions in a cementitious system. Second, the effect of silica with varying reactivities on the overall hydration in UHPC is determined and the prevailing mechanisms are identified. Additionally, the influence of the primary particle size and the agglomerate size on the particle packing density and the compressive strength of UHPC are examined. Silica fume, pyrogenic silica and silica synthesized by hydrolysis and condensation of alkoxy silanes, so-called Stoeber particles, are employed. These materials are characterized by measurements of the specific surface area, surface silanol group density, total content of silanol groups and solubility in alkaline suspension. Results indicate that Stoeber particles are by far the most reactive, followed by pyrogenic silica and the less reactive silica fume. Silica reactions are further traced in examinations on UHPC pastes and mortars by pore solution analysis, microstructure investigations (scanning electron microscopy, transmission electron microscopy and cryo scanning electron microscopy), development of the content of crystalline phases (in situ X ray diffraction), heat flow calorimetry and compressive strength measurements. Results for very short reaction times (up to 1 h) imply that silica particles might attract cations (Na+, K+ and Ca2+) from the pore solutions and form alkali silicate oligomers and calcium alkali silicate oligomers. These oligomers might be held as a layer around the silica particles and form an aqueous, amorphous gel phase. The extent of the assumed oligomerization depends on the silica reactivity. Indeed, it seems to be high enough in pastes with Stoeber particles to bind almost all alkali ions in alkali silicate oligomers. Further differences are observed between the different types of silica at short reaction times (up to around 3 d). It is concluded from compressive strength measurements, investigations of the microstructure and heat flow calorimetry that pyrogenic silica and silica fume enhance early strength and accelerate hydration, dissolution of alite and formation of C–S–H phases; whereas, Stoeber particles show minor or none of these effects. The high initial silicon concentration in the pore solution from dissolving Stoeber particles leads presumably to a selective dissolution of calcium from alite and a subsequent formation of a calcium containing aqueous silica gel phase around Stoeber particles. This process does not seem to have an enhancing effect on the hydration of alite in comparison to the other silica. In contrast, no noticeable dissolved silica was detected in the pore solution of pastes containing pyrogenic silica or silica fume. These types of silica increase the surface area for heterogeneous nucleation of C–S–H phases. The filler effect is examined by taking advantage of the adjustable and nearly monomodal size distribution of Stoeber particles. This benefit allows correlating particle sizes with calculated particle packing densities and compressive strengths. Results show, the better the dispersion of silica particles, the higher is the compressive strength. However, a clear dependence on primary particle sizes was not confirmed. All results contribute to the understanding of different (commercial) silica having varying effects on the hydration and properties of UHPCs. If an acceleration of the hydration is desired, silica with a low reactivity ought to be inserted. To fully benefit from the filler effect, silica being dispersed to primary particles should be used.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Oertel, Tina

Contributors dc:contributor

• Breu, Josef

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1053/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1053