Silica fume
Among silica fume (also: silica fume, microsilica, silica fume) is understood in the Baustoffkunde an artificial pozzolan having a high proportion of silica ( silicon dioxide SiO 2 ), which primarily as additive for the production of high-strength concrete is used.
In the product description of normal concrete, silica fume is identified with the abbreviation D.
Silica dust can transform itself into fine quartz dust or can be regarded as such with regard to its physiological effect. Intensive inhalation of silica dust can accordingly lead to silicosis .
Manufacturing
Silica fume is a by-product in the manufacture of silicon and ferro-silicon alloys. When quartz is reduced with carbon, vaporous silicon monoxide (SiO) is produced, which reacts to silicon dioxide in the presence of oxygen and which is collected as such in condensed form by cooling. The chemical composition of the silica dust differs depending on the type of alloys produced in the factory. The proportions of amorphous SiO 2 vary between 85 and 97 mass%. The remaining proportions are Al 2 O 3 , Fe 2 O 3 , CaO, MgO, K 2 O, SO 3 and C.
Silica dust is used in powder form or more often as a suspension (50 mass% solids) on the construction site.
properties
The particles contained in the silica fume are spherical and with their particle size of 0.1 to 0.2 μm, 50 to 100 times finer than cement particles. According to the standard, their specific surface should have a value of 15 to 35 m² / g (for comparison: the grinding fineness of cement is 0.2 to 0.7 m² / g). The density of silica fume is about 2.16 g / cm³.
The use of silica dust as an additive in concrete increases strength. This is due to its function as an artificial pozzolan. In the presence of hydrated lime (Ca (OH) 2 ), which is formed during the hydration of cement , the amorphous silicon dioxide reacts to form additional strength-giving and water-insoluble calcium silicate hydrates. The use of silica fume also increases the strength of the bond between cement and aggregate.
Due to its small grain size, the use of silica dust increases the tightness of a concrete. The silica fume acts as a so-called “filler”, which reduces the pore volume (or the capillary pore space) and, if the concentration is correct, increases the packing density at the same time. The penetration of gaseous or liquid pollutants such as chlorides or sulfates is made more difficult. The frost resistance of the concrete increases.
The small specific surface area of the silica dust causes a high reactivity. However, this also increases the water requirement if the optimum amount is not determined by optimizing the packing density. An amount of around 10% silica dust from the cement content can lead to stiffening or increased stickiness, which makes the use of concrete plasticizers or superplasticizers necessary in order to achieve improved workability.
As with other pozzolans, e.g. B. fly ash , a lower initial heating and hydration heat of the component is achieved through the use of silica dust.
application
Due to high material and transport costs as well as some negative properties (increase in the stickiness of the building material suspensions), silica fume is often only used for buildings with special demands on strength or durability, e.g. B. for high-strength concretes. With silica fume with the addition of superplasticizers, strengths of over 100 N / mm² with water cement values of 0.35 to 0.25 can be achieved.
Another area of application is shotcrete . Here, the use of silica dust can lead to reduced dust generation, less rebound and better adhesion due to the increase in the proportion of fine particles.
standardization
The requirements that must be met when using silica dust are regulated in the standards DIN EN 13263-1 and DIN EN 13263-2. In addition, additional provisions can be found in DIN EN 197-1 or DIN 1045-2 or DIN EN 206-1.
In DIN EN 13263-1, guide values are given for silicon dioxide (≥ 85% by mass), chloride (≤ 0.3% by mass), sulfate (≤ 2.0% by mass) and CaO content (≤ 1.0 mass%) and the content of free silicon (≤ 0.4 mass%). The loss on ignition must not exceed a mass fraction of 4.0%. The specific surface must have a value of at least 15 m² / g.
DIN EN 197-1 limits the proportion of silica dust for cements of type CEM II to a proportion of 10%. For reinforced concrete structures, this value is 11% of the cement content. The reason for this is the lowering of the pH value, which is associated with the consumption of Ca (OH) 2 in the pozzolanic reaction. As a result, the concrete can lose its passivating effect for the reinforced concrete reinforcement.
If silica dust is used as a concrete additive, this can be taken into account in the calculation of the water-cement ratio . Applying the "k-value approach" results in an equivalent water-cement ratio (w / c) eq according to DIN 1045-2 . A k S of 1.0 is usually used for silica dust .
literature
- Günter Neroth, Dieter Vollenschaar (ed.): Wendehorst building materials. Basics - building materials - surface protection. Vieweg + Teubner Verlag, Wiesbaden 2011, ISBN 978-3-8351-0225-5 .
- Jochen Stark, Bernd Wicht: Durability of concrete. Springer-Verlag, Berlin Heidelberg 2013, ISBN 978-3-642-35277-5 .
- Wilhelm Scholz, Wolfram Hiese (Ed.): Knowledge of building materials. Werner-Verlag, Cologne 2007, ISBN 978-3-8041-5227-4 .
- Peter Grübl, Helmut Weigler, Sieghart Karl: Concrete. Types, manufacture and properties. Ernst & Sohn Verlag, Berlin 2001, ISBN 3-433-01340-3 .
- DIN EN 13263-1: Silica fume for concrete - Part 1: Definitions, requirements and conformity criteria ; German version EN 13263-1: 2005 + A1: 2009. Beuth Verlag, Berlin 2009.