Silica fume for export
PROPERTIES OF SILICA FUME:
1.Physical Properties
Color of DUSTTM Silica Fume(Microsilica) range from light gray to dark gray, density around 2. 2g/cm3, lighter than cement (3.1g/cm3), similar with coal ash, and,accumulation density is generally 200 to 350kg/m3. Particles of Silica Fume are very small, most of them have a particle size of less than 1μm, average size is 0.1μm, equivalent with 1 /100 of the average diameter of cement particles. Specific surface area is between 15000 and 25000m2/kg (measured by nitrogen adsorption, or BET method). The physical properties of silica fume make tiny particles are highly dispersive, can be fully filled between cement particles,thus improve the compactness after the sclerosis of the slurry.
2.Chemical Properties
DUSTTM Silica Fume(Microsilica) is a by-product of high purity quartz, coke and wood chip reduction in the processing of silicon alloys and ferroalloys, and is a microfloor particle with high levels of non-stereotyped silicon dioxide collected from fumes of the electric arc furnace. Silica fume generally contains more than 90% SiO2, and most of them are non-stereotyped silicon dioxide, composition of which varies according to alloy varieties.
Main chemical component of silica fume is amorphous non-stereotyped silicon dioxide (SiO2), which generally accounts for more than 90% (content of SiO2 commonly used in high-performance concrete is 85%). High-fine, non-stereotyped SiO2 own high volcanic ash activity, i.e. SiO2 can react quickly with Ca(OH)2 to produce calcium hydrated silicate gel (C-S-H) to improve concrete strength and performance under the alkaline excitation of the cement hydration product Calcium Hydroxide (CA(OH)2.
DUSTTM Silica Fume(Microsilica) can be used as an auxiliary gel material to improve the microstructure of hardened cement slurry. Firstly, silica fume own very high volcanic ash activity. Although silica fume itself is basically non-hydrated with water, it can produce a gel-gel product by secondary hydration reaction under the excitation of the cement hydration product Ca (OH)2 and some other compounds. Secondly, because of the micro-aggregate characteristics of silica fume, both silica fume and its secondary hydration products can fill harmful holes in the hardened cement slurry, thus improve the microstructure of the hardened cement slurry.
Application conditions of silica fume (Microsilica) in cement slurry and concrete:
In order to make effectively use of silica fume to improve the microstructure of hardened cement slurry, many researchers worldwide have made deep study on the application conditions of silica fume in cement slurry and concrete, including water glue ratio, silica fume doping, admixture,selection and dosage of other volcanic ash doping material. There is a better water glue ratio scope in cement slurry and concrete applications, effectiveness on hardened cement slurry and concrete microstructure will be reduced if beyond the scope. As Gapparao points out, at the age of cement mortar 3d or 7d, the water glue ratio is less than 0.45 (water glue ratio is 0.35, 0.40) ,the strength of silica fume mortar test is reduced, while the glue ratio is equal to 0.45 or 0.50 ,the strength of silica fume mortar test increased. But at the age of cement mortar 28d or 90d,the strength of silica fume mortar test are almost the same under silica fume with a water glue ratio of less than 0.35, 0.40, 0.50. When water glue ratio equivalent with 0.45(No matter silicone fume content), the strength of silica fume mortar test is lower. When the water glue ratio is equal with 0.50, silica fume doping is greater than 27.5%, silica fume has a significant impact on the late strength development of mortar.
Although silica fume (Microsilica) can effectively improve hardened cement slurry and concrete microstructure, but with the increase of silica fume doping, water demand increased, self-shrinking also increased since its small particle diameter and large specific surface area.Therefore, silica fume doping is generally limited range from 5% to 10%, and to regulate water demand with high-efficiency water-reducing agent. Meantime, must pay attention to its application conditions consider compatibility among cement, silica fume and admixture. When the concrete is doped with silica fume, may lead to self-shrink of the concrete. While the concrete doped with silica fume generally have special requirement of concrete, therefore, except considering beneficial effect, we must minimize the adverse effects caused by silica fume. The effective way to solve this problem is to add silica fume together with other volcanic ash materials or other substances, so that they make up for each other to achieve better technical and economic results. At present, more popular solution including mixing ultra-fine slag with silica fume, or mixing coal ash with silica fume. Another solution is to dope silica fume into cement slurry or concrete after surface pretreat of silica fume, or directly mix silane with silica fume into cement slurry or concrete, which can improve the properties of mixed cement slurry and concrete, thus improve the microstructure of hardened cement slurry and concrete.
