Silica fume as an amorphous material does not exhibit the long-range order of a crystal. It is a by-product obtained inside electric arc furnaces used to produce silicon metal and ferro silicon. Being an ultra-fine pozzolanic, it has a particle size of 150nm. Such extreme fineness makes it an ideal ingredient for concrete. Portland cement concrete being the most popular example. Concrete that has silica fume as an ingredient is much more cohesive. Thanks to its very high surface area, it helps reduce water usage while preparing concrete and also reduce bleeding to a great extent.
Zhengzhou, Henan, China, 29th Nov 2022, King NewsWire – Silica fume as an amorphous material does not exhibit the long-range order of a crystal. It is a by-product obtained inside electric arc furnaces used to produce silicon metal and ferro silicon. Being an ultra-fine pozzolanic, it has a particle size of 150nm. Such extreme fineness makes it an ideal ingredient for concrete. Portland cement concrete being the most popular example. Concrete that has silica fume as an ingredient is much more cohesive. Thanks to its very high surface area, it helps reduce water usage while preparing concrete and also reduce bleeding to a great extent.
Metakaolin
Metakaolin, as the name suggests, is obtained from the clay mineral kaolinite. It is commonly used in the manufacture of porcelain. Although the metakaolin particle sizes are smaller than concrete, silica fume is much finer in size. As an anhydrous calcinated form of kaolinite, it provides an improved compressive strength while reducing permeability. It is popularly used in high-performance concrete, fiber cement, mortar, and stucco.
| Project | SiO₂,% | Al₂O₃,% | Fe₂O₃,% | CaO,% | MgO,% | Cl,% | total alkalinity,% | specific surface area,㎡/g |
| Metakaolin | 54.66 | 40.19 | 0.84 | 0.12 | 0.084 | 0.0037 | 0.32 | 15.1 |
| Silica Fume | 94.48 | / | / | / | / | 0.013 | / | 19.32 |
Physical Chemical Analysis
Comparison for use in Fly Ash Concrete
As per research, it was observed that the use of silica fume in fly ash concrete reduces slump extension. Whereas metakaolin increases slump extension thereby providing long-time strength. Slump extension for silica fume was only 360Mm, while metakaolin exhibited slump extension of 600Mn. Both materials provide improved resistance against chloride penetration. The diffusion coefficient of chloride ion for silica fume is 3.20, while the coefficient value for metakaolin is 7.47.
Comparison of effects on engineering properties of expansive soil
Expansive soil like bentonite or montmorillonite is used in building several structures around the world. They tend to swell or shrink based on the availability of water volume in their vicinity. It remains a fact that expansive soils damage more homes than floods and earthquakes combined. The structures made of expansive soil can develop cracks and damages based on the differential settlement of soil. Both silica fume and metakaolin are used as additive stabilizer agents to treat expansive soil. The study prepared mixes with varying concentrations of both materials. The addition of silica fume increased the Atterberg limits for the mix, while metakaolin reduced it. Silica fume decreased the specific gravity of the mix and metakaolin increased it. And in synchrony with these changes, metakaolin increased the maximum dry weight and decreased the optimum water content of the mix. The effects were a mirror opposite when silica fume was added to the mix.
Comparison of effects on alkali-silica reactions
High reactivity metakaolin and silica fume were studied for their chemistry of ASR (alkali-silica reaction) products. Both materials controlled the expansion of mortar bars equally. Typically the calcium content in ASR products keeps on increasing with time, the mixes containing added silica fume and metakaolin reduce the pace of growth. Also, the calcium-silica ratio for ASR products kept increasing, following a linear trend.
Kaolin contains 40.19% of Al203 and 54.66% of Si02, which is a high content of active ingredients and is conducive to accelerating the hydration of cement and improving the early strength of mortar or concrete; the specific surface area is 15.lm2/g and the particles are fine, which is conducive to the filling effect of the material. The content of harmful ions and total alkali is so low that it has no adverse effect on mortar or concrete.
Comparison of the fluidity of concrete
Concrete fluidity plays a vital role in the construction business. Properties like spread, slump, and slump loss are carefully controlled. It was concluded in a study that 5% to 15% presence of metakaolin provides better workability for the concrete in comparison to the use of silica fume. As an adjunct inference, the study also observed that the fluidity of metakaolin concrete could be further improved by the use of ground granulated blast furnace slag (GGBS), which is again better than concrete containing silica fume.
Concrete durability comparison
A recent study published in April 2022 reviewed the effects of silica fume and metakaolin on concrete. They checked for gas permeability, water-accessible porosity, electrical resistivity, and chloride ion diffusion. Concrete mixes with varying additive amounts were prepared using different water/binder ratios. Pozzolanic reactions were observed in the microstructure of concrete. Porosity accessible to water decreased from 11% to 1% with the introduction of silica fume in the concrete mix. However, the porosity accessible to water plummeted from 12% to 4% with metakaolin compared to the controlled concrete samples.
Chloride ions diffusion reduced with the introduction of silica fume from 54% to 75%. While the chloride ion diffusion changed from 55% to 86% with the introduction of metakaolin. It was noted that both additives significantly impact the gas permeability of concrete. Metakaolin’s presence reduces permeability from 5% to 28%. In contrast, the permeability reduced from 6% to 22% after adding silica fume. Electrical resistivity goes from 64% to 163% with silica fume and from 50% to 104% with metakaolin.
As both the additives were added, the study also concluded that there is a direct correlation between test durability and compressive strength for different concrete mixes.
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