Silica Fume or microsilica

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.

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.

Microstructure changes in Magnesium oxide

Magnesium oxide or magnesia is popularly considered a refractory material because of its stability at high temperatures. By default, it has low electrical conductivity and high thermal conductivity. It finds use in construction materials as a fireproofing agent. The study findings conclude that adding silica fume leads to denser microstructures in the concrete mix, leading to greater compressive strength.

What happens to mortar when silica fume and metakaolin are added together?

Typically when concrete is prepared, metakaolin and silica fume are added in different mixes. Metakaolin is finely ground before mixing. Silica fume is already finer than metakaolin and is added to raise the compressive strength in concrete. In a unique study, the combination of silica fume and metakaolin was added to mixes in varying proportions. The quantity of water was also regulated. While both materials strengthen the concrete when added separately, they seem to display a synergistic effect when added in combination. Such an excellent combination can prove its advantage in the production of high-performance concrete.

Comparison of effects on elasticity of concrete

Both metakaolin and silica fume are economical additives for concrete manufacture. Pozzolanic materials can be added to cement concrete to improve its strength, durability, and workability. A study explored the effect on concrete’s modulus of elasticity property upon adding silica fume or metakaolin. It was observed that the modulus value increased by 12.77% when either metakaolin or silica fume were added to constitute 10% of the mix. Replacement of cement by 10% with either of these additive materials can lead to direct savings on construction costs and improvement in the structure’s overall strength.

In addition to the above instances where both additives prove their worth, binary concrete also exhibit significant improvements. As either silica fume or metakaolin partially substitutes Portland cement, it exhibited greater strength and durability. After replacing cement with kaolin, the sand rate of the concrete can be reduced appropriately, and the amount of poly(mythical acid) high performance water-reducing agent can be increased slightly to adjust the wilderness fall to 30-50 mm; after replacing cement with 8% silica fume, the sand rate of the concrete can be reduced appropriately, and the amount of poly(mythical acid) high performance water-reducing agent can be increased slightly to adjust the wilderness fall to 30-50 mm; through steaming, the steaming strength of the base group is 69.6 MPa. The steaming strength of the base group was 69.6MPa, the steaming strength of the mix with 10% kaolinite reached 83.6MPa and the steaming strength of the mix with 8% silica fume reached 79.SMPa, indicating that replacing cement with silica fume and kaolinite can improve the steaming strength of concrete.

Silica fume, in particular, proved useful in terms of resistance to freeze-thawing and compressive strength. In contrast, metakaolin proved its worth concerning carbonation. Hence, the substitution of Portland cement with additives is economical, clean, and environmentally friendly.