The particle size is very fine, the particle size is 0.2 – 2 um, the average average is 0.5um, the surface area is 20 – 23 m2/g, which is 50-60 times of the cement. Silica powder has high activity, which can significantly improve the performance of concrete.
Performance characteristics of silica fume concrete:
(1) the strength was significantly improved. After adding silica powder to ordinary concrete, its strength can be increased by about 40% – 150% due to the mixing method (admixture), the variety of adulterated and the amount of admixture.
(2) the workability of new mixed concrete has been greatly improved. The incorporation of silica fume concrete, no segregation and bleeding phenomenon, but the slump was significantly smaller than ordinary concrete.
(3) silica fume concrete has small pore (super micro porosity), and the durability and frost resistance of the concrete are greatly improved.
(4) the improvement of other performance is also obvious. The resistance to chemical erosion, anti cavitation and impact resistance have been greatly improved.
Silica fume concrete mix design:
The performance characteristics of silica fume concrete should be fully considered when determining the mixture ratio parameters of silicon powder concrete.
1). Determination of water cement ratio
Based on the ordinary concrete without silicon fume powder, the strengthening effect of the silica powder should be fully considered.
2) determination of water consumption per unit
The amount of water per unit of silica fume concrete can be adjusted properly on the basis of the water consumption of the base concrete unit.
3) determination of the sand rate
Sand ratio is the number comparison of coarse and fine aggregate, because the influence of silica fume on it is not very significant. At the same time, the cohesiveness and water holding capacity of concrete are better after considering silica fume. Therefore, based on the sand ratio of ordinary concrete, a smaller value can be selected.
2. Control of the amount of silica fume
When the amount of silica fume is too large, it will reduce some performance and economy of concrete. Generally, it is suitable for less than 30% admixture. When the requirements of antifreeze are required, the amount of admixture should be controlled within 15%. The amount of mixing from 5% to 15% is more economical.
3. minimum ash powder dosage
According to the requirement of concrete for the mortar quantity, the total amount of cement and silica fume powder should not be less than the minimum amount of cement for ordinary concrete.
4. Principle of mix proportion design
(1) the improvement of concrete performance with silica fume is relative to that without mixing. Therefore, the design of mix proportion should be based on ordinary concrete, and the relevant parameters should be determined and adjusted accordingly.
(2) because of the same production process of silica fume concrete and ordinary concrete without silica fume, the strength guarantee rate, standard deviation and deviation coefficient are the same as those without concrete mixing.
(3) because the strength of silica fume concrete is improved significantly, it should be fully considered when determining the mixture ratio.
(4) because the density of silica fume is small and the amount of mixing is different, the density of concrete will be greatly different, so it should be calculated according to the absolute volume method.
(5) in view of the limitation of the experimental data, the calculation of the amount of admixture can be carried out according to the method of inside admixture or external admixture.
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