HPC with a compressive strength of up to 100 MPa can be produced using pure cement, but it is much easier when using high-purity silica fume. The use of silica fume is almost indispensable in manufacturing concrete with a strength exceeding 100 MPa. High-purity silica fume is used as both filler and pozzolanic materials in concrete. After using silica powder, the pore size of the hydrated slurry is greatly reduced and the pore size distribution is improved, thereby increasing the strength and reducing the permeability. For example, research results (CEB2FIP1988) show that in order to obtain a concrete strength of 70 MPa, the application of pure cement requires a gelatin ratio of 0.35. With 8% silicon, the gelatin ratio may be 0.50.
The particles of high-purity silica powder are very fine and can cause volcanic ash reaction within a few hours. Carette and Malhotra 1992) pointed out that the contribution of high-purity silica fume to concrete strength is mainly before 28 days. Therefore, it is believed that silica fume concrete is generally inferior to pure cement concrete or fly ash concrete in terms of long-term strength gain. The experimental results of silica fume on the strength development of NSC cited by Almad (1994) show that as the silicon content increases, the initial relative strength development decreases. Sandvik 1992 also found this phenomenon in 65 MPa concrete.
However, the early relative strength development of high-purity silica fume concrete is slower than that of pure cement concrete at the same cement ratio, but adding silica fume will greatly increase the strength, and the strength of silica fume concrete is higher than that of pure cement concrete. On the other hand, the early strength development of HPC is faster than that of NSC. The setting time of HPC may be delayed, but the hydration after setting will be greatly accelerated by the superplasticizer and silica fume. The result is usually a very rapid development of strength after condensation.
Reverse shrinkage of compressive strength has been reported for some low-moisture-ratio silica concrete specimens dried or cured in air (De Larrard and Aiticin 1993). This loss of strength usually occurs after 90 days of age and is often thought to be caused by internal self-drying and cracking. But laboratory and field studies by many other researchers have shown no decrease in HPC’s late-stage strength. For example, test results for all drill core samples from 3 months to 3 years old obtained from 6 different HPCs showed an increase in strength. Of course, HPC has lower long-term strength growth potential compared to NSC.
Content of water-reducing agent: If silicon is used in concrete, for example, without adding water-reducing agent, the same fluidity must be maintained. Water consumption and water-cement costs will increase, and the strength of concrete mixed with silicon will not increase. This is why in the past silicon The reason why powder is not widely used in concrete. Silica fume and water reducing agent jointly use silica fume for water dues. In other words, water consumption will not increase, and properties such as fluidity (such as concrete without silicon) and silicon concrete strength can be greatly improved. Generally, naphthalene high-efficiency water reducing agents are commonly used in China, such as 1, H, DH3, FDN, NF, N2B, etc. Its content is generally adhesive. Adjust the use of sand and gravel: Incorporating silica powder generally does not require adjusting the amount of sand and gravel. External silica fume should be deducted from the volume of gravel, such as the volume of silica fume.
