Silica Fume in Waterproof Concrete
Densified silica fume is mostly used for concrete applications. In order to evenly disperse silica fume after mixing it into the concrete, a de-densification process is required, which mixes water and silica fume at a 1:1 ratio (by mass) and with high-speed stirring. And a small amount of admixtures is often used to ensure the uniformity and stability of the silica fume slurry.
To prevent the corrosion of steel reinforcing bars
The corrosion of steel reinforcing bars is mainly due to the presence of chloride ions and oxygen. It is an electrochemical process that can cause the volume of steel bars to expand 2-7 times, depending on the final products. After the volume expansion, the concrete will crack along the bars, and the bonding force between the concrete and steel bars will decrease, resulting in the deformation of concrete members. Meanwhile, the corrosion also causes smaller effective cross-sections and larger stress. To prevent corrosion, concrete should have a certain density to block the entering of moisture and oxygen.
The porous structure of ordinary concrete is not able to isolate the steel bars from the external environment, let alone the incomplete cement hydration, the evaporation of excess water will also generate extra pores. In marine environments, when chloride ions (with a content of about 19,000 mg/L in seawater) enter the concrete, due to its acidification, the pH value of the passivation film of steel bars that forms by the cement hydration will rapidly decrease, causing the damage to the film. And the chloride ions will corrode the inner iron.
Performance improvement by silica fume
For concrete, silica fume fills the pores, forms mullite crystals, reacts with calcium hydroxide to form C-S-H. It has the following advantages: (1) It increases the pumping pressure by 15% – 30%; (2) It improves the compressive strength, flexural strength and the bonding strength among concrete, steel bars, and steel fibers, and is an essential for reactive powder concrete (also known as ultra-high performance concrete); (3) It greatly reduces the permeability of concrete, improves the resistance to the penetration of chloride ions, improves the electrical resistivity by 5 – 10 times. (4) It prevents the expansion caused by the alkali-aggregate reaction.
Dosage of silica fume
The dosage rates of silica fume vary depending on the purpose of use. And silica fume is usually used with superplasticizers. Because silica fume concrete has a high viscosity, its slump should be 2 – 3 cm higher than ordinary concrete. For mortar and concrete, the recommended dosage is 5%-8%. The suggested dosage of silica fume is 12% – 15% for underwater concrete, 5% – 15% for high-strength concrete.
Silica fume concrete has strict requirements on aggregates. The content of aggregates with flaky and elongated shapes, micas, clay should not exceed the requirements. The particle size of aggregates should be less than 3 cm for highways and bridge projects, less than 2 cm for reactive powder concrete.