Prior to develop the advantages of using silica fume to improve the concrete sulfate resistance, it may be interesting to highlight the basics form of sulfate attacks and the properties of sulfate resisting cement.
The use and deterioration of concrete in environments containing sulfates has led to the development of special sulfate resisting cements. It has also spawned significant research into the use of supplementary cementitious materials to improve sulfate resistance. Sulfate resisting cements (type V) have a low C3A content to minimize the risk off sulfate attack. However, this does not necessarily provide immunity:
- as certain sulfate react with hydrated lime and the calcium silicate hydrate the sulfate resisting cement provide less protection than expected.
- Low C3A cements are more susceptible to reinforcement corrosion attack.
The basic forms of sulfate attack are the following
The reactive aluminates in the cement will react with the gypsum in cement during hydration. This process is harmless as the ettringite does not produce expansive forces and is stable in sulfate solutions.
If the quantity of reactive aluminates in the cement is too high, then their hydrate form will be available to react with sulfates after the cement has hardened. This will produce expansive ettringite and cracking of the concrete.
Aluminate Hydrate + Calcium Hydroxide + sulfate + water => Ettringite
The second principle cause attack is the acid interaction of sulfates ions and calcium hydroxide, causing gypsum formation.
Calcium Hydroxide + sulfate + water => Gypsum
It has been shown that the cation (calcium, Magnesium, Aluminium, Ammonium) of the sulfate salt affects the type and the severity of the attack.
An alternative solution for sulfate resistance: Portland type 1 + Silica Fume
The utility of Silica Fume for enhancing the resistance of concrete to attack in sulfate environment has been examined for many years. Cement Type 1+Silica Fume have been chosen for their ability to produce a durable concrete.
Silica Fume serves to reduce the intensity of sulfate attack for the following reasons:
- By reducing the cement content (replace by silica fume), the mount of lime and C3A present in the concrete is reduced, so the amount of gypsum and ettringite then can be formed is also reduced.
- Some of the lime that is formed during hydration reacts with Silica Fume to form CSH. This further reduces the amount of lime that is available for gypsum formation.
- Reduction of concrete permeability. The higher CSH levels and the physical blocking of pores by the Silica Fume particles reduce the permeability of the concrete. Consequently, the rate of diffusion of sulfates into the cement paste, and hence the rate of sulfate attack, is reduced.