Silica Fume in Refractory Castables
The effect of silica fume on refractory castables
Silica fume consists of ultra-fine spherical particles that can fill the pores between the particles in the castables. So the water consumption and porosity of the castables are both reduced. And between particles, the sliding friction becomes rolling resistance. Because the coefficient of rolling resistance is generally much smaller than that of the sliding friction, the resisting force between the particles reduces, and leads to the reduction of viscosity of the castables and the improvement of flowability.
Silica fume reacts with water to form silanol groups (Si-OH) on the surface. During drying, a large number of silanol groups condense to siloxane bonds (Si−O−Si) after the temperature reaches 80°C, and subsequently form a siloxane network structure that provides high strength at low temperature and remains unchanged till 250°C. And this high strength provided by silica fume is not only limited under low-temperature conditions, castable products added with silica fume also have high strength at mid- and high-temperatures as the network structure has no apparent changes at temperatures up to 1200°C.
And due to silica fume’s high activity, it is easy to form mullite with Al2O3 in high alumina based refractory castables by solid state reaction at high temperature, thereby improving the strength and stability of low cement castables. Thanks to the combination of the network structure and mullite, the products can have a cold crushing strength of more than 100 MPa after firing at medium temperature (about 1000°C).
The use of silica fume in refractory castables
Using high SiO2 content silica fume to improve flowability
As silica fume has a direct influence on the flowability of refractory castables, silica fume with high SiO2 content is beneficial to the enhancement of material flowability. Under the premise of the same flow value, using low SiO2 content silica fume will inevitably lead to the increase of water content of castables. If the water content is too high, the porosity of castables left after drying will increase, causing a decrease in the bulk density and the physical strength of products.
The influence of the pH value of silica fume
A study found that the influence of pH on the flow value of the high-alumina low-cement castables showed a similar trend at different water additions: With the increase of pH value of silica fume, the flow value of the castables first increases. When the pH value of silica fume is 5.5-8.5, the castables have relatively good and stable flowability. And when the pH value exceeds 8.5, the flow value of the castables decreases significantly.
The study also showed that the hardening time of the castable is shortened as the pH value of silica fume increases, and it is shortened sharply when the pH value exceeds 8.5. When the pH value of silica fume is between 7 and 8, the castables have higher strength.
Ultra-fine silica fume has a large surface free energy, so the particles agglomerate easily. Generally, undensified silica fume is used in low cement refractory castables because of its better dispersion efficiency. In addition, using high efficient dispersant allows silica fume to give full play to its pore filling effect, so as to produce low cement castables with high density, high strength, and good high temperature performance.