Behaviour of colloidal silicas during uniaxial compaction

The pressure P transmitted by colloidal silica samples during low speed uniaxial compaction, is measured as a function of the volume fraction f of solid. In the case of pyrogenic silicas, two domains are evidenced. A first one below f ~ 0.065, is characterized by a percolation type behaviour P ~ (f - fc)k. For higher volume fractions (second domain) P scales as the void volume Vp, P ~ (Vp)- α with α ~ 3. It appears that the behaviour of the precipitated silica is very different. The P(f) curve can be fitted by two different power laws below or above f ~ 0.13. A mechanism of consolidation is proposed and the difference of the behaviour observed for the two types of silica could be attributed to the difference of the fractal dimension of the aggregates ( D < 2 or D > 2).