By Carl White
Colloidal silica post-placement pozzolan (P3) reacts with Calcium Hydroxide (CH) to form secondary Calcium Silicate Hydroxide (CSH). This increases concrete strength and reduces permeability.
CSH is the primary reaction product of hydration, contributing the most to strength and other desired properties of concrete. CH is an additional reaction product with little or no cementitious properties. It, therefore, contributes the bare minimum to the strength of hardened concrete. Considering that it is easily attacked by chemical agents and leached by water, CH is described as the weak link in concrete.
Colloidal silica reacts with CH
Colloidal silica post-placement pozzolan (P3) reacts with CH to form more CSH when it enters concrete through the capillary voids. It has a tremendous amount of pozzolanic potential, greater even than that of densified silica fume. This is considering colloidal silica post-placement pozzolan’s extremely small particle size.
During the reaction, SCP consumes readily available CH. In this way, it prevents CH from reacting with chemical agents that commonly attack concrete such as nitrates and sulphates. Readily available reactants in concrete are one of the principal factors that affect concrete’s durability in chemical attack situations. The other is concrete’s permeability which is also addressed by SCP by filling the capillary pore structures with more concrete.
CH consumed by colloidal silica
Refer to research that has demonstrated that CH is consumed by colloidal silica post-placement pozzolan (P3).
Figure 1: CH consumption by colloidal silica post-placement pozzolan (P3) within the first 24-hours of concrete placement
Figure 2: CH consumption by colloidal silica post-placement pozzolan (P3) within the first 56 days of concrete placement
A logical concern may be that colloidal silica post-placement pozzolan (P3) could exhaust the supply of CH for reaction with other pozzolans present in the concrete. Research has shown that this is not the case. Pozzolan contents in SCP-treated concrete are as high as 80% by weight of cementitious properties. Thus, the addition of colloidal silica post-placement (P3) improves the performance of concrete.
Carl White is managing director of Spraylock Africa.