The addition of silica dust to concrete remains inert initially. However once the water and cement in the concrete begin to react (hydrating), two chemicals are produced:
- Calcium silicate hydrate (CSH), which is responsible for the strength of concrete
- Calcium hydroxide (CH) or free lime, which is a byproduct of the reaction
Calcium hydroxide fills the gaps between calcium silicate crystals and in low-grade cements is responsible for being leached out.
The Pozzolanic reaction between silica fume and free lime produces more calcium silicate hydrate which adds to the compressive strength of the concrete as well as compaction and impermeability.
- The word Pozzolanic originates from an old town in Italy called Pozzuoli where ancient Romans produced mortars for construction by combining volcanic ash with lime stone.
Volcanic ash had a similar composition to silica fume which its combination with lime stone produces calcium silicate hydrate which is the bonding material of cement therefore after thousands of years these ancient buildings are still standing.