What is the activity index?
The activity index is a measure of the reactive strength of a pozzolanic or latent hydraulic addition (such as fly ash, ground granulated blast-furnace slag or silica fume) relative to pure Portland cement. The index indicates the extent to which the addition contributes to the strength development of concrete.
How is the activity index determined?
The activity index is determined by comparing two concrete mixes:
- Reference mix — with 100% Portland cement as the binder
- Test mix — in which part of the cement has been replaced by the addition under test (for example, 25% fly ash)
After 28 days of curing, the compressive strength of both mixes is measured:
Activity index = (compressive strength of test mix / compressive strength of reference mix) × 100%
An activity index of 85% means that the test mix with fly ash achieves 85% of the strength of the mix with Portland cement alone.
Application
The activity index is relevant in:
- Concrete mix design — determines how much cement can be replaced by an alternative binder without unacceptable loss of strength
- Quality control — fly ash and GGBS must meet minimum activity index requirements under European standards (BS EN 450 for fly ash, BS EN 15167 for GGBS)
- Sustainable construction — using fly ash and GGBS reduces CO₂ emissions because less cement is needed
Typical values
| Addition | Activity index at 28 days |
|---|---|
| Fly ash (PFA) | 75–95% |
| GGBS (ground) | 80–110% |
| Silica fume | 100–130% |
| Metakaolin | 90–110% |
Silica fume and GGBS can achieve an index above 100%, meaning they actually increase the concrete strength compared with Portland cement alone.
Related terms
- Concrete
- Cement
- Fly ash
- GGBS
- Concrete quality
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