Cement and Combination Designations
BS 8500 does not treat fly ash or ground granulated blastfurnace slag (GGBS) as an optional add-on percentage on top of a Portland cement figure. Instead it defines named cement and combination designations, each covering a Portland cement, or a factory-made blended cement, used together with a specific proportion range of a secondary cementitious material — and each designation is specified and used as a cement type in its own right.
Reading the designations
| Designation | What it is |
|---|---|
| CEM I | Portland cement, no secondary cementitious material |
| CIIA-V, CIIB-V | Portland cement combined with fly ash (V), at increasing fly ash proportion, A < B |
| CIIIA, CIIIB | Portland cement combined with GGBS (III), at increasing GGBS proportion, A < B |
The letter suffix (A, B) broadly indicates the proportion band of the secondary material, increasing from A onward; BS 8500-2 sets out the exact designations and their composition limits, and additional designations exist beyond this short list, including combinations with silica fume and limestone.
Why combinations, not ‘% replacement’
This is a different framing from a simple percentage-replacement approach: under BS 8500, once a combination like CIIB-V or CIIIA is chosen, it is specified and used as the cement type for that mix — with its own exposure-class durability limits — rather than as CEM I plus an added percentage of a supplementary material bolted on afterward. Fly ash used in these combinations must comply with BS EN 450, and GGBS with BS EN 15167, when supplied and combined at the concrete plant, as opposed to a factory-blended cement that already contains the secondary material.
Why use a combination at all
Combinations with fly ash or GGBS are widely used because they can improve resistance to some forms of chemical attack, reduce heat of hydration in mass sections, and lower embodied carbon compared with CEM I alone, though they can also change setting time and early-age strength gain. See what changed in BS 8500:2023 for how low-carbon multi-component cements have developed further.
Frequently asked questions
Is CIIB-V the same as adding fly ash to CEM I concrete on site?
No — under BS 8500 the combination is specified and designed as its own cement type from the start, with its own durability limits, rather than fly ash being added as an afterthought to a CEM I mix.
Do all combinations behave the same as CEM I?
No — combinations with a higher proportion of fly ash or GGBS often gain strength more slowly at early ages and can have longer setting times, which affects formwork striking times and cold-weather working, even though 28-day and later performance is often comparable or improved.
Where are the exact composition limits for each designation?
BS 8500-2’s cement and combination designation tables give the current composition limits — check the edition current for your project rather than relying on a remembered percentage.