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In the UK, GGBS is normally supplied as a separate material and added at the concrete mixer. However, it can also be blended with Portland cement in a cement factory and marketed as ‘Portland slag’ or ‘Blastfurnace’ cement. The British Standard for 'Concrete' [BS 8500] allows either approach to be used.
These alternative routes have little effect on the properties of the finished concrete and the savings in carbon dioxide emissions are broadly similar. Generally, adding the GGBS at the concrete mixer:
A major advantage of adding GGBS at the concrete mixer, rather than at the cement factory, is the flexibility to vary the proportion and thereby optimise the technical performance of the concrete. The most widely used GGBS proportion is 50% but it is advantageous to be able to vary the proportion to meet specific requirements, e.g.:
| Property | Requirement | % GGBS |
| sulfate resistance | high sulfate resistance | 66 to 80% |
| chloride ingress | high chloride resistance | 66 to 80% |
| heat of hydration | controlling temperature rise | 50 to 70% |
| setting time | to avoid extended finishing times, for applications such as power-floated floors |
25 to 40% |
| early-strength | high early strength | 20 to 40% |
| very low early-strength gain, e.g. for secant piling | 81 to 95% | |
| to avoid retardation in cold weather | 30 to 40% |
Some users, e.g. precast concrete factories producing only a small range of products that require similar concrete properties, will not want to incur the expense of the additional silo needed to blend at the mixer and may prefer a factory-blended cement.
For further information, comparing the alternative approaches, see:
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For further information, about the within-mixer approach and how it is used within BS8500, see: