Latent hydraulic — literally ‘hidden water-setting’ — describes a material that does have the capacity to harden with water, but does not start on its own. An activator is needed, usually an alkaline environment, before the reaction begins.
The classic example is granulated blast furnace slag, the by-product of pig iron production, which takes on a glassy structure when quenched rapidly with water. Ground to a powder it contains the right calcium, silicon, aluminium and magnesium compounds, yet with water alone virtually nothing happens.
How activation works
As soon as Portland cement clinker meets water, calcium hydroxide is formed and the mix reaches a high pH. At that high pH the glassy slag dissolves and forms the same CSH gel as the cement itself — the glue that gives hardened cement paste its strength. Blast furnace cement (the CEM III group in the European cement standard) therefore always contains a proportion of Portland clinker to act as the starter. Calcium sulfate and strong alkalis can also serve as activators.
Difference from a pozzolan
Both need an activator, but they do not work the same way:
- Latent hydraulic (blast furnace slag): the material can form a binder itself; the activator only sets the reaction going.
- Pozzolan (fly ash, silica fume, trass, natural pozzolans): does not react with water alone, but consumes calcium hydroxide to form CSH gel. Without that lime, nothing happens.
What it means on site
- Slower strength development. Concrete with a high slag content gains strength more slowly, especially in cold weather. Allow longer striking times and take extra care against frost in the first days.
- Less heat of hydration. An advantage in massive sections such as raft foundations and basement walls, where the heat would otherwise cause cracking.
- Better resistance to sulfates and chlorides. This is why blast furnace cement is a common choice for foundations, basements and marine or hydraulic works.
- Lighter colour and a lower carbon footprint than pure Portland cement, because it contains less clinker. Slag and fly ash are the main routes by which the concrete industry reduces its emissions.
- Curing is not optional. Slower-hardening concrete is particularly sensitive to drying out; keep the surface damp or covered for several days.
Related terms
- Blast furnace cement
- Blast furnace slag and steel slag
- Cement
- Ready-mixed concrete
- Concrete quality
- Calcium aluminate cement
- Concrete shrinkage
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