GGBS proportions

On its own GGBS hardens slowly and, for use in concrete, it needs to be activated by combining it with an activator (typically Portland cement CEM I but alkali activation is also used).

A typical combination is 50% GGBS with 50% CEM I, but percentages of GGBS anywhere between 20 and 80% are commonly used. The greater the percentage of GGBS, the greater will be the effect on concrete properties.

Setting times

With GGBS, the setting time will be extended slightly, perhaps by about 30 minutes. The effect will be more pronounced at high levels of GGBS and/or low temperatures.

An extended setting time is advantageous in that the concrete will remain workable longer and there will be a reduced risk of cold joints. This is particularly useful in warm weather.

Water demand

The differences in rheological behaviour between GGBS and Portland cement CEM I may enable a small reduction in water content to achieve an equivalent consistence class.

Consistence (Slump)

While concretes containing GGBS have a similar, or slightly improved consistence to equivalent Portland cement CEM I concretes, fresh concrete containing GGBS tends to require less energy for movement. This is thought to be related to the relatively smooth glassy surface texture of the GGBS particles compared to the rough texture of Portland cement CEM I particles.

This makes it easier to place and compact, especially when pumping or using mechanical vibration. In addition, it will retain its workability for longer. Early age temperature rise

The reactions involved in the setting and hardening of concrete are exothermic and generate significant heat and can produce large temperature rises, particularly in thick-section pours.

This can result in early age internally and externally restrained thermal cracking. Replacing Portland cement CEM I with GGBS reduces the temperature rise and helps to avoid early-age thermal cracking. The higher the percentage of GGBS, the lower the rate of reaction and consequently the slower the rate at which heat is developed. This will result in reduced peak temperatures and reduced thermal strain.

Strength gain in GGBS concrete

With the same cementitious material content (the total weight of Portland cement CEM I plus GGBS), similar 28-day strengths to Portland cement CEM I will normally be achieved when using up to 50% GGBS.

At higher GGBS percentages the cementitious content will need to be increased to achieve equivalent 28-day strength. The rate of reaction and strength development of GGBS concrete is slower than equivalent concrete made with Portland cement CEM I but continues at a significant rate for much longer (data indicates continued development after many years). Typically the 7-day strength of a ggbs concrete will be 55-65% of the 28-day strength compared to 70-80% for a similar CEM I Portland cement concrete.

The reduction in early-strength will be more noticeable at high GGBS levels and low temperatures. Similarly low cement content concrete, kerb backing etc, the reduction will be  magnified.

Under normal circumstances, the striking times for concretes containing up to 50% GGBS, do not increase sufficiently to significantly affect the construction programme. However, concretes with higher levels of GGBS will not always achieve sufficient strength after one day to allow removal of vertical formwork, particularly at lower temperatures, lower cementitious contents and in thinner sections.

Relative strength development of concrete with different cement types

At 7 Days

At 28 Days

At 56 Days

Colour

Ground granulated blast-furnace slag is off-white in colour and substantially lighter than Portland cement CEM I.

This whiter colour is also seen in concrete made with GGBS, especially at CEM I replacement levels of 50% and above. The more aesthetically pleasing appearance of GGBS concrete can help soften the visual impact of large structures such as bridges and retaining walls.

For coloured concrete, the pigment requirements are often reduced with GGBS and the colours are brighter.