GGBS
Ground Granulated Blast Furnace Slag (GGBS) has become a familiar and valuable constituent in concrete as a cementitious material.
What is GGBS?
GGBS (Ground Granulated Blast-furnace Slag) is a by-product from the blast-furnaces used to make iron.
Blast-furnaces operate at temperatures of about 1,500°C and are fed with a carefully controlled mixture of iron ore, coke and limestone. The iron ore is reduced to iron and the remaining materials form a slag that floats on top of the iron.
This slag is periodically tapped off as a molten liquid and if it is to be used for the manufacture of GGBS it has to be rapidly quenched in large volumes of water. The quenching optimises the cementitious properties and produces granules similar to a coarse sand. This ‘granulated’ slag is then dried and ground to a fine powder.
Although normally designated as ‘GGBS’ in the UK, it can also be referred to as ‘GGBFS’ or ‘slag cement’.
Use of GGBS in concrete
GGBS is utilised in the majority of UK ready-mix deliveries due to the wide range of technical benefits it imparts to concrete including:
- Lower early-age temperature rise, reducing the risk of thermal cracking in large pours
- Minimising the risk of detrimental internal reactions such as Alkali Silica Reaction and Delayed Ettringite Formation
- High resistance to chloride ingress thus reducing the risk of steel reinforcement corrosion
- High resistance to attack by sulphates and other chemicals
- Better workability, making placing and compaction easier
- Pleasing off-white colour
- Reduced embodied carbon emissions
For further information on these attributes, visit the Cementitious Slag Makers’ Association.
On its own GGBS hardens slowly and, for use in concrete, it needs to be activated by combining it with an activator. This is typically Portland cement CEM I, but alkali activation can also be used.
A typical combination is 50% GGBS with 50% CEM I, but percentages of GGBS between 20 and 80% are commonly used. The greater the percentage of GGBS, the greater will be the effect on concrete properties.
Impact of GGBS on 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 can be 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.
Impact of GGBS on 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.
Impact of GGBS on 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; they generate significant heat and can produce large temperature rises, particularly in thick-section pours. This can result in early age 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). This can be taken advantage of by specifying for a 56 dayconcrete strength as opposed to a 28-day strength, see Specifying Sustainable Concrete: Using 56 Day Concrete Strengths.
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 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.
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.
Specification
Where possible, specifiers should avoid specifying particular cements or combinations, as most concrete plants will not stock all cementitious materials and there is usually a range of ways to meet the required concrete performance.
Where the technical performance characteristics of GGBS are desired, this should be addressed through specifying the required exposure class in accordance with BS 8500-1.
Where a project has low carbon aspirations, this should be realised by agreeing a carbon range or limit for the concrete, or ideally whole concrete package, rather than specifying specific proportions of individual constituents. This allows higher GGBS contents to be used where they deliver the greatest technical and environmental benefit, while avoiding its use where it is less suitable, or delivers fewer benefits.
Refer to Specifying Sustainable Concrete: BS8500 for further information.
Availability
GGBS is a globally-traded product. It is stocked at many plants across the UK and the suppliers have indicated they have long term contracts in place.