Frequently Asked Questions
Technical Support Line 44 (0) 1724 294221
- What is the difference between GGBS and PFA?
- Do national and international specifications recognise and permit the use of GGBS?
- Can I get certification that GGBS complies with British Standards?
- How does the slag processor ensure that GGBS has the right cementitious quality?
- How is GGBS delivered and stored?
- Does GGBS harden in concrete in the same way as Portland cement?
- So concrete can be made using only GGBS with no CEM I Portland cement?
- What ratio of GGBS to CEM I should be used?
- Does the water demand of concrete change when GGBS is used?
- Does GGBS concrete set in the same time as CEM I concrete?
- Can I powerfloat GGBS concrete?
- Can I use a water reducing admixture in GGBS concrete?
- And what about air entraining agents?
- And other admixtures?
- Does concrete with GGBS achieve the same strength as concrete with CEM I?
- If GGBS concrete gains strength more slowly than CEM I concrete,
- What about soffit and side shutters?
- Can you use GGBS in concrete which is in contact with potable water?
- What about the environment?
- How much will concrete with GGBS cost?
Answers
1. What is the difference between GGBS and PFA?
PFA is a material produced by burning coal in power stations and is composed mainly of silica. It reduces the amount of water required to produce a given consistency. At later stages it takes part in a pozzolanic reaction with the excess hydroxide released by CEM I Portland cement. It is considerably finer than a CEM I PC and has a grey colour which produces a darker concrete.
2. Do national and international specifications recognise and permit the use of GGBS?
Certainly. GGBS has its own British Standard BS 6699. The Complementary British Standard to BS EN 206-1, BS 8500, Concrete, refers to and permits the use of GGBS. Virtually all standards that specify cementitious material for use in concrete and mortar permit the use of GGBS.
3. Can I get certification that GGBS complies with British Standards?
Two types of certification are provided. The first confirms that the material complies with the GGBS specification BS 6699. The second advises the blend levels appropriate to a strength class with any particular CEM I Portland cement source i.e. 32.5, 42.5 or 52.5
4. How does the slag processor ensure that GGBS has the right cementitious quality?
The slag processor closely monitors the chemical composition of the granulated slag and is able to reject or blend material ensuring a consistent feed blend composition to his plant. GGBS quality is controlled by testing concrete made with production samples. The compressive strength from the test specimens is compared to a target and the trend obtained from these results determines the required fineness of the GGBS. By increasing the fineness a higher concrete strength is obtained; decreasing fineness results in a lower strength.
5. How is GGBS delivered and stored?
GGBS is delivered in bulk road tankers, similar to those used for CEM I Portland cement. It is blown into silos by air pressure. Storage silos must have fully operational de-dusting apparatus together with high level alarms and pressure release valves. Appropriate safety procedures must be followed when discharging the powder. GGBS flows in the same manner as CEM I Portland cement and similar handling equipment is used. The material must be protected against moisture ingress and if this is done it has an almost indefinite shelf life.
6. Does GGBS harden in concrete in the same way as Portland cement?
Yes. As GGBS is a hydraulic material, hydration will take place.
7. So concrete can be made using only GGBS with no CEM I Portland cement?
In theory, yes. However, the hydration (setting and hardening) of GGBS on its own is too slow to be of practical use. When CEM I and GGBS are used together the release of alkalis causes the hydration process to accelerate resulting in a much improved rate of gain of strength in the concrete.
8. What ratio of GGBS to CEM I should be used?
For general use, the proportion of GGBS which replaces the CEM I PC is between 40% and 55%. This level is usually governed by the performance of the CEM I PC and the type and quality of the aggregates. However, depending on the specific application, higher addition rates enhance the benefits of using GGBS. By going to 70% GGBS a greater degree of durability is imparted and temperature rise is further reduced. By reducing the addition rate to 30%GGBS sufficient early age strength may be achieved for operations such as early handling of precast units.
9. Does the water demand of concrete change when GGBS is used?
10. Does GGBS concrete set in the same time as CEM I concrete?
Depending on the amount of GGBS used, section thickness and ambient temperature, increases in setting time can be observed. This is beneficial in reducing the risk of cold joints in large pours and giving a slightly longer window to place, compact and finish the concrete.
11. Can I powerfloat GGBS concrete?
Yes, in a very similar way as CEM I PC concrete. Normally, between 20% and 50% GGBS is used for powerfloating projects, depending on the floor size and the time of year. However, when using higher percentages of GGBS, particularly in cold weather, it may be necessary to change the finishing regime, e.g. place the concrete late in the afternoon and powerfloat next morning. When using any type of concrete in flooring it is very important to ensure consistent workability of the material, as variation in the workability of different batches will cause different setting rates. As with all concrete curing is essential to ensure maximum potential performance is achieved.
12. Can I use a water reducing admixture in GGBS concrete?
Water reducing admixtures including high range water reducers work particularly well with GGBS concretes.
13. And what about air entraining agents?
Air entraining agents are compatible with GGBS, however slight adjustments in the dosage rate may be required to achieve the target air content. The air content of a concrete should be tested and appropriate adjustments made.
14. And other admixtures?
Normally all admixtures work with GGBS. If you are unsure either check with us or with your admixture supplier. Care should be taken when using retarders. As GGBS increases setting time a reduction in the 'standard' CEM I PC dosage may be required.
15. Does concrete with GGBS achieve the same strength as concrete with CEM I ?
This is very much dependent on the source of CEM I PC, the type and quality of the aggregate and the mix design. Generally, when an addition rate between 40% and 55% GGBS is used the compressive strength of standard cured cubes tested at 28 days is similar to CEM I PC concrete. At earlier ages i.e. 7 days, the strength of blended cement concrete is lower, usually around 50% to 60% of its 28 day strength. CEM I PC concrete has a 7 day strength of around 75% of its 28 day strength. Whereas CEM I PC concrete gains little strength after 28 days, GGBS continues to gain significant strength beyond 28 days.
16. If GGBS concrete gains strength more slowly than CEM I concrete, do I need to keep my shutters up longer?
Generally no. As strength gain is temperature dependent, concrete sections above 250 - 300mm thickness will generate sufficient heat to provide strength gain to enable vertical shutters to be stripped within the same timescale as CEM I PC. However if high addition rates are being used and with low ambient temperatures extended striking times may be required. In order to determine accurate insitu strengths for striking purposes, temperature matched curing should be carried out. This involves curing cubes in a water bath which follows the same temperature profile as the structure. The temperature of the structure is monitored by placing thermocouples in the concrete. Alternatively maturity determination techniques can be used.
17. What about soffit and side shutters?
CIRIA report 136 gives details of striking times. The use of temperature matched curing may allow an improvement of those times.
18. Can you use GGBS in concrete which is in contact with potable water?
Yes, GGBS is approved by the water industry for applications involving potable water.
19. What about the environment?
As GGBS is a product of the iron industry, most of the energy required to produce it has already been expended in the production of the iron. Similarly, the amount of CO2 generated is much less being typically 1/15th of that generated by the production of CEM I PC .Whilst minerals are quarried specifically for the production of CEM I PC, no quarrying is carried out specifically for the production of GGBS thus natural materials are being conserved. The use of GGBS in concrete is in line with the Government's sustainability strategy.
20. How much will concrete with GGBS cost?
Concrete containing GGBS will cost no more than the equivalent concrete produced with CEM I PC, and will almost certainly be less than concrete with sulfate resisting Portland cement.