Granulated blastfurnace slag (ggbs) is a by-product of iron smelting. It is made by quenching selected molten blastfurnace slag to form granules. The granulated slag may be interground or blended with Portland cement clinker at certain cement works to produce:
n Portland-slag cement CEM II/A-S with a slag content of 6 - 35% conforming to BS EN 197-1
n Or, more commonly, blastfurnace cement CEM IIIA, which contains 36 - 65% ggbs, conforming to BS EN 197-1 .
Alternatively, the granules may be ground down separately to a white powder with a fineness similar to that of cement and combined in the concrete mixer with CEM I cement to produce a blastfurnace cement.
Mixer combinations of typically 40 - 50% ggbs with CEM I have the notation CIIIA and, at this level of addition, 28-day strengths are similar to those obtained with CEM I 42,5N.
As ggbs has little hydraulic activity of its own but is activated by the calcium hydroxide and other alkaline solutions produced by the hydration of Portland cement, it is referred to as 'a latent hydraulic binder'. Cements incorporating ggbs generate less heat and gain strength more slowly. The strengths at early ages are lower than those obtained with CEM I.
Table 3: Properties of hardened concrete incorporating pfa or ggbs - summary of comparisons with Portland cement CEM I
|
Property
|
Pfa
|
Ggbs
|
Comment
|
|
Long-term strength
(as a proportion of 28-day strength)
|
Greater with good curing
|
Greater with good curing
|
Depends on materials used and
curing
|
|
General physical properties
of hardened concrete
(modulus, creep)
|
Similar properties
|
Similar properties
|
Primarily depends on concrete
strength at loading
|
|
Resistance to carbonation- induced corrosion
|
Similar resistance
|
Similar resistance up to 50%
|
Depends on concrete strength class, exposure and curing conditions
|
|
Resistance to chloride-
induced corrosion
|
Greater resistance
|
Greater resistance
|
For equal w/c and well cured
|
|
Seawater attack
|
Similar performance
|
Similar performance
|
Primarily depends on concrete quality
|
|
Sulfate resistance
|
Greater resistance with
25 - 40%
|
Greater resistance with over
60%
|
See BRE Special
Digest 1
|
|
Freeze-thaw resistance
|
Similar performance except
at
early age
|
Similar performance except at
early age
|
Depends on strength at time of
exposure to freezing
|
|
Abrasion resistance
|
Similar performance except
at
early age
|
Similar performance except at
early age
|
Depends on strength
at time of
exposure to abrasion
|
|
Alkali-silica reaction
|
Often used for minimizing
risk of damage
|
Often used for minimizing risk
of damage
|
Requires selection of suitable proportion
of
pfa or ggbs
|
|
NOTE
The durability of concrete depends on the correct
proportions of pfa or ggbs incorporated. Guidance is given in BS 5328, BS EN 206-1 ,
BS 8500 and in BRE Special Digest 1.
|
|||
Blastfurnace cement, either the manufactured CEM III/A or the mixer combinations CIIIA, may be used for all purposes for which CEM I is used but, because it has a lower early development of strength, particularly in cold weather, it may not be suitable where early removal of formwork is required. It is a moderately low-heat cement and can, therefore, be used to advantage to reduce early heat of hydration in thick sections.
When the proportion of ggbs is 66 - 80% the notation CEM III/B applies for the manufactured cement and CIIIB for a mixer combination. This was previously known as high slag blastfurnace cement and is specified because of its lower heat characteristics or to impart resistance to sulfate attack.
Because the reaction between ggbs and lime released by the Portland cement is dependent on the availability of moisture, extra care has to be taken in curing concrete containing these cements or combinations in order to prevent premature drying out and to permit the development of strength.
BS 146 continues in revised form to allow for UK provisions not included in the European Standard EN 197-1 for blastfurnace slag cements. There is currently no equivalent EN relating to ggbs as an addition and, accordingly, BS 6699 continues to apply.
Pulverized-fuel ash cements and fly ash cements
The ash resulting from the burning of pulverized coal in power station furnaces is known in the concrete sector as pulverized-fuel ash (pfa) or fly ash.
This ash is fine enough to be carried away in the flue gases and is removed from the gases by electrostatic precipitators to prevent atmospheric pollution.
The precipitated material is a fine powder of glassy spheres that can have pozzolanic properties, i.e. when mixed into concrete it can react chemically with the calcium hydroxide (lime) that is released during the hydration of Portland cement. The products of this reaction are cementitious, and in certain circumstances pfa or fly ash can be used to replace part of the Portland cement in concrete.
The properties of fly ash for use as a cementitious component in concrete are specified in BS EN 450 with additional UK provisions for pfa made in BS 3892: Part 1. Pfa conforming to Part 2 of the same BS (Part 2 ash) is more coarse and is generally regarded as an inert addition used, for example, to modify properties of aggregate such as their gradings.
Fly ash, in the context of BS EN 450, means 'coal fly ash' rather than ash produced from other combustible materials. Fly ash conforming to BS EN 450 can be coarser than that conforming to BS 3892 : Part 1. However, fly ash to BS EN 450 can be used, in accordance with BS EN 206-1 and BS 8500 as a 'Type II addition' (pozzolanic or latent hydraulic material) in order to improve certain properties or to achieve special properties.
Substitution of these types of cement for Portland cement is not a straightforward replacement of like for like, and the following points have to be borne in mind when designing pfa concrete:
n Pfa reacts more slowly than Portland cement. At early age and particularly at low temperatures pfa contributes less strength; to achieve the same 28-day compressive strength the amount of cementitious material may need to be increased - typically by about 10%. The potential strength after three months is likely
to be greater than CEM I provided the concrete is maintained in a moist environment, such as in underwater structures or concrete in the ground