Table 38 Typical compositions of granulated blastfurnace slag and Portland cement

Granulated Portland cement blastfurnace slag (%)

Granulated Portland cement blastfurnace slag (%)

Lime 41

68

Silica 35

22

Alumina 11

S

Iron Oxide 1

3

Other 12

2

Table 3.9 Composition of low early strength blastfurnace cements

to British Standard BS EN 197: 2004

Composition Type

CEM III/A

CEM III/B CEM III/C

(%)

(%) (%)

Portland cement clinker

3S-64

2G-34 S—19

Blastfurnace slag

36-6S

66-8G 81—9S

Minor constituents

G-S

G-S G-S

Concrete manufactured from a blend of Portland and granulated blastfurnace slag cements has a lower permeability than Portland cement alone; this enhances resistance to attack from sulfates, weak acids and to the ingress of chlorides which can cause rapid corrosion of steel reinforcement, for example in marine environments and near roads subjected to de-icing salts. Sulfate attack is also reduced by the decrease in tricalcium aluminate content. The more gradual hydration of granulated blastfurnace slag cement evolves less heat and more slowly than Portland cement alone; thus a 70% granulated blastfurnace slag mix can be used for mass concrete, where otherwise a significant temperature rise could cause cracking. The slower evolution of heat is associated with a more gradual development of strength over the first 28-day period.

However, the ultimate strength of the mature concrete is comparable to that of the equivalent Portland cement. The initial set with granulated blastfurnace slag blends is slower than for Portland cement alone, and the fresh concrete mixes are more plastic, giving better flow for placing and full compaction. The risk of alkali-silica reaction caused by reactive silica aggregates can be reduced by the use of granulated blastfurnace slag to reduce the active alkali content of the concrete mix to below the critical 3.0 kg/m3 level. The classes for low early strength blastfurnace cements are listed in Table 3.10.

Portland fly ash and pozzolanic cements Pozzolanic materials are natural or manufactured materials containing silica, which react with the calcium hydroxide produced in the hydration of Portland cement to produce further cementitious products. Within the UK, natural volcanic pozzolanas are little used, but fly ash, formerly termed pulverised-fuel ash (PFA), the waste product from coal-fired electricity-generating stations, is used either factory mixed with Portland cement or blended in on site.

Table 3.10 Strength classes of low early strength blastfurnace cements to British Standard BS EN 197-4: 2004

Strength classes

Compressive strength (MPa)

2 day early

7 day early

28 day standard

strength

strength

strength

32.5 L

>12

32.5-52.5

42.5 L

-

>16

42.5-62.5

52.5 L

>10

-

>52.5

Portland fly ash cement, cures and evolves heat more slowly than Portland cement; it is therefore appropriate for use in mass concrete to reduce the risk of thermal cracking. Additions of up to 25% fly ash in Portland cement are often used; the concrete produced is darker than with Portland cement alone. Concrete made with blends of 25-40% by weight of fly ash in Portland cement has good sulfate-resisting properties. However, in the presence of groundwater with high magnesium concentrations, sulfate-resist-ing Portland cement should be used. Fly ash concretes also have enhanced resistance to chloride ingress, which is frequently the cause of corrosion to steel reinforcement.

The fly ash produced in the UK by burning pulverised bituminous coal is siliceous, containing predominantly reactive silica and alumina. In addition to siliceous fly ash, the European Standard EN 197-1: 2000 does allow for the use of calcareous fly ash, which additionally contains active lime, giving some self-setting properties. The range of fly ash suitable for concrete is defined in the standard BS EN 450-1: 2005. Natural pozzolanas of volcanic origin and industrial pozzolanas from other industrial processes in Europe are used with Portland cement and are categorised under EN 197-1: 2000 as pozzolanic cements.

Portland limestone cement

The addition of up to 5% limestone filler to Portland cement has little effect on its properties. The addition of up to 25% limestone gives a performance similar to that of Portland cement with a proportionally lower ce -mentitious content; thus, if equivalent durability to Portland cement is required, then cement contents must be increased. The two categories of limestone for Portland limestone cement are defined by their total organic carbon (TOC) content; LL refers to a maximum of 0.20% and L to a maximum of 0.50% by mass.

Silica fume

Silica fume or microsilica, a by-product from the manufacture of silicon and ferro-silicon, consists of ultra-fine spheres of silica. The material, because of its high surface area, when blended as a minor addition to Portland cement, increases the rate of hydration, giving the concrete a high early strength and also a reduced permeability. This in turn produces greater resistance to chemical attack and abrasion. Silica fume may be added up to 5% as a filler, or in Portland silica fume cement to between 6 and 10%.

Burnt shale

Burnt shale is produced by heating oil shale to 800°C in a kiln. It is similar in nature to blastfurnace slag, containing mainly calcium silicate and calcium aluminate, but also silica, lime and calcium sulfate. It is weakly cementitious. The European Standard EN 197-1: 2000 allows for the use of burnt shale as a filler to 5%, or between 6 and 35% in Portland burnt shale cement.

Fillers

Fillers up to 5% by weight of the cement content may be added to cements to the standard EN 197-1: 2000. They should be materials which do not increase the water requirements of the cement. Fillers may be any of the permitted alternative main constituents (e.g. granulated blastfurnace slag, pozzolanas, fly ash, burnt shale, silica fume or limestone), or other inorganic materials, providing that they are not already present as one of the main constituents. The most common fillers are limestone and either raw meal or partially calcined material from the cement-making process.

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