THE EFFECT OF SILICA FUME ON THE PROPERTIES OFCONCRETE AS DEFINED IN CONCRETE SOCIETY REPORT 74,

CEMENTITIOUS MATERIALS

Des King, United Kingdom

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THE EFFECT OFSILICA FUME ON THE PROPERTIES OF CONCRETE AS DEFINED

IN CONCRETE SOCIETY REPORT 74, CEMENTITIOUS MATERIALS

Des King

Hon. FICT, Dip M, Chartered MCIM, DMS Consultant, United Kingdom

Keywords: Silica fume, CSTR74, properties of concrete, effects, cementitious materials, high performance concrete.

Abstract. The new Concrete Society Publication, Cementitious Materials, CSTR74, covers the effects of cementitious materials - blast furnace slag (GGBS), fly ash (FA), limestone fines and silica fume (microsilica) on the properties of concrete. This paper focuses on the effects of silica fume on all the main properties of concrete in the fresh and hardened state as defined in the publication.

1. INTRODUCTION

Silica fume can be used either as a densified or undensified powder, a slurry, as a combination at the concrete mixer, or part of a factory-blended cement.The Report provides detailed information and references for further reading, on the effect of these materials on the fresh and hardened properties of concrete. Generally, the comparisons are made against a concrete made with 100% CEM I (Portland cement).

The following terms are used with specific meanings:

- blending: mixing together cementitious components e i ther at the concrete mixer or in the cement factory (including inter-grinding):

- composite cement: a binder where the cementitious components are blended in a cement factory (including inter-grinding)

- combination: a binder where the cementitious components are blended at the concrete mixer- strength class: characteristic compressive strength of test cubes at 28-days- w/c: water/cementitious ratio

The paper provides detailed information and references for further reading, on the effect of silica fume (SF) on the fresh and hardened properties of concrete. Generally, the comparisons are made against a concrete made with 100% OPC (CEM1 Portland cement). The paper contains extracts from the new UK Concrete Society Report No. 74 (CSTR74), Cementitious Materials of which the author was a member of the working party.

2. PRODUCTION

The raw materials for the production of silica fume are by-products from the production of silicon metal, and these by-products are further processed to produce cementitious materials for use in concrete.

Des King

Production of silica fume

Silica fume is a by-product of the manufacture of silicon metal and ferro-silicon alloys. The process involves the reduction of high purity quartz (SiO2) in electric arc furnaces at temperatures in excess of2,000°C. Silica fume is a very fine powder consisting mainly of spherical particles or microspheres of mean diameter about 0.15 microns, with a very high specific surface area (15,000–25,000 m2/kg).Each microsphere is on average 100 times smaller than an average cement grain. At a typical dosage of 10% by mass of cement, there will be 50,000–100,000 silica fume particles per cement grain.

3. CHEMICAL AND PHYSICAL PROPERTIES

Indicative values for the chemical and physical properties of silica fume compared with other cementitious materials are given in Table 1.

Table 1: Typical properties.

Fly ash GGBS Silica fumeFineness (m2/kg) 450 350 to 550 15,000 to

35,000Bulk density(kg/m3) 1300 1200 1350-1510Specific gravity 2.2 2.9 2.2

Main elementsSilicon (% as SiO2) 38 to 55 30 to 40 > 85Aluminium (% as Al2O3) 20 to 40 5 to 20 < 2Iron (% as Fe2O3) 6 to 16 < 2 <1Calcium (% as CaO) 1.8 to 10 35 to 40 <1Magnesium (% as MgO) 1.0 to 3.5 5 to 18 <1Sodium (% as Na2O) 0.8 to 1.8 < 1 <1Potassium (% as K2O) 2.3 to 4.5 < 1 <1Titanium (% as TiO2) 0.9 to 1.1 < 2 -Chloride (% as Cl) <0.01 < 0.1 < 0.3Loss on ignition (%) 3 to 20 < 3 < 4Sulfate (% as SO4) 0.42 to 3.0 < 2.5 < 0.3Free calcium oxide (%) <0.1 to 1.0 < 1 < 1

Des King

Cementitious Properties

The materials differ in their chemical reactivity. GGBS is a latent hydraulic binder, i.e. when mixed with water, it slowly sets and hardens. With GGBS alone, the rate of hardening is relatively slow and for normal concrete applications, GGBS needs to be activated by combining it with CEM I. Fly ash and silica fume are pozzolanas, i.e. they do not react with water alone but do react chemically with the calcium hydroxide produced by the hydration of CEM I to form calcium silicate hydrates (C-S-H) which bind the concrete together. Limestone is chemically, relatively inert but limestone fines, because of their fine particle size, can contribute towards strength by a physical, void-filling mechanism. There is evidence that fine calcium carbonates act as nucleation sites, accelerating the hydration of CEM I, improving early strength development.

Silica fume is a highly reactive pozzolana that converts all or most of the liberated calcium hydroxide to C-S-H

As a result of their chemical reactivity and also fineness, the materials differ in the proportion of the cementitious content at which they can be used, with indicative proportions being:

GGBS typically 50% but can be up to 70% or more fly ash typically 30% but can be up to 50% or more limestone fines typically 15% but can be up to 20% or more silica fume typically <8% but can be up to 12.5% or more

4. USE IN CONCRETE

Silica fume can either be added separately at the concrete mixer, where they are referred to as‘additions’ or else be incorporated into a factory-produced composite cement. The following sections describe the way in which Standards deal with these two alternative approaches and the relative merits and practicalities.

The European standard for cements BS EN 197-1 covers a wide range of cements other than CEM I. These include:

• CEM II-S Portland-slag cement (6-35% blastfurnace slag)• CEM II-D Portland- silica fume cement (6-10% silica fume)• CEM II-V Portland-fly ash cement (6-35% fly ash)• CEM II-L(LL) Portland-limestone cement (6-35% limestone)• CEM III Blastfurnace cement (36-95% blastfurnace slag)• CEM IV Pozzolanic cement (11-55% pozzolan (including fly ash)

BS EN 13263, Silica fume for concreteEach of these standards has two parts:

- Part 1: Definitions, specifications and conformity criteria- Part 2: Conformity evaluation

Within BS EN 206-1, additions may be used under the k-value concept, with the term water/cement ratio being replaced by water/(cement + (k × addition)) ratio. Specific k-values are allocated for silica fume to BS EN 13263 and for fly ash to BS EN 450-1. However the k-value approach is rarely (if ever) used in the UK. Silica fume concrete is normally designed for high durability or strength characteristics and the BS 8500 recommended limiting values for cement content and w/c, are generally satisfied, even without taking the silica fume into account.

Table 2 summarizes the ways in which additions may be used according to BS EN 206–1 and BS8500.

Table 2: The use of additions in concrete.

Addition Standard for Suitability for use in Rules for counting towards the addition concrete is confirmed by cementitious content, are in

Fly ash BS EN 450 BS EN 206-1, clause 5.1.6 BS EN 206-1, clause 5.2.5.2.2or BS 8500-2, clause 4.4

GGBS BS EN 15167 BS 8500-2, clause 4.4 BS 8500-2, clause 4.4Silica fume BS EN 13263 BS EN 206-1, clause 5.1.6 BS EN 206-1, clause 5.2.5.2.3

Des King

Current availability and use for silica fume

Global consumption of silica fume exceeds 1 million tonnes per annum. Silica fume is generally dark grey to black or off-white in colour and can be supplied as a densified powder or slurry depending on the application and the available handling facilities. For use in the UK, it is normally supplied as slurry, consisting of 50% powder and 50% water. In powder form silica fume is available in bulk, large bags and small bags. If required in bags, these can be tailored to suit the customers’ needs for handling and batch weight per cubic metre of concrete. Other applications include fibre cement, gypsum cement, refractory mortars and castables and in the use of specialised ultra high strength precastsections where strengths of over 200 N/mm2 can be designed.

5. COMPATIBILITY WITH ADMIXTURES

Generally chemical admixtures can be used with silica fume in the same way as for conventional concretes. Silica fume is normally used with a super-plasticiser. As is the case for OPC cements the performance of admixtures may depend upon the properties of the individual source of the cementitious material and tests should be carried out to establish the appropriate dosage levels.

To maximise the full strength producing potential of silica fume in concrete it is recommended that it should always be used with a dispersant admixture such as high range water reducing agent (HRWRA). The dosage will depend on the amount of silica fume and the type of admixture used; see Jahren. The dosage of air-entraining admixture to produce a required volume of air in concrete usually increases with the amount of silica fume. The amount of silica fume and the type of mixing were found to have no significant influence on the development and stability of the air-void system, see Pigeon et al.

6. WATER DEMAND

Silica fume can be expected to produce an increased water demand, which is normally countered by the use of admixtures.

The water demand of concrete containing silica fume increases with increasing amounts of silica fume, see Carette and Malhotra and Scali et al. This increase is caused primarily by the high surface area of the silica fume. To achieve a maximum improvement in strength and durability, silica-fume concrete should contain a high- range water reducing admixture.

6. WORKABILITY/CONSISTENCE

Fresh concrete containing silica fume is more cohesive and less prone to segregation than concrete without silica fume. Experience has shown that it is necessary to increase the initial slump of concrete with silica fume by approximately50mm above that required for conventional CEM I concrete to maintain the same apparent workability, see Jahren. Silica fume addition has been used to assist in pumping long distances, especially vertically. Concrete was pumped in a single operation to a height of 601 metres at the Burj Khalifa project in Dubai; so far the world’s tallest building.

Fig. 2. Casting of precast concrete units for the sea defences at Cleveleys, UK using a proprietary concre containing silica fume.

7. SETTING TIME

Unlike other SCM’s such as slag and fly ash, silica fume does not significantly affect setting time.

8. BLEEDING

After concrete has been placed there is a tendency for the solids (aggregates and cementitious) to settle and displace the water, which is pushed upwards. If the process is excessive, the water appears as a layer on the surface. The tendency of a concrete to bleed is affected by the constituents and their proportions, particularly the grading of the fine aggregate, the water content and any admixtures. Excessive bleeding can produce a layer of weak laitance on the top of the concrete and may result in plastic settlement cracks but bleeding can also be beneficial in avoiding plastic shrinkage cracks, which can form on concrete placed on hot or windy days, where the rate of evaporation of moisture from the surface exceeds the rate of bleeding, see Concrete Society Good Concrete Guide 8.

Concrete containing silica fume shows significantly reduced bleeding. As silica fume dosage is increased, bleeding will be reduced. This effect is caused primarily by the high surface area of the silica fume to be wetted; there is very little free water left in the mixture for bleeding, see Grutzeck, Roy and Wolfe-Confer. Jahren points out that because of the reduced bleeding, care should be taken to prevent early moisture loss from freshly placed silica-fume concrete, particularly under conditions that promote rapid surface drying.

9. HEAT OF HYDRATION

Silica fume has effectively a similar heat of hydration to CEM I.

A reduction in the early age temperature rise can reduce the risk of early-age thermal cracking and detailed guidance is provided in CIRIA Report C660, Early-age thermal crack control in concrete. However a slower release of heat can reduce the initial rate of strength gain. This may necessitate longer periods before striking formwork and/or removal of props especially when casting thin, exposed sections in winter conditions in cooler climates.

CIRIA Report C660 suggests that silica fume should be considered equivalent to CEM I in regard to heat generation. It also points out that silica fume, when used with a high range water reducing admixture, can achieve equivalent strength with a reduced binder content (subject to any minimum limit on binder content) and thereby lower heat output. Papworth et al calculated theoretically the temperature rise for silica fume concrete and concluded that by reducing the total binder content of the mix, it can reduce the temperature rise. They then compare predicted temperature rises with those measured in practical situations. A solution to providing high early strength couple with low heat was demonstrated in massive silica fume concrete pours at the Hanford Nuclear Encapsulation facility in the USA.

10. SENSITIVITY TO CURING

Concrete stiffens and hardens through the hydration reaction between cement and water. Strength and microstructure, which depend on the degree of hydration, can be adversely affected if concrete is allowed to dry out at early ages and hydration is prematurely arrested. The layer close to the surface seems to be the most sensitive, as evidenced by the large effect of curing on abrasion-resistance. At greater depth, in the region approaching the level of the reinforcing steel, the effect of curing appears to be less critical, see CIRIA Report C530. With silica fume, the concrete may contain very little free water and with significantly reduced bleed there needs to be extra emphasis on curing so that the surface layer retains the water needed for development of the properties of the concrete.

To obtain the full benefits of silica fume concrete, proper curing procedures must be followed. Because concrete containing silica fume shows significantly reduced bleed (as dosage is increased) there is very little free water left in the mixture for bleeding, see Grutzeck, Roy and Wolfe-Confer. This extra emphasis on curing helps to retain the water needed for development of the desired properties of the concrete. Experience shows that under conditions of fast evaporation (wind and sun), curing measures must be taken immediately after placing the concrete.

CEM I8% SF50% GGBS15% LF30% FA70% GGBS

CO

MP

RE

SS

IVE

STR

EN

GTH

11. EFFECT ON CONCRETE STRENGTH

Strength DevelopmentFigure 1, shows indicative strength development, at a f ixed w /c r a t i o , where concretes containing fly ash, limestone fines or high-replacement GGBS generally have lower 28-day strength than CEM I whilst silica fume can give increased 28-day strength. Figure 3 shows indicative longer-term strength development, where for concretes required to achieve a specified 28-day strength, those containing GGBS, fly ash and silica fume generally show increased ultimate strengths.

150%

100%

50%

0%0 7 14 21 28 35 42 49 56 63 70 77 84 91

DAYS

Figure 3. Indicative Strength Development (at fixed w/c and relative to the 28-day strength ofCEM I)

Notes:1. The relative strengths are indicative and will vary significantly, with mix design and materials2. At fixed w/c, the workability/consistence will vary, according to the water-demand3. This graph is derived from experience and data contributed by the authors of CSTR74

Concrete made with silica fume follows the conventional relationship between compressive strength and w/c but strength is increased at a given w/c ratio when silica fume is used. High early compressive strength (in excess of 25N/mm2 at 24 hours) can be achieved. With proper concretedesign, very high 28-day strengths can be produced, using normal ready-mixed concrete plants and in the USA and Asia 100–130N/mm2 concretes are used in tall buildings. Cementitious contents are generally > 400 kg/m3 and w/c in the range 0.30 to 0.40. An example in South East Asia is the new79 floor East Island Centre in Hong Kong where the volume of concrete was reduced by 15% through the use of Grade 100 MPa self-compacting concrete. Not only were there significant sustainabilitybenefits but also the client benefited commercially through the additional floor space opened up forrental in this expensive part of Hong Kong.

Fig 4. One Island East Tower in Hong Kong under construction. C100 silica fume concrete in the columns and core walls means only 8 columns are required on each floor.

Fig 5. The Burj Khalifa in Dubai is currentlythe tallest building in the world where the silica fume concrete contributed to record pumping heights of over 600m in a single operation

12. STRIKING TIMES

The initial strength gain of concrete with cementitious materials such as slag and fly as is likely to be slower than CEM I concrete, particularly with higher percentages and this can sometimes require formwork striking times to be extended, especially at lower temperatures. Factors that need to be taken into account before removing formwork are:- the temperature of surface concrete should not fall below freezing until it has developed a

strength of 5 MPa- the concrete needs to have developed sufficient strength to resist mechanical damage during

formwork removal.- For soffit formwork, sufficient strength is required for the concrete to be self-supporting.

General guidance on striking of slab soffits and backpropping can be found in the Construct ‘Guide to flat slab falsework and formwork' (CONSTRUCT).

Experience indicates that the time of striking formwork is not significantly affected by the use of silica fume by itself.

Str

engt

h as

% o

f tha

t ach

ieve

d at

28-

days

100%

CEM I 8% SF 15% LF30% FA 50% GGBS 70% GGBS

0%1 day 3 days 7 days 28 days

Fig 6. Indicative early strengths shown as percentage of 28-day strengthNotes:

1. The relative strengths are indicative and will vary significantly, with mix design and materials2. The early-strengths are relative to 28-day strength3. The cementitious content required to achieve a specified 28-day strength will not be the

same for the different combinations4. This graph is derived from experience and data contributed by the authors of this CSTR 74

Experience indicates that the time of striking formwork is not significantly affected by the use of silica fume by itself.

13. TENSILE STRENGTH

The tensile (flexural) strength of concrete increases with compressive strength, with the tensile strength being typically only about 10% of the compressive strength. Compared with CEM I concrete the ratio of tensile strength tends to be slightly lower for silica fume concrete.

As the compressive strength of silica fume concrete increase, the tensile strength also increases, but at a gradually decreasing ratio, see Goldman. Hooton reports that the splitting-tensile strengths as a percentage of compressive strength of 10% silica fume concrete (w/c = 0.35) ranged from 8.5% to8.9% at ages of 28 to 182 days, whereas similar concrete without silica fume ranged from 9.4% to10.7%.

14. EFFECT ON CONCRETE DURABILITY

PermeabilitySilica fume can produce very large reductions in water permeability of up to one order magnitude ormore, depending on the mix design and dosage of silica fume.

The reduction in the size of capillary pores increases the probability of transforming continuous pores into discontinuous ones, see Philleo. Because capillary porosity is related to permeability, see Powers et al [Ref. 85], the permeability to liquids and vapours is thus reduced by silica fume addition. Silica fume can produce very large reductions in water permeability, up to an order of magnitude or more, see Hooton. Data for mortar and concrete show a similar trend in that silica fume reduces permeability, see for example Scheetz, Grutzek and Strickler, Mehta and Gjørv and Delage and Aitcin. Maage and Maage and Sellevold reported a reduction in permeability of about one order of magnitude for silica fume dosages of 5% to 12%; the most improvement was with the lowest dose that was used with the lowest w/c ratio. Measurement of the water permeability for quality concrete

(40N/mm2) is often impossible because of the measuring equipment limitations and leakage around the permeability cells, see for example Hustad and Loland and Hooton. El-Dieb and Hooton were able to measure a water permeability of 1.9 × 10-16 m/s for a 0.29 w/c concrete with 7% silica fume plus 25% GGBS. The mechanism involved is due primarily to the high pozzolanic reaction linked withimprovement in the interfacial transition zone.

15. PROTECTION TO EMBEDDED STEEL (CARBONATION)

Steel embedded in concrete is protected against corrosion by the alkalinity of the cement paste. Despite any reduction in calcium hydroxide, resulting from the incorporation of silica fume, the pH of the cement paste remains at an adequately high level to protect steel. Carbonation can reduce the alkalinity and protection to the steel. Silica fume concrete tends to show greater carbonation than CEM I mixes of equivalent 28 day strength.

For detailed information on the effect of cementitious materials on the protection to embedded steel against carbonation-induced corrosion, the reader should refer to Concrete Society TR61 “Enhancing reinforced concrete durability” .

For silica fume, Vennesland and Gjørv, Fidjestol found a good correlation between carbonation depth and w/c. Byfors and Skjolsvold showed that if mixes with the same 28-day strength are compared, those containing silica fume carbonate more. Little field data is available for concrete containing silica fume but Skjolsvold has reported on cores drilled from structures and concludes that the mean carbonation depth in the silica fume concrete was appreciably greater than in the CEM I concrete. The variability of depths of carbonation was also higher for the silica fume concretes.

16. PROTECTION TO EMBEDDED STEEL (CHLORIDES)

If chloride permeates the concrete to the depth of the reinforcement it can initiate corrosion of the steel. Concrete made with silica fume is generally substantially more resistant to chloride diffusion than CEM I concrete and for reinforced concrete structures exposed to chlorides, its use will give enhanced durability.

For detailed information on the effect of cementitious materials on the protection to embedded steel against chloride-induced corrosion, the reader should refer to Concrete Society TR61 “Enhancing reinforced concrete durability”.

Detwiler, Fapohunda and Natale used the ASTM C 1202 electrically-accelerated test to compare the chloride-ion transport of concrete with and without 5% silica fume and cured at different temperatures to the same degree of hydration and found that the silica-fume concrete performed better than the comparable control concrete for any given curing temperature. Hooton et al found an increase in chloride penetration resistance for concrete containing 7% silica fume by AASHTO T 259, ASTM C1202, and bulk diffusion tests for both ambient-temperature and 70°C curing. McGrath and Hooton found beneficial effects of silica fume on chloride diffusion coefficients. Whiting and Detwiler studied optimum silica-fume concrete mixtures for bridge decks. They found that the use of silica fume reduced the chloride diffusivity by a factor of 3 or more compared with similar concrete not containing silica fume. Increased dosages of silica fume above 6 to 8% by mass provided little additional benefit. In a field survey of bridge deck overlays in Illinois, Detwiler, Kojundic and Fidjestol found that the depth of penetration of chlorides after eight years of exposure was considerably less for a silica-fume concrete overlay than for a dense concrete overlay without silica fume. Results for ASTM C 1202 testing were consistent with those determined in detailed chloride concentration profiles on companion cores.

Fig 7. Oresund Bridge 15% fly ash and 5% SF for durability in a marine environment

17. SULFATE RESISTANCE

Sulfate attack of concrete occurs through both chemical and physical processes. Two main reactions are involved, these being the reaction of sulfate ions with hydrated calcium aluminates forming ettringite, and the combination of sulfate ions with free calcium hydroxide forming gypsum. The first reaction is of more practical significance. Considerable increases in volume result from both reactions causing expansion and disruption of the hardened concrete. More recently, a second form of sulfate attack, called thaumasite attack has been recognised as a problem after the discovery of its effects on the foundations of some motorway bridges in the UK. Thaumasite is a calcium silicate sulfo-carbonate hydrate, which forms at temperatures below 15°C by a reaction between cement paste hydrates, carbonate and sulfate ions. Its formation reduces the cement paste to a soft mulch. Unfortunately, conventional Sulfate Resisting Portland Cement offers no protection against the Thaumasite form of sulfate attack. GGBS, fly ash and silica fume can substantially increase the resistance to both forms of sulfate attack compared with CEM I concrete.

Detailed recommendations for avoiding sulfate attack using GGBS and fly ash can be found in Building Research Establishment: Special Digest 1:2005, Concrete in aggressive ground and the recommendations of this Digest have been adopted by BS 8500:2006. These documents do not provide any guidance for utilising the increase in sulfate resistance from incorporating silica fume. For limestone fines concretes, the recommendations for limestone fines concretes are generally more onerous than for CEM I.

Silica fume is very effective in reducing or preventing the attack from sodium sulfate, see for example Carlsson et al., Mather (Ref, Mehta, Hooton, Cohen and Bentur and Sellevold and Nilsen). The primary mechanism by which silica fume improves sodium sulfate resistance is by reduction of permeability, see Khatri, and it may be augmented by reduced calcium hydroxide contents due to pozzolanic reaction. Performance of silica-fume concrete exposed to ammonium sulfate is mixed Popovic et al. Performance when exposed to magnesium sulfate is less than that of paste without silica fume Cohen and Bentur.

18. RESISTANCE TO ACIDS

All types of cement are susceptible to attack by acids and, in highly acid solutions (e.g. pH less than3.5), dissolution of the cement matrix and subsequent loss of integrity of concrete will occur. Some moorland waters with low hardness, containing dissolved carbon dioxide and with pH values in the

range 4–7, may also be very aggressive to concrete, particularly with continuously flowing water. In general, in such aggressive conditions, the quality of the concrete has been considered to be of greater importance than the type of cementitious material.

Luther suggests that silica fume will increase the resistance of concrete to dilute acids and chemical attack through reduced permeability and through reduced content of calcium hydroxide, see. Silica- fume concrete is not completely impervious to all aggressive chemicals, especially in the case of concentrated acid attack on the surface; however, research and field performance show that at a low w/c, silica-fume concrete can be used effectively to prevent significant damage by many types of chemical attack including sewage and silica fume concrete has been specified for use in sewer and outfall pipes in many countries, see for example Shanghai Institute of Building Science report.

19. FREEZE-THAW RESISTANCE (FOR COLD WEATHER CLIMATES)

Concrete, which is saturated with water, can be damaged by repeated cycles of freezing and thawing, and the use of de-icing salts greatly exacerbates the likelihood of attack. Unfortunately, there is no reliable laboratory test for establishing freeze-thaw resistance and concrete that is known to have reliable freeze-thaw resistance under real-life conditions, can often fail when tested by currently available laboratory test methods, see Muller. Freeze-thaw damage usually shows up as scaling of the surface, exposing the underlying coarse aggregate. Concretes that may be exposed to freezing and thawing while saturated, need to be suitably proportioned and the concrete well cured after placing see Gunter et al and Luther et al. Air entrainment of the concrete will significantly improve the resistance of concrete to freeze-thaw damage. Silica fume concrete should be air entrained if exposed to freezing-and-thawing conditions.

Numerous investigators have shown that it is possible to produce freeze thaw resistant, air-entrained concrete containing silica fume (see Aïtcin and Vezina; Caldarone et al; Cohen and Olek; Malhotra; Sonebi and Khayat. In these studies satisfactory freeze thaw durability factors were obtained at w/c ratios up to 0.60. Yamato et al and Malhotra et al suggest that, for freeze thaw resistance, the silica fume content should be limited to a maximum of 15%. Saucier et al conclude that it is vital to check admixture compatibility and to perform tests with the actual materials to be used in order to ensure that the air-entraining admixture is effective and that a stable air-void system can be produced. ACI234R-2006 concludes that the data for silica-fume concrete without entrained air is conflicting and recommends that silica fume concrete should be air entrained where adequate resistance to freezing- and-thawing conditions is required.

20. ALKALI-SILICA REACTION

Alkali-silica reaction (ASR) is a reaction between the hydroxyl ions in the pore water within concrete and certain forms of silica, which occur as part of some aggregates. The product of the alkali-silica reaction is a gel which imbibes pore fluid and expands; in some instances this expansion induces internal stress in the concrete of such magnitude that extensive macro-cracking of the concrete occurs. The damage occurs in parts of the concrete structure exposed to moisture. Silica fume can reduce the risk of damage due to ASR.

The use of silica fume in sufficient quantity and properly dispersed in concrete, either on its own or in combination with other pozzolans or GGBS, can be an effective means of combating ASR, as evidenced by decades of field experience in addition to laboratory tests. Gudmundsson and Olafsson report the effect of silica fume on the expansion of materials from Iceland, where the entire production of cement contains 7–10% silica fume in order to reduce the ASR caused by a combination of highly reactive natural aggregates and the local high alkali cement.

21. ABRASION RESISTANCE

Numerous factors influence the abrasion resistance of floor slabs, the most significant being curing, strength class and trowelling intensity. Curing appears to be more important for abrasion resistance than changes in constituent materials. For example, the difference between immediate exposure and seven days covered with polythene can be as much as an order of magnitude in terms of abrasion resistance. When a good standard of curing is applied, e.g. wet curing for seven days or the application of a 90% efficiency resin compound. Silica fume can increase abrasion resistance through increased strength.

Silica fume will increase the abrasion-erosion and abrasion resistance of concrete through increased strength of the matrix and the improved bond between matrix and aggregates. The excellent resistance of silica fume concrete to abrasion erosion damage was reported by Holland and McDonald based on investigations performed at the U.S. Army Corps Engineers Waterways Experiment Station (WES). The abrasion-resistance test method used was developed by the WES (see Liu, which is now ASTM C 1138. This method uses steel balls in water stirred by paddles at 1200 rpm to simulate the abrasive action of waterborne particles such as silt, sand, gravel, and boulders. In the UK proprietary special concretes have been developed for specific applications such as waste transfer stations, sea defence construction and hard wearing floors. Applications of silica fume concrete in dams has proven beneficial to increase service life and thin surface applications containing silica fume have been developed for industrial floors and highway repairs.

Fig 8. Otterburn Army Training Estate: 10% silica fume was used in concrete hardstandings to increase the resistance to abrasion by tracked vehicles

22. RESISTANCE TO FIRE

There is no evidence to suggest that the type of cementitious material will have a large effect on the resistance to fire. In some cases of extremely low permeability concrete, explosive spalling has been reported

A number of researchers have demonstrated that the fire performance of silica-fume concrete is little different from that of conventional concrete, see for example Jensen and Aarup and Dumuolin and Behloul. Properties such as thermal conductivity and specific heat do not change significantly, and there is evidence that properties during the fire and residual properties are actually better for silica- fume concrete. Research by Phan and Carino indicates that for high-strength concrete, the relative amount of residual strength may be less than that in conventional-strength concrete. This is in part due to the reduced content of calcium hydroxide in concrete with pozzolans, which means that less water is released at high temperatures, see Chan, Peng and Chan, Chan, Peng and Anson and Saad et al.

In some cases of extremely low permeability, silica fume concrete, explosive spalling has been reported. While silica fume content as such is not the governing parameter, the low permeability often associated with a low w/c and silica fume may require preventative measures. Section 6.2, Spalling, in Part 1-2 of Eurocode 2 (General rules – Structural fire design) suggests that no special precautions need be taken for concretes up to C80/95 provided that the silica fume content is below 6% by weight of cement. For higher silica fume contents, and for concrete grades above C80/95, variousprecautionary measures are recommended including the provision of at least 2kg/m3 of monofilament polypropylene fibres.

23. EFFECT ON CONCRETE’S PHYSICAL PROPERTIES

ColourAlthough the curing time and formwork type can have some effect, the colour of concrete is principally determined by the colour of the cementitious material. Although ‘white cements’ are available at a price, CEM I is normally a shade of grey. GGBS is an off white powder and its use, particularly at high levels, lightens the colour of concrete. Fly ash (particularly with higher carbon contents) is generally darker than CEM I and the use of this or silica fume tend to darken the colour. Portland limestone cements or combinations containing limestone fines are normally slightly lighter than the corresponding CEM I.Most silica fumes range from light to dark grey. Because SiO2 is colourless, the colour is determined by the non-silica components, which typically include carbon and iron oxide. In general, the higher the carbon content, the darker the silica fume. The carbon content of silica fume is affected by many factors relating to the manufacturing process, such as: use of wood chips versus coal, wood chip composition, furnace temperature, furnace exhaust temperature, and the type of product (metal alloy) being produced. Almost white coloured silica fume is available for use in architectural concrete.

Elastic ModulusGGBS, fly ash or silica fume usually increase the ultimate modulus, but the magnitude of the increase is generally not significant in terms of design. Limestone fines has little effect.

Sellevold et al found that the dynamic modulus of elasticity increases with increasing silica-fume content in pastes. Helland, Hoff and Einstabland concluded that the stress-strain behaviour of silica- fume concrete was similar to that of concrete without silica fume. Several other researchers have reported that the static modulus of elasticity of silica-fume concrete is apparently similar to that of concrete without silica fume of similar strength, see for example Luther and Hansen; Løland and Hooton. However, Burg and Ös reported that concrete incorporating 15% silica fume as an addition had a higher modulus of elasticity than the control concrete without silica fume, regardless of curingconditions. Wolsiefer reported a modulus of elasticity of 43.1kN/mm2 and a Poisson’s ratio of 0.21 for a 98N/mm2 compressive strength silica-fume concrete. Saucie studied five silica-fume concretes and found Poisson’s ratios ranging in value from 0.208 for 92N/mm2 concrete to 0.256 for 113N/mm2

concrete. Iravani obtained Poisson’s ratios ranging from 0.16 to 0.20, including 0.18 for a 105N/mm2

strength, 31.7kN/mm2 modulus silica-fume concrete, and 0.19 for a 120N/mm2 strength, 37.1kN/mm2

modulus silica-fume concrete cured for 3 weeks at 100% relative humidity and then for 5 weeks at50% relative humidity. The range of these values for Poisson’s ratio (0.16 to 0.256) are similar to what would be expected for CEM I concrete.

CreepUnder conditions of no moisture loss, lower creep values will be found when using GGBS or fly ash in concrete in comparison to CEM I concrete of a similar strength class. This is generally associated with the greater strength gain of the GGBS or fly ash concretes during the period under load. Such conditions are likely with concrete which is remote from the cover zone, particularly in large sections. In many other practical situations (i.e. beams, columns, slabs), where there is significant long-term drying, the strength gain may be negligible and the creep characteristics of the different types of concrete will be similar. The creep of silica-fume concrete should be no higher than that of concrete of equal strength class without silica fume. Where the load is applied at an age less than 28-days, the lower early-strengths of GGBS or fly ash concretes may result in increased creep, see Bamforth et al.

Limited published data and the different nature of the creep tests used by various investigators make it difficult to draw specific conclusions on the effect of silica fume on the creep of concrete. Wolsiefer examined concretes loaded from both 12 hours and 28 days up to 4 months. He found that the silica fume concretes exhibit less creep than would have been expected from control concrete of equivalent strength. Penttala studied high strength concretes with 10% silica fume addition and found that the creep was 20% less than theoretical predictions. Tomaszewicz concluded that the creep coefficient of a C80 silica fume concrete was lower than a control C30 concrete.

Tensile Strain CapacityConcrete Society Digest 2, Mass concrete suggests that concrete containing GGBS or fly ash might exhibit marginally more brittle failure characteristics, with the tensile strain capacity being slightly

lower than for a similar strength class CEM I concrete. However data on the effects of cementitious materials on the tensile strain capacity of concrete is very limited.

Drying ShrinkageBecause of the differing conditions under which tests have been carried out, it is difficult to make direct comparisons between the effects of different cementitious materials.

Long-term total shrinkage of silica-fume concrete is comparable to that of concrete without silica fume of otherwise similar composition. Jahren found that shrinkage was of the same order as that of control concretes with emphasis on good curing. Drying shrinkage data on concrete containing 20% silica fume have been published by Wolsiefer, who found that shrinkage values for the silica fume specimens were 24% lower than those of similar but lower strength concrete made without silica fume. Khatri and Sirivivatnanon reported that the incorporation of 10% silica fume as replacement for CEM I reduced the long-term drying shrinkage of the concrete after 28 days; however, it increased the early-age shrinkage after an initial curing of 7 days in lime water.

Autogenous ShrinkageAutogenous shrinkage occurs under sealed conditions (no moisture loss), because of chemical shrinkage (volume reduction due to the hydration reaction) and self-dessication (internal consumption of water by the hydration reaction). It is generally only considered to occur in concretes with w/c less than 0.45, see CIRIA C660 and much of the published data on autogenous shrinkage is for concrete with silica fume because it is typically used in low w/c, high-strength concretes. On the basis of limited evidence, it has been suggested that autogenous shrinkage is reduced in direct proportion to the percentage of fly ash in the mix, is increased by 8% for every 10% of ggbs present, and is increased by 10% for every 1% of silica fume included.

Kanstad et al. measured the autogenous shrinkage for concrete with and without silica fume and incorporated their results into a mathematical model to predict the cracking behaviour. They found that the autogenous shrinkage of silica-fume concrete within the first 2 or 3 days is greater than for concrete without silica fume, consistent with the findings of Springenschmid, Breitenbücher and Mangold. When the other factors contributing to cracking were taken into account, however, such as development of tensile strength, they found that silica-fume concrete is no more likely to crack than concrete without silica fume. Whiting and Detwiler and Whiting, Detwiler and Lagergren studied the effects of w/c and silica fume content on the tendency of concrete to crack under conditions of full restraint and found that the concrete’s tendency to crack was highly sensitive to the length of the curing period. For concrete that was moist cured for only one day, the tendency to crack increased with the silica-fume content. For concrete that was moist cured for seven days, there was no relationship between silica fume content and tendency to crack. They recommended that specifications for silica-fume concrete bridge decks require 7-day, continuous moist curing.

Coefficient Of Thermal ExpansionIt is well established that the coefficient of thermal expansion of concrete is largely influenced by the coarse aggregate used. Thus the type of cementitious material will have little influence on this property.

Thermal Conductivity And DiffusivityNo published data are available on the influences of the thermal conductivity and diffusivity properties of concrete. However, as with coefficient of thermal expansion, these properties are influenced mainly by the aggregate type. The thermal conductivity of fly ash is similar to that of quartz sand, e.g. 0.8 to0.6 w/K.m, and as a result is likely not have any significant effect on the resulting concrete. Limited measurements by Tolloczko on concretes containing 0, 50 and 70% GGBS showed no significant differences between the concretes.

Thermal MassThe cementitious material will have only a marginal effect on the thermal mass of concrete and other factors such as aggregate-type, w/c and aggregate/cement ratio will have much more effect.

24. EFFECT ON ENVIRONMENTAL IMPACTS

GeneralInternationally, there exist numerous detailed life cycle analyses for CEM I (see e.g. Nisbet, Josa and CEMBUREAU, GGBS (see e.g. BRE and Marcea) and fly ash (see e.g). Detailed life cycle analyses for silica fume and limestone fines are more difficult to find.

In these life cycle analyses, by-products such as blastfurnace slag, fly ash and silica fume are often not assigned any of the burdens of the primary process that produce them but are sometimes allocated a proportion of the primary burdens on a basis such as economic value; see BS EN ISO14044. These materials do however, fully bear the environmental burdens associated with any further processing, transport, etc. that is necessary to make them suitable for use in concrete.

Significant reductions in environmental impacts occur for GGBS, fly ash and silica fume, because:

- their manufacture does not require the quarrying of virgin minerals- their use in concrete avoids them being disposed of to landfill- they use much less energy in their manufacture than CEM I.

The last point also applies for limestone fines. There may also be some effect on other environmental indicators such as water-use or acidification, but it is generally accepted that the most important reduction is in carbon dioxide emissions, which will be dealt with in detail in the following section.

It should also be noted that there might be indirect effects. Where their use enhances durability, this can lead to longer service life and reduced lifetime impact. The higher strengths achievable with silica fume concrete may enable reductions in the overall volume of concrete coupled with less volume of high CO2 CEM1 thus reducing overall emissions ; see Chan, et al.

Embodied Carbon Dioxide For Cementitious MaterialsThe CEM I component of the mix is responsible for the majority of the embodied carbon content of concrete. Based on UK information, indicative embodied CO2 (ECO2) values for the main constituents of concrete are provided in Table 3. The figures for CEM I, GGBS and fly ash are derived using data for the calendar year 2007 (BCA) and are ‘cradle-to-factory-gate’ values, which do not include transport from place of manufacture to concrete plants. The CO2 emissions associated with the mixing, transfer and storage of silica fume slurry, from the production site in Norway to the port in the UK (Elkem/Enviros) are 14 kg per tonne of slurry, which equates to 28 kg per tonne of silica fume powder.

Table 3: Embodied carbon contents for main constituents of reinforced concrete

Material Embodied CO2(kg / tonne)

Portland cement, CEM I 930

Addition or cement constituent

GGBS 52Fly ash 4

Limestone fines 32Silica fume powder 28Minor additional constituent 32

Aggregate 4Reinforcement 427

Embodied Carbon Dioxide For ConcreteCEM I (Portland cement) is the most energy intensive constituent of concrete and the overall embodied CO2 content can be significantly reduced through the use of additions or composite cements, see Figure 27. In assessing the options for reducing the embodied CO2, it is necessary to take into account, see Concrete Industry Alliance report:

the replacement levelany increase or decrease in cementitious content to achieve the required strength transport distances

The Concrete Centre has prepared a table to help specifiers to calculate the ECO2 content of their particular chosen mix design. The information will be updated and specifiers should refer to it on the web to gain the latest information available.

25. SPECIAL APPLICATIONS

Ternary BlendsTernary cements are blends of CEM I with two additions. The three cementitious materials can be introduced separately into the concrete mixer but it is also possible to produce a ternary blend by combining an addition with a composite cement in the concrete mixer. In Holland, fly ash is used with Blastfurnace cement and in Ireland, GGBS is used with Portland Limestone cements, under the provisions of the Irish National Annex to EN 206, see NSAI. Currently, BS 8500 does not contain any guidance for the use of ternary blends but some guidance can be found in the Concrete Society Guide for the design of concrete structures in the Arabian Peninsula.

Dhir et al measured strength development, permeation properties, carbonation rates, heat of hydration, Ca(OH)2 content and porosity on ternary blends, based on GGBS, fly ash, silica fume, limestone fines and metakaolin. Their report provides a broad database, on how cement and additions can be combined and the likely impact on concrete properties. They found that although the properties of ternary blend concretes showed some differences, they were broadly similar to those of the corresponding binary CEM I/GGBS and CEM I/fly ash blends.

Silica fume has frequently been combined with Portland cement and either fly ash or GGBS to produce ternary blends, such as that used in the Hong Kong, Tsing Ma Bridge as shown in. In ternary blends, the amount of silica fume is typically somewhat less than would be used without the fly ash or GGBS. Bleszynski, et al reported that ternary blend concrete exhibits greater overall durability performance than CEM I concrete and binary blends, with ternary blends giving better placing and finishing characteristics than binary silica fume blends. The paper also concludes that the incorporation of GGBS into silica fume concrete reduces the water demand (or water reducing admixture dose); the combination of silica fume and GGBS offers increased resistance to ASR expansion and chloride ingress than the use of one of these materials alone. Broadly similar effects were found by Leung and Wong/ Several investigators have explored the possibility of using silica fume in combination with fly ash, to compensate for the slower strength development associated with the fly ash. Mehta and Gjørv and Carette and Malhotra found that a ternary blend with silica fume could offset the reduction of early strength caused by the use of fly ash on its own. Thomas et al found that a fly ash/silica fume ternary cement had many advantages over CEM I. These advantages include increased resistance to chloride ion penetration, alkali-silica reaction, and sulfate attack and also improved placing and finishing characteristics, improved long-term properties, and reduced cost compared with Portland silica fume cement. Ternary blends, however, may render concrete paving more vulnerable to freeze-thaw scaling if the concrete is not proportioned and placed properly.

Fig 9. Hong Kong, Tsing Ma Bridge 65%GGBS and 5%SF for durability in marine exposure

Thomas et al investigated ternary and quaternary blends produced from CEM I, Portland limestone cement and a ‘SCM’ (a blend of 2 part of GGBS with 1 part of fly ash). They measured strength, chloride permeability and freeze-thaw resistance using laboratory tests and also cast external slabs to assess freeze-thaw susceptibility. The trends in performance were similar to what would be expected from binary blends at equivalent levels of addition.

Fig 10. Rapid Chloride Penetration Tests on Grade 35 concretes [Ref 251]

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