Con.wuction and Building Materials, Vol. 10, No. 5, pp. 309-314, 1996 Copyright 8 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0950-0618/96 $15.00+0.00

095%0618(95)MO14-3

Benefits of slag and fly ash

Jan Bijen

DeliI University of Technology, Faculty of Civil Engineering and Intron, Institute for Materials and Environmental Research, 2600 GA Delft, The Netherlands

Received 24 January 1995

The use of ground granulated blast furnace slag and powder coal fly ash as an addition to either cement or concrete is well-established. Concrete made with these secondary raw materials as a part of the binder does show distinctive advantages over concrete with Portland cement only. Especially, the performances with respect to chloride-initiated corrosion of rebars, alkali-silica reaction and sulphate attack are substantially improved. These improvements and their causes are discussed. Copyright Q 1996 Elsevier Science Ltd.

Keywords: concrete; slag; fly ash

Ground granulated blast furnace slag has been used as a cementitious agent for concrete for more than a century. Blast furnace slag cement, a composition of ground slag, Portland clinker and gypsum/anhydrite, was introduced on the market in Germany in 1888. The history of powder coal/fly ash (bituminous coal) is younger: combustion of powdered coal was introduced after the second world war.

Nowadays the use of slag and fly ash in concrete, either as a constituent of cement or as an addition (mineral admixture), is widespread. In some countries blast furnace slag cement has the main share of the market. In the Netherlands for instance blast furnace slag cement with 65 to 70% of slag has a market share of about 60%. It is applied to almost all kinds of concrete structures, while it is the exclusive material for structures in marine environments.

Evidently the use of slag and fly ash offers benefits with respect to the costs of manufacturing of concrete, because these raw materials are produced as by-prod- ucts or waste materials and can replace purpose-made Portland clinker. The same applies for the effects on the environment of cement and concrete products. Fewer primary energy and raw materials are required in producing concrete, while the durability of structures is improved. In a cradle-to-grave life cycle analysis, concrete products with slag or fly ash will score favourably in comparison with Portland cement concrete.

This paper does not cover these benefits, but rather concentrates on benefits with respect to concrete prop- erties, mainly durability. It starts with a brief discussion of the effects of slag and fly ash on the chemistry/miner- alogy and the microstructure of concrete. Subsequently the beneficial influence on permeability/diffusivity, chlo- ride-initiated corrosion, chemical attack and alkali silica reaction is discussed. Finally some disadvantages of the use of slag and fly ash are considered in order to give the reader not only the pros, but also the cons.

Chemicalhineralogical effects

Both in slag and fly ash the amorphous glass is the active part. The glass in blast furnace consists of mono- silicates like those in Portland clinker, Q” type. The fly ash glass consists of cross-linked silica-tetrahedra, Q4 type, see Figure 1, for coordination. When activated the blast furnace slag dissolves.

In fly ash the Si-0-Si links have to be broken. Fly ash does not dissolve but decomposes. The decomposed remains of the fly ash react with lime, which is gener- ated by the Portland clinker hydration, and water to form calcium silicate hydrates. Slag only has to be acti- vated (latently hydraulic), while fly ash in addition needs lime (pozzolanic). The activation of slag occurs at relatively low pH’s, e.g. less than 12, while at 2O’C fly ash needs a pH of more than 13, see Figure 2.

Because the alkalinity of Portland cement at 20-C develops to a level higher than 13 only with time, fly ash activation shows an incubation time, see Figure 3. The Portland clinker hydration generates lime, while fly ash consumes it.

In general the pore water is somewhat less alkaline if slag or fly ash is applied. This is not because the total amount of alkaline constituents is less, but because more sodium and potassium are bound in the cementi- tious phase. Figure 4 shows an example of the hydroxyl concentration development in time.

si 0- 0-

si 0

-OSiO- -oyiosi - SioiioSi SiosioSi SiosioSi

0- 0- 0- 0- 0 Si

d Q’ Q2 Q’ Q4

wm

Figure 1 Tetrahedral coordination of Si atoms

309

310 Benefits of slag and fly ash: J. Bijen

‘I-

-1 *Silica fume NaOH (20°C) + Silica fume NaOH (40°C)

2 + Fly ash NaOH (20°C) t Fly ash NaOH (40°C)

zl f

PH Pore diameter (pm)

Figure 2 Effect of pH and temperature on the concentration of dissolved silicium in NaOH solution for fly ash and silica fume2

Figure 5 Pore size distribution of cement paste, with Portland cement CEM I 42.5 with and without fly ash after one year of hard- ening at 20-C w/(c+pfa) = 0.45 (6))

II- Days I

I ’ 0 - *‘kc. (w/c = 0.45)

3 5 7 142’ 28

r]%aw.d

a^ + 20% (m/m) class F fly

9 - t20% (m/m) fine sand 9 8 _ g

w&c + pfa) = 0.45

2 7-

d 6-

+ 5

13.67

‘3.6’

‘3.55

13.41

13.37 I., .,=

‘g 4-

I J.‘, ‘3.07

12.77 ‘0 100 IO00

Hours

Figure 3 Development of the OH concentration in the pore water of a Portland cement paste (CEM 1142.5) with fly ash and fine quartz sand. Temperature 2o’C, w/(c+pfa) = 0.453

‘0 ‘00 loo0 Hours

Figure 4 The OH concentration as a function of the type of cement (p.c. = Portland cement CEM I/42.5, pbfc = Portland blast furnace slag cement CEM III/B 42.5 LH HS, pfa = class F fly ash, w/(pc+pfa) = 0.45’

Because of the lower alkalinity of the pore water and the decreased availability of free lime the combined use of slag and fly ash or other pozzolans is not simply an accumulation of the benefits of both.

Microstructure

The pore size distribution of concrete with ground gran- ulated slag or fly ash is substantially finer than of pure Portland cement concrete. Figure 5 shows the pore size distribution for a Portland cement paste with and

300

^M 240 G?

1 :%;I$) class F fly ash

E I80

i? ! ‘4

wl(c + pfa) = 0.45

ref. p.c.-A . . ‘\ \

ref. -‘; ‘\\

IO r -) OPC, 7 davs

. 1 I I I I I

0 I

20 40 60 80 ‘00 120

Distance from interface (km)

Figure 6 Degree of orientation of CH at the paste-granite interface for Portland cement CEM I/42.5 and Portland blast furnace slag cement CEM III/B 42.5 LH HS (age: 7 days)4

without fly ash as measured with mercury porosimetry. Initially the pore size distribution is more coarse, but later the opposite is the case. For blast furnace slag cement the pore size distribution is finer from the very beginning. The percentage of gel pores is higher, that of capillary power lower. Total porosity is not influenced substantially.

The additions have a major effect on the transition zone between aggregate and cementitious matrix or between reinforcement and matrix. This is illustrated in Figure 6 for blast furnace slag cement, where the so- called lime orientation index is shown as a function of distance from the interface. The porous interfacial zone can extend up to 40 urn into the matrix and cover 20 to 30% of the volume of the paste. When fly ash or slag is applied the thickness of the zone can be greatly reduced. Likely causes for this reduction are the presence of finer solid material, better particle packing leading to less segregation and possibly less syneresis of the cement gel. Obviously also less lime is available to crystallize at the interface where favourable conditions for precipitation prevail.

Permeabilityhliffusivity

The substantial effect of slag and fly ash on the pore structure is not reflected so much in the permeability, but rather in the diffusivity of ions.

Benefits of slag and fly ash: J. Bijen 311

Table 1 Permeability coefficient at different pressure gradients for NaCl solution at 20’C5

Permeability coefficient ( 10-i2 m.s-i) Pressure gradient (MPa) OPC-0.40 OPC-OSO PBFSC-0.40 PBFSC-0.50

I.0 3.1 6.4

2.5 1.6 3.5 5.0 2.4 3.2 1.5 1.6 4.3 9.5 1.3 4.4

OPC =CEM I 32.5 (ENV 197 Cement) PBFSC =CEM III B/42.5 LH HS

2.9 _ 2.6 5.9 3.6 6.5 4.4 4.1 3.5 3.9

Table 2 Typical diffusivities of chloride ions in concrete at 20-C for respectively IV/C = 0.4 and n/c = 0.55 after 6 months of (wet) harden- ing in IO-l2 m* s-i

Portland blast furnace Portland fly ash w/c Portland cement slag cement” cementb

0.4 4 0.1 0.2 0.55 10 0.3 0.5

U 70% slag CEM III B/42.5 LH HS b 25% fly ash CEM 11

The effect on the permeability of slag cement concrete relative to Portland cement concrete is shown in Table 1, while Table 2 illustrates the large effects on chloride diffusivity. This large effect is mainly caused by the ‘densification’ of the matrix; the reduced thickness of the interfacial zone has only a limited effect4 in the range of say a factor of 2. The reduced diffusivity is also reflected in other related properties, such as the electri- cal resistivity, as shown in Figure 7.

The effects of slag and fly ash are strongly dependent on the amount of additions applied. Figure 8 shows the effect of the slag content on chloride diffusivity. Although this paper discusses beneficial effects in general terms, it must be borne in mind that substantial effects are mostly only present at higher addition percentages.

Protection of steel reinforcement

The reduction in diffusivity has a major beneficial effect with respect to the protection of reinforcement against

1 + OPC, 25% ily ash

I A OR v opt

I I I IO 100 1000

Time (days)

Figure 7 Ohmic electrical resistance of concrete with and without fly ash6. OPC = CEM Y42.5, OPBFSC = CEM III/B 42.5 LH HS

‘rOPC=CEMl42,5 OPBFSC = CEM III IB 42.5

3

Slag content (96 m/m)

Figure 8 Dependence of diffusion coefficient of chloride and sodium ions on the slag content of Portland blast furnace slag cement paste (water/cement ratio 0.60, temperature 21-C) according to Brodersen’. * Percentage by mass

440 r 400.

z 360.

fj 320.

,x 280.

,g 240.

2 200.

.s 160.

:z 120.

I5 80.

40

0

I: DC = 4 5 x IO-” m2/s - II: DC = 210 x IO-‘* ml/s

Ill

- 111: DC = 0.2 x 1O-‘2 m*ls

/

/ _ / _ _ / _

/f, II

I 6, 20 40 60 80 100 120 140

Cover (mm)

Figure 9 Illustration of corrosion initiation time due to chloride ions at 35-C in concrete produced with respectively sulphate-resisting Portland cement (type V according to ASTM), ordinary Portland cement (type I) and Portland blast furnace slag cement with 65% mm-i slag CEM III/B 42.5 LH HS assuming: penetration according to Fick’s second law; effective diffusion coefficients: I DC sulphate resisting = 4.5 X IO-i2 cm2 s-i; II DC ordinary Portland = 2.0 X lo-l2 cm2 s-i; III DC Portland blast furnace slag = 0.2 X IO-i2 cm2 s-i; corro- sion initiation threshold chloride concentration 0.3% on cement mass8

chloride-initiated corrosion. The rate of penetration of chloride into concrete is greatly reduced. Figure 9 shows a prediction made in 1982 for the Saudi Arabia-Bahrain Causeway (now King Fahad Causeway) with respect to the initiation of corrosion, assuming a critical corrosion concentration of 0.3% by mass of cement. Although the prediction was based on diffusion only and did not take into account salt water absorption it clearly demonstrates the enormous effect of the slag cement. The 12 km bridges of this causeway were built with Portland blast furnace slag cement, CEM III/B 42.5 LH HS. Figure 10 shows a chloride profile of a test pile 6, 7 and 124 years respectively after erection in the Gulf. This pile was uncoated, i.e. the concrete was fully exposed. By means of a computer simulation, taking into account both the ingress of chlo- ride due to water absorption (convection) and chloride diffusion, a chloride diffusivity of 0.1 X 1 O-12 m2 s-1 was calculated. Reality appears to confirm the predictions fully, certainly if we take into account that the first 10 mm is subject to (salt) water absorption and drying and more permeable than the bulk of the concretes. Nine years after completion, the concrete structures of the

312 Benefits of slag and fly ash: J. Bgen

5 w core ( 1994) =‘

i4

+ core (1988; ref. 2) * Core (1988; ref. 2)

s + Core ( 1989; ref. 3)

g3 E 2 -2 d

0 IO 20 30 40 50 60 70 80 90 100

Average depth (mm)

Figure 10 Chloride profile of fully exposed test pile respectively 6, 7 and 12.5 years after positioning in the Gulf

King Fahad Causeway appear to be in an excellent condition with no sign of reinforcement corrosion.

The effects of slag and fly ash on chloride penetration are much greater than the most well-known concrete quality parameter, the water/cement ratio. Going from a water/cement ratio of 0.55 to 0.4 the diffusivity of chloride ions changes by a factor of 2 to 3. However, as shown in Table 2, the cement type can make a differ- ence of a factor of 25.

The effect of slag and fly ash is not limited to the initiation of corrosion by chlorides, it also increases the critical corrosion concentration beyond which unduly high corrosion progress occurs. Figure I1 shows the macrocell current measured for concretes with different chloride contentslo. With the blast furnace slag cement the critical corrosion concentration could be as high as 2% of mass cement. This is related to the high electrical resistivity and the increase in anodic polarization re- sistance”. This means with respect to the time to corro- sion initiation a substantial improvement on top of the slower ingress of chloride discussed above.

It is sometimes argued that due to the lower lime buffer in concretes with slag or fly ash, especially at the interface with steel, protection of steel at a certain chlo- ride concentration is less. The results of laboratory investigations as well as the performance of structures in practice do not support that opinion. A study of

“; 1.6

2

g 1.2

‘g I.0

;

g 1.4 i

0.8

0.6

n Portland cement w/c = 0.65 + Portland cement w/c = 0.45 A Blast furnace cement w/c = 0.65 l Blast furnace cemenl w/c = 0.45

~icJ J z 0 0.5 I I.5 2 2.5 3 3.5

Chloride content (96 m/m on cement)

Figure 11 Macrocell current density as a factor of chloride contentlo: Portland cement = CEM I/42.5; blast furnace cement = CEM III/B 42.5 LH HS

Dutch marine structures built with blast furnace slag cement, some of them older than 50 years, proves that theory, laboratory evidence and practice are in line with each other’*. In fact the present performance of the King Fahad Causeway proves that with these type of cements or additions durable reinforced concrete struc- tures can be built with nothing else to protect the steel than concrete, even in the most aggressive natural environmentsg.

Doubts have been raised against the use of slag and fly ash in prestressed and post-tensioned concrete. Blast furnace slags contain sulphides which should catalyse the development of hydrogen atoms in the case of corrosion of prestressed steel in concrete which provokes hydrogen embrittlement. Tests and practical experience over 50 years have shown this concern is not justified and slag cement can be used freely.

Fly ash contains carbon particles. Carbon is more noble than steel and it was suggested that this endan- gers service life. Tests have shown that for concrete in which the high pH is maintained negative effects of the coke particles are not present even at high concentra- tions of carbon. When the pH is decreased due to carbonation a negative effect of fly ash cannot be excluded if combined with blast furnace slag cementl3.

On the basis of these experiments it is allowed in the Netherlands to apply fly ash in Portland cement concrete for prestressed concrete, but not in combina- tion with blast furnace slag cement. From practice in countries where fly ash is used in prestressed and post- tensioned concrete no detrimental effects of fly ash on stress corrosion of prestressed steel have been reported.

Chemical attack

In general, concretes with slag or fly ash are more re- sistant to chemical attack than concretes without these additions.

Agents dissolving or decomposing the binder

Exposed to weak acids, such as aggressive carbon- dioxide-containing water, concrete with slag or fly ash

Time (years)

Figure 12 Surface deterioration of concrete as a function of time upon immersion in strongly aggressive carbon dioxide-containing water of 100 gl 1 of lime solving CO,: PC = Portland cement; TrC = Trass Portland cement; HOC = Portland blast furnace slag cement

Benefits of slag and fly ash: J. Bijen 313

deteriorates less, as shown in Figure 22. Obviously this is due to the decrease in free lime and the increase in gel content. For strongly acid environments, as can be present in sewer systems (biogenic sulphuric acid attack) the difference with Portland cement concrete may be small.

Swelling reactions

In environments where sulphate attack is likely, blast furnace slag cements are prescribed in the Netherlands. Fly ash can have a similar effect, but in a few cases no beneficial effects have been observed. In the case of addition of fly ash to sulphate resisting Portland cement more swelling is sometimes observed“+, although resis- tance remains high enough for application in sulphate- containing soils. Causes for the improved resistance mentioned in the literature are the strongly decreased mobility of ions e.g. sulphatesi5, the reduced availability of C,A and the decrease in the thickness of the inter- facial zone. The interfacial zone is known as an area where expansive ettringite is formed. It is not that no ettringite is formed, but there is less swelling. Destruc- tive swelling due to secondary ettringite formation, which is stimulated strongly at increasing curing tem- perature, can be prevented by using slag and/or fly ashl6.

For another swelling reaction, alkali silica reaction, slag and fly ash have a similar effect as for sulphate attack. Causes mentioned in literature are:

l reduced mobility of ions; l lower alkalinity; l lower free lime content notably in the vicinity of the

reactive aggregate; l lower thickness of the interfacial zone.

In a number of countries slag or fly ash is recom- mended or prescribed if aggregates of which alkali- aggregate reaction is suspected, are applied; see e.g. Table 3’7.

Disadvantages

The addition of slag or fly ash also has some disadvan- tages. In general these can be reduced or counteracted fully by measures well known in concrete technology.

Curing sensitivity

The hydration of concretes with slag or fly ash is slower than that of Portland cement concrete of the same 28 day strength. Therefore the curing time must be

Table 3 Recommendations concerning the type of cement to he used to minimalize damage due to alkali-silica reaction in concrete struc- tures (German Recommendations”)

Cement type Alkali content

% (m mm’) Na,O-equivalent Slag content

% m m-l

Portland cement Blast furnace slag cement

10.60 _ 11.10 250 (50-65) 12.0 265

extended relative to Portland cement concrete. If this is taken into account and curing is ceased at the same maturity as for the corresponding Portland cement concrete no adverse effects are present. If not, a thicker, more permeable concrete skin can be expected. It should be noted that for concrete in aggressive environ- ments usually compositions with a low water/cement ratio are used. The lower the water/cement ratio the less vulnerable to early dry conditions these concretes appear to be. N.B. There appears to be a remarkable difference with rapidly reacting pozzolan like silica fume, where an increase in curing sensitivity can occur at decreasing water/cement ratios.

Another disadvantage sometimes mentioned in the literature is an increased carbonation rate. This is partly bound up with strength development and partly with the lime buffer capacity available. Concretes with a low hydration rate will carbonate faster with those with a high hydration rate if exposed to the environment at an early stage; this is related to curing. Slag or fly ash containing concretes will also have a lower lime content than Portland cement concretes. Thus the amount of material to be carbonated is lower. These factors explain the higher carbonation rates observed in labo- ratory experiments in which the concrete is stored at constant humidity and temperature. In practice the difference is much less or absent. This is due to the on average higher moisture content of concrete with slag or fly ash, which is related to the finer pore system*s. In general it can be stated that carbonation under the present rules of concrete standards will never be a cause of corrosion of concrete, regardless of whether slag or fly ash is used.

Differences between concretes with and without slag and fly ash are met with respect to the resistance to frost-thaw de-icing salt cycles, as shown in Figure 13.

In general, it can be stated that for a certain strength and air content the resistance of concrete with slag in scaling tests is initially lower, but at higher numbers of cycles the difference is small or the blast furnace slag performs even better. The causes of this difference are not fully clear. An explanation may be that if concrete with slag is carbonated, the concrete becomes more

n 320, OPC + 280, OPC 40 fly ash A 320, OPBFSC v 280. OPBFSC 40 fly ash

Number of cycles

Figure 13 Loss of mass of concrete with Portland cement CEM I/42.5 or ordinary Portland blast furnace slag cement CEM III/B 42.5 with and without pfa during de-icing tests6

314 Benefits of slag and fly ash: J. Bijen

porous while in the case of Portland cement the po- rosity decreases. Another factor may be that the pore system of uncarbonated concrete with slag or fly ash is substantially finer, so that a lower space factor for entrained air has to be applied than for Portland cement. Nevertheless the initially higher scaling rate is a reason not to apply blast furnace slag cement for top surfaces in roads in the Netherlands.

In the case of fly ash the absorption of air entraining agents by the coal particles (coke) in the fly ash has to be taken into account. For fly ash concretes of the same 28 day strength as reference Portland cement concrete the differences appear to be minor.

Conclusions

Slag and fly ash have some major beneficial effects on the performance of reinforced concrete structures. The most important is the enormous reduction of the rate of penetration of chloride ions into concrete and the increase in the critical chloride concentration concern- ing chloride-induced corrosion.

Due to these advantages concrete structures can have a long corrosion-free service life under the most aggres- sive natural environmental conditions with no other protection for the reinforcement than concrete.

Other important benefits are the resistance against sulphate attack and the inertness with respect to alkali- silica reactive expansion.

There are some disadvantages certainly, but these can be overcome by taking measures well known in concrete technology.

References Grutzeck, W. and Sarakar, L. Mineral admixtures: reactions, micro-structure and macro-properties. Adv. Cement Concr. July 1994, 292-328 Larbi, J.A., Fraaij, A.L.A. and Bijen, J.M. The chemistry of the pore Buid of silicy fume-blended cement systems. Cement Concr. Res. 1990,20, 506516 Fraaij, A.L.A. Bijen, J. and de Haan, Y.M. The reaction of fly

4

5

6

I

8

9

10

11

12

13

14

1.5

16

17

18

ash in concrete. A critical examination. Cement Concr. Res. 1989, 19, 235-246 Larbi, J. The cement paste-aggregate interfacial zone in concrete. Thesis, Delft University of Technology, Civil Engineering Department, 1991 van der Wegen, G., Bijen, J. and van Selst, R. Behaviour of concrete affected by seawater under high pressure. in Proc. f&h International Conference on Ojjshore Mechunics und Arctic Engineering, Vol. III-B, 1991, pp. 607-613 Fly ash as addition to concrete. CUR-report 144, Gouda, The Netherlands, 1991 Brodersen, H.A. Zur Abhlngigkeit der Transportvorgange verschiedener lonen im Beton von Struktur und Zusammensetzung des Zementsteines (Dependence of transport of various ions in concrete of structure and composition of cement stone. Dissertation RWTH Aachen, 1992 Bijen, J. Advantages of the use of portland blast furnace slag cement for marine concrete. in Proc. Symposium Technology of Concrete when Pozzolan Slugs wd Chemical Admixtures we used. Monterey, ACI, 1985, pp. 485-500 Bijen, J. Durability aspects of the King Fahd Causeway. Concrete in Hot climates, Ed. M.J. Walker, E & EN Spon, London, 1992, pp. 231-244 Bakker, R., Van der Wegen, G. and Bijen, J. Reinforced concrete, an assessment of the allowable chloride content. In Third CANMET/ACI Int. Conf Durubility of Concrete, Supplementary papers, Nice, France, pp. 185-199 Raupach, M. On chloride-induced macrocell corrosion of steel in concrete. Deutscher Ausschufi fur Stahibeton, Heft 433, Beuth Verlag, Berlin, 1991 Wiebenga, J.G. Durability of concrete structures along the North Sea coasts of the Netherlands. AC1 pulication Sp-65 (Performance of concrete in marine environment.) ACI, Detroit, 1980, pp. 437452 Contribution zum 29 Forschungskolloguium, Deutscher AusschuB fur Stahlbeton, 24, 25 March 1994, Delft University of Technology Communications SchieBl, IBAC RWTH Aachen, Germany, and Dr Plumat, Lafarge, France Bakker, R.F.M. On the case of increasing resistance of concrete made from blast furnace slag cement to the alkali-silica reaction and the sulphate corrosion. Dissertation, RWTH Aachen, 1981 Heinz, D. Schadigende Bildung Ettringite-ahnlinche Phaser in warme-behandelton Morteln und Beton (Deterioration by ettrin- gite-like phases due to heat treatment of mortars and concrete). Thesis RWTH Aachen, December 1986 Recommendations Alkali-Silica Reaction in Concrete (Richtlinie Alkali-Reaktion im Beton). Deutscher AusschuB fur Stahlbeton, Beuth Verlag, Berlin, December 1986 Bijen, J., Van der Wegen, G. and Van Selst, R. Carbonation of portland blast pance slag cement concrete with fly ash. In Proc. Third Int. Conf on Fly Ash Silica Fume, Slag und Muteriul Pozzoluns in Concrete, Vol I, SP 114-31, AC1 Detroit, 1989