Mineral-based secondary binders, utilization, and considerations in mix design. Exercise 5

Transcription

1 Mineral-based secondary binders, utilization, and considerations in mix design Exercise 5

2 Mineral-based secondary binders Fly ash filler secondary binder (pozzolan) Ground granulated blastfurnace slag (GGBS) secondary binder (hydraulic) used especially in massive and sulphate-resistant structures Silica fume secondary binder (pozzolan) used especially in structures requiring good chemical resistance or great strength

3 Fly ash is one of the residues generated in combustion of fine ground coal. It is generally captured by electrostatic precipitators (ilmanpuhdistin) or other particle filtration equipments before the flue gases reach the chimneys. The collected fly ash is fine (made of small pieces, grains etc) and pozzolanic.

4 Blast furnace slag is formed when iron ore (malmi) or iron pellets, coke (koksi) and a flux (sulatusaine) (either limestone or dolomite) are melted together in a blast furnace. When the metallurgical smelting process is complete, the lime in the flux has been chemically combined with the aluminates and silicates of the ore and coke ash to form a non-metallic product called blast furnace slag. During the period of cooling and hardening from its molten state, BF slag can be cooled in several ways to form any of several types of BF slag products. Usually cooled quickly using water (granulation) or air cooled by projecting it into the air by a rotating drum (pelletising ). To obtain a good slag reactivity or hydraulicity rapid cooling is needed. The glass content (lasimaisuusaste) of slags suitable for blending with Portland cement typically varies between %.

5 Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production and consists of very small spherical particles. Because of its chemical and physical properties, it is a very reactive pozzolan.

6 The use of secondary binders is regulated by the Concrete code Supervise the quality control Determine the maximum amounts in concrete Advise on the use of concretes containing secondary binders Give guidelines to concrete quality control

7 AVERAGE PHYSICAL PROPERTIES OF SECONDARY BINDERS: Fly ash Silica fume GGBS Cement specific surface area [m 2 /kg] (apparent) density [kg/m 3 ] bulk density [kg/m 3 ]

8 AVERAGE CHEMICAL COMPOSITION [%]: Fly ash Silica fume GGBS Cement CaO SiO Al 2 O , Fe 2 O ,5 1 3 Others (MgO, K 2 O, Na 2 O, S, SO 3, loss on ignition)

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10 Grounds on using fly ash As a filler improves concretes Worability Coherence (koossapysyvyys) Pumpability Stability during transport As a binder improves concretes Strength May hinder air-entrainment

11 The pozzolanic reaction in concrete Simplified equation: Water (H) + cement (C, S) C-S-H (CaO SiO 2 H 2 O) + CH (Ca(OH) 2 ) Pozzolan (S) + CH (Ca(OH) 2 ) C-S-H (CaO SiO 2 H 2 O)

12 AVERAGE CHEMICAL COMPOSITION [%]: Fly ash Silica fume GGBS Cement CaO SiO Al 2 O , Fe 2 O ,5 1 3 Others (MgO, K 2 O, Na 2 O, S, SO 3, loss on ignition)

13 The reaction of fly ash in concrete is more susceptible to temperature: In cool conditions the reaction is slow When heat treated fly ash takes part in the initial strength development Fly ash does not does not have an ifluence on Plastic shrinkage (cover large concrete pours!!) Drying shrinkage (avoid mix composition with too much water and small aggregate) Modulus of elasticity (kimmomoduuli) Creep (viruma)

14 Influence on strength

15 Concretes containing GGBS Used as a binder in normal concrete Used as a binder in special cases Massive structures The use of GGBS reduces the amount of heat released Structures susceptible to sulphates Concrete is considered sulphate resistant when 70 % of binder is comprised of GGBS.

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17 Concretes containing GGBS GGBS works normally as a slow reacting secondary binder Early strength development depends on the amount of slag used In ready-mixed concretes the slow strength gain can be compensated by using rapid hardening cement. The reaction of GGBS in concrete is susceptible to temperature: In cool conditions the reaction is very slow At normal temperature the reaction is slow When heat treated GGBS takes part in the initial strength development When heat treated at high temperatures, GGBS increases the early strength development

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19 Other characteristics Reduces the amount of water needed Does not have an effect on thermal expansion, modulus of elasticity nor creep (viruma). But may affect the final shrinkage when used in large quantities (> 60 %).

20 Grounds on using silica fume As a binder improves concretes Strength Compactness Chemical resistance Durability On special cases (very rarely) when used as filler, improves Coherence (koossapysyvyys) Stability during transport

21 A water reducing admixture is used with silica fume. Because of the improved coherence, the risk of plastic shrinkage is increased. Thus curing should commence right after placing and compaction

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23 Concretes containing silica fume (SF) The reaction of silica fume in concrete is susceptible to temperature: In cool conditions the reaction is very slow When heat treated, strength development of SF concrete is accelerated more compared to a normal concrete The largest use of silica fume is for the purpose of producing high strength or high performance concretes Curing is very important when using SF

24 Other properties Does not affect the shrinkage Because of the very fine composition of SF, a respirator (hengityssuojain) should be worn Colour

25 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic ie. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code 3. Divide the binder into active and non active (i.e. filler) parts when the activity index of the secondary binder is below 1. If the activity index is 1, all of the additive is considered as binder. 4. Replace filler (from the aggregate) part by volume with the non active part of the secondary binder.

26 In Exercise 1 ( ) we proportioned concrete: cement 280 kg/m 3 water 167 kg/m 3 aggregate 1936,8 kg/m 3 sand 522,9 kg/m 3 gravel 484,2 kg/m 3 coarse gravel 929,7 kg/m 3 air 20 dm 3 Proportion the concrete using GGBS: CEM I 50/50

27 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code

28 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code 3. Divide the binder into active and non active (i.e. filler) parts when the activity index of the secondary binder is below 1. If the activity index is 1, all of the additive is considered as binder.

29 Divide the binder into active and non active parts using the activity index, i.e. into binder and filler. The effective amount of binder in a concrete containing secondary binders can be estimated with equation: B eff = C + k additive * ADDITIVE In wich B eff is the effective binder amount [kg/m 3 ] C is the amount of cement [kg/m 3 ] ADDITIVE is the amount of the additive [kg/m 3 ] k additive is the activity index of the additive

30 GGBS:CEM I 50/50 Activity index of GGBS: From the old (2004) standard

31 New 2012 concrete code p.102:

32 B eff = C + k additive * GGBS B eff = C + k additive * GGBS = 280 kg (from the original mix design) B eff = C + 0,8 * C = 280 kg C = 155,6 kg = GGBS The active part of GGBS: 0,8*155,6 = 124,4 kg The rest (155,6 kg 124,4 kg) 31,2 kg is considered as filler

33 Since 31,2 kg of GGBS is filler we need to reduce this amount from the amount of the finest aggregate (in this case sand). In the original mix design the finest aggregate was sand 522,9 kg/m 3 522,9 31,2 = 491,7 kg The initial water amount does not change (167 kg/m 3 ). Because the density of the GGBS is different than the density of the aggregate and cement, we need to calculate a new amount of aggregate using the basic equation of concrete:

34 Density of GGBS is 3000 kg/m 3 Calculate a new amount of aggregate = 1000 (155,6/3, ,6/3, /1 +20) = 710,9 dm 3 710,9 * 2,68 = 1905,3 kg/m 3

35 Finally we can calculate (check) the amounts (kg) and volumes (dm 3 ) of each component Cement 155,6 kg 50,2 dm 3 GGBS 155,6 kg 51,9 dm 3 Water 167 kg 167 dm 3 Agg. 1905,3 kg Sand 491,7 kg 183,5 dm 3 Gravel 353,4 kg 131,9 dm 3 Coarse gravel 1060,2 kg 395,6 dm 3 Air dm 3 Total 1000 dm 3 OK!

36 2. We require a mix of strength class C30 and a slump of 140 mm. The mix design is to be done with ordinary Portland cement with cement strength of 49,5 MPa and a maximum amount of fly ash in exposure class XC3. Grading of the aggregate is presented in the handed out forms.

37 New strength classes cylinders cubes The proportioning strength (suhteituslujuus) K s can be calculated as: K s = 1,2*K*42,5/N N is the test strength of the cement = 1,2*37*42,5/49,5=39,1

38 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e. equivalent amount of cement

39 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e.. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code

40 Exposure Maximum allowable quantity of additives [%] class GGBS Fly ash SF

41 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e.. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code 3. Divide the binder into active and non active (i.e. filler) parts when the activity index of the secondary binder is below 1. If the activity index is 1, all of the additive is considered as binder.

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43 Mix design of concrete containing secondary binders 1. Carry out the mix design as usual in order to find out the basic i.e.. equivalent amount of cement 2. Use a wanted amount of secondary binder following the regulations in the Concrete code 3. Divide the binder into active and non active (i.e. filler) parts when the activity index of the secondary binder is below 1. If the activity index is 1, all of the additive is considered as binder. 4. Replace filler (from the aggregate) part by volume with the non active part of the secondary binder.

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45 Solution: The maximum amount of fly ash: 100/2,22 = 45,0 %

46 Divide the binder into active and non active parts. B eff = C + k additive * ADDITIVE According to the concrete code, the activity index of FA is 0,4 when proportion of fly ash to cement 0,33 and 0 when > 0,33 Thus: B eff = C + k additive * FA eff B eff = C + k additive * FA eff = 325 kg B eff = C + 0,4 * (0,33 * C) = 325 kg C = = 287,1 kg, FA eff = 0,33 * 287,1 = 94,7 kg Fly ash over the 33 % amount is considered non-active with activity index of 0. The amount of this FA is = 12 % 0,12 * 287,1 = 34,5 kg FA = FA eff + Fa non.e = 94,7 + 34,5 = 129,2 kg

47 Of the fly ash active (acts as binder) is: FA act = FA eff * 0,4 = 94,7 * 0,4 = 37,88 kg Rest is filler 129,2-37,9 = 91,3 kg Since 91,3 kg of FA is filler we need to reduce this amount from the amount of the finest aggregate In the original mix design the finest aggregate was sand 372 kg/m ,3 = 280,7 kg The density of FA is different than the density of the aggregate so we need to calculate a new amount of aggregate using the basic equation of concrete: 1000 (287,1/3, ,2/2, /1 +20) = 650,2 dm 3 = 1742,6 kg

48 Proportion the concrete using the maximum amount of silica in the same exposure class.

49 Mix design of concrete containing silica fume The maximum amount of silica in exposure class XC3 is 11,1 % B eff = C + k additive * SI = C + 1*(0,111*C) = 325 kg C = 292,8 kg SI = 32,5 kg

50 The activity factor of silica is 1 when the water/cement ratio is > 0,45 (181/325=0,56)

51 Since the activity index is > 1, all of the silica is considered as binder. No filler replacement needs to be made Calculate a new amount of aggregate using the basic equation of concrete: 1000 (292,8/3,1 + 32,5/2, ) = 689,8 dm 3 = 1848,6 kg