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

Mineral-based secondary binders, utilization, and considerations in

mix design

Exercise 5

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

Fly ashis 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.

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 80-100%.

Silica fumeis 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.

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

AVERAGE PHYSICAL PROPERTIES OF SECONDARY BINDERS:Fly ash Silica fume GGBS Cement

400 20000 350 350-4502300 2200 3000 3100(apparent) density [kg/m3]

specific surface area [m2/kg]

1000-1200

100-1800

1100-1800

200 - 600

bulk density [kg/m3]

AVERAGE CHEMICAL COMPOSITION [%]:Fly ash Silica fume GGBS Cement

CaO 5 - 40 60SiO2 50 90 35 20Al2O3 25 0,5 10 5Fe2O3 10 0,5 1 3Others 10 9 14 8(MgO, K2O,

Na2O, S, SO3, loss on ignition)

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

The pozzolanic reaction in concrete

• Simplified equation:Water (H) + cement (C, S) → C-S-H (CaO SiO∙ 2 H∙ 2O) + CH (Ca(OH)2)

Pozzolan (S) + CH (Ca(OH)2)

→ C-S-H (CaO SiO∙ 2 H∙ 2O)

AVERAGE CHEMICAL COMPOSITION [%]:Fly ash Silica fume GGBS Cement

CaO 5 - 40 60SiO2 50 90 35 20Al2O3 25 0,5 10 5Fe2O3 10 0,5 1 3Others 10 9 14 8(MgO, K2O,

Na2O, S, SO3, loss on ignition)

• 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)

Influence on strength

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.

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

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 %).

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

• 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

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

Other properties

• Does not affect the shrinkage• Because of the very fine composition of SF, a

respirator (hengityssuojain) should be worn• Colour

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.

In Exercise 1 (21.9.2012) we proportioned concrete:– cement 280 kg/m3

– water 167 kg/m3

– aggregate 1936,8 kg/m3

• sand 522,9 kg/m3

• gravel 484,2 kg/m3

• coarse gravel 929,7 kg/m3

– air 20 dm3

Proportion the concrete using GGBS: CEM I 50/50


1. Carry out the mix design as usual in order to find out the basic i.e. equivalent amount of cement


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:

Beff = C + kadditive * ADDITIVE

In wich Beff is the effective binder amount [kg/m3]

C is the amount of cement [kg/m3] ADDITIVE is the amount of the additive [kg/m3] kadditive is the activity index of the additive

GGBS:CEM I 50/50

Activity index of GGBS:

From the old (2004) standard

New 2012 concrete code p.102:

Beff = C + kadditive * GGBS

Beff = C + kadditive * GGBS = 280 kg (from the original mix design)

Beff = 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

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/m3 522,9 – 31,2 = 491,7 kg

The initial water amount does not change (167 kg/m3).

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:

• Density of GGBS is 3000 kg/m3

Calculate a new amount of aggregate= 1000 – (155,6/3,1 + 155,6/3,0 + 167/1 +20)= 710,9 dm3

710,9 * 2,68 = 1905,3 kg/m3

Finally we can calculate (check) the amounts (kg) and volumes (dm3) of each component

Cement 155,6 kg 50,2 dm3

GGBS 155,6 kg 51,9 dm3

Water 167 kg 167 dm3

Agg. 1905,3 kgSand 491,7 kg 183,5 dm3

Gravel 353,4 kg 131,9 dm3

Coarse gravel 1060,2 kg 395,6 dm3

Air -- 20 dm3

Total 1000 dm3

OK!

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.

New strength classes

cylinders cubes

The proportioning strength (suhteituslujuus) Ks can be calculated as:Ks = 1,2*K*42,5/N N is the test strength of the cement

= 1,2*37*42,5/49,5=39,1


1. Carry out the mix design as usual in order to find out the basic i.e.. equivalent amount of cement


Maximum allowable quantity of additives [%]Exposureclass GGBS Fly ash SF





4. Replace filler (from the aggregate) part by volume with the non active part of the secondary binder.

Solution:

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

Divide the binder into active and non active parts. Beff = C + kadditive * 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: Beff = C + kadditive * FAeff

Beff = C + kadditive * FAeff = 325 kgBeff = C + 0,4 * (0,33 * C) = 325 kg→ C = = 287,1 kg→ FAeff = 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 45-33 = 12 %

0,12 * 287,1 = 34,5 kgFA = FAeff + Fanon.e = 94,7 + 34,5 = 129,2 kg

Of the fly ash active (acts as binder) is:FAact = FAeff * 0,4 = 94,7 * 0,4 = 37,88 kg

Rest is filler 129,2-37,9 = 91,3 kgSince 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/m3 372 – 91,3 = 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 – (287,1/3,1 + 129,2/2,3 + 181/1 +20)= 650,2 dm3

= 1742,6 kg

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

Mix design of concrete containing silica fume

The maximum amount of silica in exposure class XC3 is 11,1 %

Beff = C + kadditive * SI

= C + 1*(0,111*C) = 325 kgC = 292,8 kgSI = 32,5 kg

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

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 + 32,5/2,2 + 181 + 20) = 689,8 dm3

= 1848,6 kg

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

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