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Concrete Technology 2/5

Aalto UniversitySchool of EngineeringDepartment of Civil and Structural EngineeringBuilding Materials Technology

Ground granulated blast furnace slag GGBSGGBS is obtained by quenching molten iron slag (a by-product of iron and steel-making) from a blast furnace inwater or steam, to produce a glassy, granular product that isthen dried and ground into a fine powder.

Temperature in the molten iron slag reaches 1400 oCAnnual production is about 0.6 million ton in FinlandCooling methods

-slow cooling of slag melts results in an unreactivecrystalline material consisting of an assemblage ofCa-Al-Mg silicates

Concrete Technology 2


-to obtain a good slag reactivity or hydraulicity, the slagmelt needs to be rapidly cooled or quenched below 800 °Cin order to prevent the crystallization of merwinite andmelilite-pelletized slag (swelling by a smaller water amount)-granulated slag (swelling in large water amount)

Granulated slag-swelling is produced by water sprays-final cooling in large water pool-granulated particles are lifted from the pool-fast cooling prevents the crystallization and theparticles possess a glassy outer surface


Pelletized slag-molten slag is poured on a plate-water sprays cause swelling in the slag-revolving cylinder brakes the slag into particles-water is sprayed into the pellet heap to prevent theparticles from sintering together

Pelletized slag versus granulated slag-pelletized has larger grains-pelletized is more porous-pelletized is lighter and dryer-pelletized is more crystalline and easier to grind

Utilization possibilities

-supplementary binder in cement-blast furnace slag cement 80 % of the cement content-grinding

-preferably separate grinding from the clinker-specific surface area 260 - 300 m2/kg (800 m2/kg)-preferably granulated slag < #4 mm, largergrains are used as aggregates

-as concrete aggregate


Mineral composition

Hydration products-C-S-H (C/S-ratio is smaller than in OPC)-AFt and AFm phases-hydrogarnet C3AH6

-hydrotalcite M6ACH12

-C-A-H, C-M-H, and small amounts of Ca(OH)2


Oxide Formula Blast furnace slag (%) OPC (%)Calcium oxide CaO 39 62Aluminum oxide Al2O3 10 5Silicon dioxide SiO2 36 20Magnesium oxide MgO 11 2Iron, manganese, potassium, and titanium oxides

The hydraulic properties of GGBS improve when thealkalinity of the slag increases

2

when p > 1 GGBS is alkalinep< 1 GGBS is acidic

Properties of GGBS-latently hydraulic binder-not a pozzolanic material because it needs only smallamount of activator to change to hydraulic and doesnot consume the activator

Activators-Portland cement, CaO, alkali, alkali sulfate-Ca(OH)2 , CaCl2, gypsum or CaSO4, NaOH



Blast furnace slag cement is a low-heat cementHydration heat of GGBSw/c = 0.4, specific surface area 250 m2/kg (Blaine)

Binder combination [%] Heat of hydration [kJ/kg]Slag Portland cement 3 days 7 days20 80 264 31030 70 234 28540 60 219 25550 50 205 24760 40 188 23470 30 163 19775 25 155 18880 20 142 17685 15 126 163


Hydraulicity of GGBS increases when the glass content inthe slag increases

-cooling technology (rate of cooling)-temperature-chemical composition

Effects on fresh concrete-increases the slump of fresh concrete lowerwater-cement ratio higher strength-cohesion of the mix decreases, if slag amount is large-water segregation on the surface increases-better compaction lower air content-somewhat increased density

Effect of GGBS on the consistency of concreteQc : Qagg : Qw = 1 : 3 : 0.42

Effects of GGBS on hardened concreteStrength gain

-lower early strength-higher final strength


Binder [%] Consistency

Slag Portland cement sVB Slump [cm]

0 100 1.3 13.3

70 30 0.9 14.8

Heat treatment-suits for GGBS very well-efficiency improves with increasing GGBS amount-higher temperatures can be applied

Flexural strength-the ratio between flexural and compressive strengthis higher in GGBS concretes

Durability against chemical loads-if slag amount is > 50%, concrete possesses goodchemical durability (especially against sulfates)-smaller Ca(OH)2 amount pH of pore water isdiminished lower capacity to hinderreinforcement corrosion


Admixtures when applying GGBS-no interference with plasticizers and air-entrainment-freeze-thaw durability can be degraded especiallyduring salt loads

Advantages and disadvantages in the use of GGBSAdvantages

-smaller hydration heat, smaller risk for cracking inmassive structures-smaller water to binder ratio, higher strength-half the price compared to Portland cement-improved durability against chemical loads-improved flexural/compressive strength ratio-improved impermeability at early age


Disadvantages-additional quality control costs-slow and small hydration heat, not suitable in precastfactories nor during winter concreting-faster carbonation rate, decreased service life span dueto reinforcement corrosion-carbonation changes pore structure unfavorably withrespect to durability

-permeability increases at later age (carbonation)-pore structure becomes more coarse-salt-freeze durability deteriorates

-inferior workability and cohesion in fresh concrete



Condensed silica fume- Silica fume is an amorphous (non-crystalline)

polymorph of silicon dioxide, silica.- ultrafine powder collected as a by-product of the

silicon and ferrosilicon alloy production- consists of spherical particles with an average particle

diameter of 150 nm, about 1/100 part of the diameterof average cement particle

- main field of application is as pozzolanic material forhigh performance concrete

- annual production about 1.2 million tons- density about 2200 kg/m3

- particle density 200…300 kg/m3

Component Si FeSi75%

FeSI75%HR

FeSi50%

FeCrSi CaSi SiMn

SiO2 94 89 90 83 83 54 25Fe2O3 0.03 0.6 2.9 2.5 1.0 0.7 1.8Al2O3 0.06 0.4 1.0 2.5 2.5 0.9 2.5CaO 0.5 0.2 0.1 0.8 0.8 23.2 4.0MgO 1.1 1.7 0.2 3.0 7.0 3.3 2.7Na2O 0.04 0.2 0.9 0.3 1.0 0.6 2.0K2O 0.05 1.2 1.3 2.0 1.2 2.4 8.5C 1.0 1.4 0.6 1.8 1.6 3.4 2.5S 0.2 0.1 2.5MnO 0.06 0.2 0.2 36LOI 2.5 2.7 3.6 2.2 7.9 10



Effects on fresh concrete- stabilizing effect cohesion improves and the

segregation of aggregates and bleeding is diminishedunderwater concreting, shotcrete, pumped concrete,light weight concrete

- if used < 5% of cement weight only a smalleffect on water demand in normal concrete

- if used in larger amount, plasticizers orsuperplasticizers are needed

- the slump loss with time is directly proportional toincrease in the silica fume content due to theintroduction of large surface area in the concrete mixby its addition. Although the slump decreases, themix remains highly cohesive.

- increased tendency for plastic shrinkage surfacecracking

- moist curing or a plastic covering sheet

Effect on hardened concreteCompressive strength

-water-binder ratio v/(c + k s)-efficiency coefficient k = 2…5 (s = silica content)

Hardening temperature-heat treatment improves early strength-disadvantageous in winter concreting-8% silica can raise compressive strength to 100 MPa-30% silica can raise compressive strength to 200 MPa



Compactness-no influence on the total porosity-increase in finer porosity-remarkably increased impermeability of concrete

Freeze-thaw durability-decrease in freezable capillary water-pore size distribution-if there is very small amount of freezable water at-20 oC good freeze-thaw durability

Durability against chemical loads-denseness of silica concrete-small amount of Ca(OH)2

Improved reinforcement bond

Concrete admixtures


According to ASTM

1. Chemical admixtures-Accelerators (K)-Retarders (H)-Plasticizers (N) (water reducing admixtures)-Superplasticizers (Nt)

2. Air-entraining admixtures3. Pozzolanic supplementary binders4. Other admixtures

-Expansion producing admixtures -Flocculating admixtures-Color pigments -Organism growth inhibitors-Water proofers -Set regulating admixtures-Agents improving pumpability -Damp proofing admixtures-Corrosion inhibitors -Antifreezing admixtures-Liquid repelling admixtures

Overall review

Retarders-impure salts of Na, Ca, or NH4

-hydroxycarboxyl acids and their salts-carbohydrates containing sugars-inorganic compounds: phosphates, fluorites, lead orzinc oxides

Accelerators-CaCl2 causing reinforcement corrosion.calcium formiate-trietanolamine


Water-reducing admixtures (plasticizers)-salts of Na, Ca, or NH4

-lignosulfonates-hydroxylcarboxyl acids and their salts (hydroxylatedpolymers)

ApplicationOrdinary plasticizers

-improved strength-easier and faster compaction works-improved durability properties-economical advantages


Accelerating plasticizers-winter concreting-improving form circulation rate

Retarding plasticizers-large and massive concreting structures-prevention of cold joints (construction joints)-hot weather concreting

Plasticizers have a natural tendency to retard concretesetting and the beginning of hardening process.Operation principle

-based on the adsorption and dispersion mechanisms in particlesin water solution

-does not affect the total hydration heat evolution-applies on both plastic and stiff concretes



-the addition time of the admixture has an effect-compatibility of the binder and plasticizer

Bleeding-lignosulfonates decrease-hydroxycarboxylate acids increase

Air content in concrete-lignosulfonates increase 1 - 3 %-hydroxycarboxylate acids do not increase-usually when air-entraining admixtures are applied togetherwith plasticizers, the dosage of air-entraining agent can bereduced

Strength-with identical cement content. Air content, and freshconcrete consistency, plasticizers increase strength by 10 – 20 %

-application to save cement

Durability-if v/c is small, concrete possesses small permeability andporosity and, therefore, the durability properties are good

-plasticizers containing chlorides-strength is also related to freeze-thaw durability

Setting-1 h < plasticizer < 1.5 h-retarding plasticizer > 1.5…3.5 h-accelerating plasticizer < 1…3.5 h


Workability-slump value can be doubled-hydroxycarboxylate admixtures are more efficient compared tolignosulfates

-with large dosages there is a retarding effect-workability time is decreased, hot temperature decreases theworkability further (addition time of the plasticizer oradditional dosage at the building site)

Shrinkage and creep-variable shrinkage results depending on the binder combination-same holds to creep results


concrete technology 2/5 - aalto technology 2/5 ... concrete technology 2. concrete technology 2 ......

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