Concrete Technology 2/5. Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology

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1 /5 Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology

2 Ground granulated blast furnace slag GGBS GGBS is obtained by quenching molten iron slag (a byproduct of iron and steel-making) from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder. Temperature in the molten iron slag reaches 1400 o C Annual production is about 0.6 million ton in Finland Cooling methods -slow cooling of slag melts results in an unreactive crystalline material consisting of an assemblage of Ca-Al-Mg silicates

3 -to obtain a good slag reactivity or hydraulicity, the slag melt needs to be rapidly cooled or quenched below 800 C in order to prevent the crystallization of merwinite and melilite -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 the particles possess a glassy outer surface

4 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 the particles 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

5 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 m 2 /kg (800 m 2 /kg) -preferably granulated slag < #4 mm, larger grains are used as aggregates -as concrete aggregate

6 Mineral composition Oxide Formula Blast furnace slag (%) OPC (%) Calcium oxide CaO Aluminum oxide Al2O Silicon dioxide SiO Magnesium oxide MgO 11 2 Iron, manganese, potassium, and titanium oxides 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

7 The hydraulic properties of GGBS improve when the alkalinity of the slag increases 2 when p > 1 GGBS is alkaline p< 1 GGBS is acidic Properties of GGBS Activators -latently hydraulic binder -not a pozzolanic material because it needs only small amount of activator to change to hydraulic and does not consume the activator -Portland cement, CaO, alkali, alkali sulfate -Ca(OH)2, CaCl2, gypsum or CaSO4, NaOH

8 Blast furnace slag cement is a low-heat cement Hydration heat of GGBS w/c = 0.4, specific surface area 250 m 2 /kg (Blaine) Binder combination [%] Heat of hydration [kj/kg] Slag Portland cement 3 days 7 days

9 Hydraulicity of GGBS increases when the glass content in the slag increases -cooling technology (rate of cooling) -temperature -chemical composition Effects on fresh concrete -increases the slump of fresh concrete lower water-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

10 Effect of GGBS on the consistency of concrete Qc : Qagg : Qw = 1 : 3 : 0.42 Binder [%] Consistency Slag Portland cement svb Slump [cm] Effects of GGBS on hardened concrete Strength gain -lower early strength -higher final strength

11 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 strength is higher in GGBS concretes Durability against chemical loads -if slag amount is > 50%, concrete possesses good chemical durability (especially against sulfates) -smaller Ca(OH)2 amount ph of pore water is diminished lower capacity to hinder reinforcement corrosion

12 Admixtures when applying GGBS -no interference with plasticizers and air-entrainment -freeze-thaw durability can be degraded especially during salt loads Advantages and disadvantages in the use of GGBS Advantages -smaller hydration heat, smaller risk for cracking in massive 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

13 Disadvantages -additional quality control costs -slow and small hydration heat, not suitable in precast factories nor during winter concreting -faster carbonation rate, decreased service life span due to reinforcement corrosion -carbonation changes pore structure unfavorably with respect to durability -permeability increases at later age (carbonation) -pore structure becomes more coarse -salt-freeze durability deteriorates -inferior workability and cohesion in fresh concrete

14 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 diameter of average cement particle - main field of application is as pozzolanic material for high performance concrete - annual production about 1.2 million tons - density about 2200 kg/m 3 - particle density kg/m 3

15 Component Si FeSi 75% FeSI 75%HR FeSi 50% FeCrSi CaSi SiMn SiO Fe2O Al2O CaO MgO Na2O K2O C S MnO LOI

16 Effects on fresh concrete - stabilizing effect cohesion improves and the segregation of aggregates and bleeding is diminished underwater concreting, shotcrete, pumped concrete, light weight concrete - if used < 5% of cement weight only a small effect on water demand in normal concrete - if used in larger amount, plasticizers or superplasticizers are needed - the slump loss with time is directly proportional to increase in the silica fume content due to the introduction of large surface area in the concrete mix by its addition. Although the slump decreases, the mix remains highly cohesive.

17 - increased tendency for plastic shrinkage surface cracking - moist curing or a plastic covering sheet Effect on hardened concrete Compressive 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

18 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 o C good freeze-thaw durability Durability against chemical loads -denseness of silica concrete -small amount of Ca(OH)2 Improved reinforcement bond

19 Concrete admixtures According to ASTM 1. Chemical admixtures -Accelerators (K) -Retarders (H) -Plasticizers (N) (water reducing admixtures) -Superplasticizers (Nt) 2. Air-entraining admixtures 3. Pozzolanic supplementary binders 4. 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

20 Overall review Retarders Accelerators -impure salts of Na, Ca, or NH4 -hydroxycarboxyl acids and their salts -carbohydrates containing sugars -inorganic compounds: phosphates, fluorites, lead or zinc oxides -CaCl2 causing reinforcement corrosion.calcium formiate -trietanolamine

21 Water-reducing admixtures (plasticizers) -salts of Na, Ca, or NH4 -lignosulfonates -hydroxylcarboxyl acids and their salts (hydroxylated polymers) Application Ordinary plasticizers -improved strength -easier and faster compaction works -improved durability properties -economical advantages

22 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 concrete setting and the beginning of hardening process. Operation principle -based on the adsorption and dispersion mechanisms in particles in water solution -does not affect the total hydration heat evolution -applies on both plastic and stiff concretes

23 -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 together with plasticizers, the dosage of air-entraining agent can be reduced

24 Strength Durability Setting -with identical cement content. Air content, and fresh concrete consistency, plasticizers increase strength by % -application to save cement -if v/c is small, concrete possesses small permeability and porosity and, therefore, the durability properties are good -plasticizers containing chlorides -strength is also related to freeze-thaw durability -1 h < plasticizer < 1.5 h -retarding plasticizer > h -accelerating plasticizer < h

25 Workability -slump value can be doubled -hydroxycarboxylate admixtures are more efficient compared to lignosulfates -with large dosages there is a retarding effect -workability time is decreased, hot temperature decreases the workability further (addition time of the plasticizer or additional dosage at the building site) Shrinkage and creep -variable shrinkage results depending on the binder combination -same holds to creep results