Concrete Technology 2/4 - Aalto · Concrete Technology 2/4 Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology

Concrete Technology 2/4

Aalto UniversitySchool of EngineeringDepartment of Civil and Structural EngineeringBuilding Materials Technology

Hydration in a closed systemWater is not transferred with the surroundingsVariables v volume fraction (0…1)

hydration degree (0…1)1- solid concentration (0…1)

Volume of unhydrated cement

Solid volume of the gel

Concrete Technology 2

)1)(1( αθ −−=cv

( ) wcgsv ραθ−ρ−+αθ−≅ /126,0)25,01()1(

αθ−≅ )1(6,1gsv


Volume of gel water

Volume of capillary water

Volume of the contraction pores

vgw c w≅ ⋅ −0 75 0 26 1, , ( ) /ρ θ α ρ

vgw ≈ −0 6 1, ( )θ α

vkw c w≅ − ⋅ − − −θ ρ θ α ρ θ α0 26 1 0 6 1, ( ) / , ( )vkw ≅ − −θ θ α1 4 1, ( )

vp c w≅ ⋅ −0 25 0 26 1, , ( ) /ρ θ α ρvp ≈ −0 2 1, ( )θ α


Transfer of water can take place with the environment

The water volume transferred between the environment andstructure (cement + water) is denoted by v the formulaeof capillary water and porosity are changed

Hydration in an open system

structuretheofinsidefromdiffusionofratesurfacethefromnevaporatioofrate

vvvv kwkw ∆∆αθθ +=+−−≅ )1(4,1'

vvvv pp ∆∆αθ −=−−≅ )1(2,0'

pkw vvv ≤≤− ∆

Conversion of the volume variables to weight based variables(kg/ concrete m3)

-total volume of the system

-the volume variables are multiplied by the totalvolume of the system


332,032,0

11

mlQQ

cvQ

cvQ

QQv

wcc

wcc

w

w

c

ctot

+≈

+≅

+=+=

ρρρρ

Volume of hydration productsInitial porosity



c

wcw

cw

ρ

ρθ+

=

)1()1( αθ −⋅−=cv


Volume of gel water




αθ ⋅−⋅= )1(6,1gsv

αθ ⋅−⋅= )1(6,0gwv

αθθ ⋅−⋅−= )1(4,1kwv

αθ ⋅−⋅= )1(2,0pv


Maximum degree of hydration

1. No external water

vkw=0

2. Wet curing

1)1(4,1max ≤

−⋅=

θθα

vkw+vp=0 1)1(2,1max ≤

−⋅=

θθα



Hydration of condensed silica fume-chemical shrinkage is 22 ml/100 g silica fume-amount of gel water is 0.50 g/ g silica fume-water amount consumed in the reaction 0 g/ g silica

Densities of the components-ρc= 3100-3150 kg/m3

-ρw= 1000 kg/m3

-ρs= 2200 kg/m3

Volume of hydration products when condensedsilica fume is applied


Initial porosity

coefficient

cs

cw

cw

s

w

c

w ⋅++=

ρ

ρ

ρ

ρθ

csk

⋅+=

4,11

1




Volume of gel water


)1()1( αθ −⋅−⋅= kvc

αθ ⋅−⋅+⋅= )1()7,06,1(cskvgs

αθ ⋅−⋅+⋅= )1()6,16,0(cskvgw

αθθ ⋅−⋅+⋅−= )1()6,14,1(cskvkw


Volume of the unreacted condensed silica fume

Maximum degree of hydration

1. No external water

vkw=0


αθ ⋅−⋅+⋅= )1()7,02,0(cskv p

)1()1(4,1 αθ −⋅−⋅⋅⋅=cskvs

1)1()6,14,1(

max ≤−⋅+⋅

=θ

θα

csk

2. Wet curing

vkw+vp=0


1)1()9,02,1(

max ≤−⋅+⋅

=θ

θα

csk

Capillary pores-represent that part of the total volume of the cement pastethat is not filled by the hydration products-volume of capillary pores is dependent on water-cementratio and degree of hydrationVolume proportions during different phases of hydration

If v/c > 0,38 - 0,42, the hydration products of cement pastecannot fill all of the capillary pores.Capillary pores- have approximately diameter < 1,3 µm- shape is variable- can be connected with each other


- are distributed randomly in the cement gel- have a major effect on permeability and freeze-thaw

durabilityAs hydration proceeds the connectivity to other capillarypores can be broken and they are connected to each otheronly through gel pores- v/c -ratio- curing in wet conditionsThe hydration degree and water-cement ratio which makesit possible to break the connectivity of the capillary poresare- if v/c ≥ 0,7 not even complete hydration degree enables

the interception



- when cement is finely ground v/c < ∼0,8- applying coarse cements v/c < 0,7

The average curing time (hardening time) for capillarypores to break a continuous pore system

v/c Time0.40 3 d0.45 7 d0.50 14 d0.60 6 months0.70 1 year

>0.70 impossible

Concrete is not usually considered to be proper if capillarypores form a continuous pore system.

Gel pores- spaces between gel particles- φ ≅ 15...20 Å (Å = 10 -10 m)- a decade larger compared to water molecules ⇒ they are

affected by partial pressure and mobility of water vapor -- the volume content of gel pores is 28 % of gel total

volume depending on the hardening conditions.



-the volume content of gel pores is somewhat dependenton cement type but does not depend on

-water-cement ratio nor-hydration degree

⇒ Hydration does not affect on the previously generatedhydration products and the gel composition isinvariant of the age at which it is forming

-the surface area of the cement gel is about 200 000m2/kg, while the surface area of unhydrated cement isabout 300 m2/kg

-the surface area of cement gel which has hydrated inhigh temperature is about 7000 m2/kg ⇒ Thedimensions and morphology of the hydration productsare quite different ⇒ the structure is microcrystalline

Supplementary binders

Fly ash-coal ash-peat ash pozzolanic reaction-wood ash

Commonly used in Finland is coal ash produced in heat andelectricity power plants

-2-15 % unburnt coal residues-collected from the exhaust gases-bottom ash not suitable


Composition of coal ash in a typical power plant


Component Variation area%

Average value%

Requirement

SiO2 40 - 50 48(S + A+ F)Together> 70 %

Al2O3 20 - 30 23

Fe2O3 8 – 13 10

CaO 4 – 10 7

MgO 3 – 6 4 5.0 %

Na2O+K2O 1 - 4 2.5

SO3 0.5 – 1.5 0.8 2.5 %

Cl- - 0.03 0.1 %

Loss of ignition 0.5 - 8 2 8 %

Maximum allowable supplementary binder amounts


Seosaineiden käytön sallitut enimmäismäärät riippuenrasitusluokista ja käytettävästä sementistä on esitetty taulukossa4.4.

Taulukko 4.4 Betonin valmistuksessa suurimmat sallitutseosainelisäykset lasketaan seuraavista kaavoista, joissaQII on sementin sisältämät seosaineet [%].

Rasitusluokka Suurin sallittu seosainelisäys [%]Masuunikuona Lentotuhka Silika1)

X0XC1…XC3XS1XD1, XA1

10020

−,

Q-100 II

100650

−,

Q-100 II

9Q-100 II

XF1, XF3

100750

−,

Q-100 IIXC4,XS2, XS3XD2, XD3

10080

−,

Q-100 II

XF2, XF4 10050

−,

Q-100 II

14Q-100 II

1) Lisäksi on varmistuttava, että syntyvän sideaineseoksenklinkkeripitoisuus on vähintään 65 % rasitusluokissa X0, XC1…XC3, XS1, XD1 ja XA1 ja 80 % muissa rasitusluokissa.

Particle size distribution and properties-between the distributions of fillers and cement-50 – 70 % spherical ash particles, part of which are hollow-density of the solid content 2.2 kg/m3

-particle density 1.4 – 1.5 kg/m3

-dusty-pozzolanic binder having efficiency coefficient of 0.2 – 0.4 incompressive strength compared to Portland cementUtilization of coal fly ash1. Raw material in clinker production

-intermediate product-already in fine particle form-residual coal acts as additional energy source


2. As a supplementary binder with cement-maximum content in CEM IIA is 10 %-maximum content in CEM IIB is 35 %-maximum content with CEM I is 60 %-mixed with cement before or after grinding

3. Substitute for natural filler-aggregate-pneumatic transport and water tight filler silos-good homogeneity


Properties in fresh concreteCoal fly ash improves

-workability-pumpability-cohesion

Good quality coal fly ash-possesses a small loss of ignition and fine particle size-decreases water demand of the mix

When coal fly ash is applied-concrete becomes denser-air content is decreased-color is “greener”-hydration heat is decreased


-decreases cement content-reacts with and binds calcium hydroxide-improves durability in some cases

Properties in hardened concrete-latent hydraulic properties (pozzolanic reaction withCa(OH)2, 1kg fly ash binds 875 g of Ca(OH)2)

-when used as supplementary binder as a replacementof cement early age strength decreases and finalstrength increases-when used as a filler the pozzolanic reactionimproves strength (not necessarily 28 d strength)-heat treatment improves hardening rate (even earlystrength)


-improves chemical durability of concrete, especiallydurability against sulfates-decreases the susceptibility for alkali-aggregatereaction and improves the volume stability ofcements containing too large amounts of MgO

Large fly ash content decreases Ca(OH)2 content in hydratedconcrete which increases the risk of reinforcement corrosion.

Maximum fly ash content is restricted to 60 % of Portlandcement (CEM I).


Large coal content in fly ash-diminishes the pozzolanic reaction-increases water demand-causes color variations-coal particles absorb admixtures, for example, air-entraining admixture dosage must be increased 1.2 –2 fold

Fly ash and admixtures-plasticizers and retarding plasticizers generatehigher 91 d strength and decrease water segregation,Young’s modulus, and 90 d shrinkage. Setting timeincreases by 1 – 2 hours.


Advantages of fly ash as a filler-diminished water demand-particle composition is finer which diminishes watersegregation-smaller density

Disadvantages of fly ash-quality control-difficulties in quality variations-cumbersome handling compared to natural fillers-seasonal variation which causes storage needs-transport costs-additional dosage amount of admixtures


Utilization of wood fly ash

• Recycled material from energy production of wood andpaper industry

• Annual production about 300 000 tons in Finland• Raw material consists of bark, saw dust, and different

fiber or leftover sludges• Has similar properties as fly ash produced from coal in

electricity and heat production




SO3[%]

Cl-[%]

SiO2[%]

Na2O-equivalent

[%]

SiO2+Al2O3+Fe2O3

[%]

MgO[%]

CaO[%]

Loss ofignition([%])

Wood ash 1 8 0,4 35 4,1 35 + 18+ 3 = 56

3 15 C (7)

Wood ash 2 3 0,2 43 3,3 43 + 22+ 4 = 69

3 15 A (1)

Wood ash 3 10 1,3 20 4,9 20 + 14+ 4 = 38

3 31 A (2)

Wood ash 4 8 0,3 28 5,4 28 + 17+ 6 = 51

5 15 A (3)

Wood ash 5 2 0,2 36 3,7 36 + 29+ 2 = 67

2 14 B (5)

Limit valuesin

(SFS-EN450-1)

for fly ash

< 3,0 % < 0,1 % reactive> 25 %

< 5 % > 70 % < 4 % free< 2,5 %react.

< 10 %

A < 5 %B 5-7 %C 7-9 %



Properties


Density[kg/ dm3]

Powder density[kg/dm3]

Activityindex28 d

Expansionof paste

14 d[µm]

Settingtime[min]

Wood ash1 2,79 0,69-0,72 0,86 67

Wood ash2 2,61 0,85-0,87 0,61 13

Wood ash3 3,00 0,95-0,97 0,84 64 210

Wood ash4 2,77 0,75-0,78 0,75 60

Wood ash5 2,62 0,68-0,72 0,83 30 225

Fly ash 0,96 54

Class B 2,22 240Class C 2,14

Portlandcement 1 84 200

Compressive strength


Compressive strength

Freeze-thaw durability

Quality control


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Concrete Technology 2/4 - Aalto · Concrete Technology 2/4 Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology