Concrete Hydration 04

8/16/2019 Concrete Hydration 04

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HYDRATION

Lect-3


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• Stiffening - Loss of consistency by the plastic cement paste• Free water plasticity

• Loss in free water due to the adsorption by the C-S-H and evaporation

• Setting - Solidification of the plastic cement paste

• Initial set: eginning of the solidification !unwor"able#• Final set: Complete solidification

• Hardening - Strength gain with time• Higher C$S and C$ % content Higher early strength

• Higher proportion of C&

S

• Slow hardening Lower early strength and higher final strength

• Lower rate of heat of hydration

• Fineness also influences the strength development and heat evolution

PHYSICAL ASPECTS OF SETTING AND HARDENING

PROCESSES


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'ypical composition of cement

C3S (55%)

C3A (10%)

C2S (20%)

C4AF

(8%)

CSH2(5%)


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'wo mechanisms of hydration

• 'hrough-solution hydration !'SH#

• (issolution of anhydrous compounds

• Formation of highly insoluble hydrates in solution• )recipitation of hydrates from the supersaturated solution

• 'opochemical or solid-state hydration !SSH#

• *eaction at the surface of anhydrous cement compounds

• Initially 'SH and then SSH

+ehta and +onteiro


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%t the beginning of mi,ing the paste has a structure which consists of

cement particles with water-filled space between them. %s hydration

proceeds the gels are formed / they occupy some of this space

0el )ores: &12 of the total gel volume have diameter of 3.345-3.3&3

6m. !very small-loss or gain of water is difficult#

Capillary )ores: 4&.5 6m diameter with varying si7es shapes /

randomly distributed in the paste

8olume of capillary pores decreases as hydration ta"es place. 9ater in

capillary pores is mobile can not be lost by evaporation or water can

get into the pores. 'hey are mainly responsible for permeability.

- wc ratio

capillary porosity

- degree of hydration

H!"#t$& 'ec#&$*


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• C&S / C$S: ;3-132 of cement is composed of these two compounds /

most of the strength giving properties of cement is controlled by these

compounds.

• higher early strength-higher heat generation !roads cold

environments#• Higher C&S>lower early strength-lower heat generation !dams#

H!"#t$& 'ec#&$*

H ! t$ ' $


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• +oderate reaction rate

• High strength

• High heat liberation

HYDRATION OF SLILICATESH!"#t$& 'ec#&$*

H ! t$ ' $


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• Slow reaction rate

• Low initial but high later strength

• Low heat liberation

• O+e"#,, "e#ct$& C3S . C2S . /#te" C-S-H . CH

HYDRATION OF SLILICATESH!"#t$& 'ec#&$*

{ { {

} } }

2 3 2 3

water calcium hydroxidedicalcium silicate C-S-H

2 3 2 8

2C S 4H C S H 3CH

2C S 9H C S H CH

+ → +

+ → +

123

678


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C$%: is characteristically fast reacting with water / may lead to a rapid

stiffening of the paste with a large amount of the heat generation !Flash-Set#-

!?uic"-Set#.

In order to prevent this rapid reaction gypsum is added to the clin"er.

0ypsum C$% /water react to form relatively insoluble Calcium-Sulfo-

%luminates.

If there is no gypsum>flash-set more gypsum>ettringite formation increases which will causecrac"ing

%lso Calcium-Sulfo %luminates are prone !less resistant# to sulfate attac" /

does not contribute much for strength. 'he cement to be used in ma"ing

concretes that are going to be e,posed to soils or waters that contain

sulfates

C@%F: 'he hydration of ferrite phase is not well understand. Ferrite phase

has lesser role in development of strength. 'he hydration products are

similar to C$%. %lumina / iron o,ide occur interchangeably in the hydration

products

HYDRATION 'ECHANIS'


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FOR'ATION OF CALCI' SLPHOAL'INATES AND

AL'INOHYDRATE

"# & ec #& *

• Attringite !%Ft# formation:

• Fast reaction low strength and very high heat liberation

• +onosulfoaluminate !%Fm# formation:

• %fter the depletion of sulfates and increase in aluminate

ions %Ft reacts further to become %fm

• calcium aluminohydrate

J/g)1!2-H" HS#C2HHS3C#C32323

=∆→++

1243323 HS#3C4H#2CHS#C →++

hydroxide alumi$um-%erric

3

hydrate alumi$ateumtetracalci

134

water hydroxidecalcium

%erritealumi$o umtetracalci

4 &)H"#'&)H"#'CH14CH2#&C +→++


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F"*#t$& !"#t$& "!ct +e" te t$*e

!"#t$&


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'ICROSCOPIC STRCTRE OF CE'ENT HYDRATE

CO'POND

Attringite +onosulfoaluminate


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'ICROSCOPIC STRCTRE OF CE'ENT HYDRATE

CO'POND

'$c"t"ct"e C-S-H e,


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'ICROSTRCTRE OF CALCI' HYDRO6IDE OR

PORTLANDITE


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'ORPHOLOGY OF HYDRATION PRODCTS IN 7 DAYS

Boung et al. 41

Cement particle

coated with C-S-H

which is surrounded

by ettringite needles

)latelets of

monosulfoaluminate

Large crystals ofcalcium hydro,ide


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C-S-H %gglomerations of C-S-H

particles of 4-433 nm.

H

He,agonal crystals of

calcium hydro,idemonosulfoaluminate and

calcium aluminohydrate of

about 4 6m.

CCapillary pores and voids of

43 nm to 4 6m.

'$c"t"ct"e !"#te! ce*e&t #te

HEAT OF HYDRATION


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• Hydration process of cement is accompanied by heat generation

!e,othermic#.

• %s concrete is a fair insulator the generated heat in mass concrete may

result in e,pansion / crac"ing. 'his could be overcome by using suitable

cement type.

• It could also be advantages for cold weather concreting.

• 'he heat of hydration of =)C is on the order of 15-4&3 calgr.

• %bout 532 of this heat is liberated within 4-$ days / ;52 within ; days.

• y limiting C$S / C$% content heat of hydration can be reduced.

HEAT OF HYDRATION


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Stage I: *apid evolution of heat lasts about 45 minutes !C$ % and some C$S reaction#.

Stage II: (ormant period lasts until initial set occurs in & to @ hoursStage III: *apid reaction of C$S during the acceleration period with the pea" being reached

at about 1-43 hours much after final set at @-1 hours and hardening has begun

Stage I8: *ate of reaction slows down until steady state is reached in 4&-&@ hours

Stage 8: Steady state


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END

Concrete Hydration 04

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