Mechanism of Sulphate Resistance

7/31/2019 Mechanism of Sulphate Resistance

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PR E SE N T ER : A B B A S E L T AH I R E L SA K H I

L A BO R AT O R Y SU P E R V I SO R / CH E M I ST

Q AT A R N A T I O N A L CE M E N T C OM P AN Y

M ECH ANI SM OFSULPH ATE RESI STANCE


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Cement concrete is the most widely used buildingmaterial due to its satisfying performance in strengthrequirements. But when dealing with durability

aspects of concrete, the chemical attack which resultsin,

Deteriorationof concrete.

Cracking ofconcrete.

Volumechange.


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M echanism of Sulphate Attack

Calcium hydroxide & alumina bearing phase of hydrated cement

vulnerable to attack by Sulphate ions, Ettringite & Gypsum are primary

products of the chemical reaction between a Sulphate bearing solutionsand cement hydration products. Failure by expansion of concrete in

presence of Sulphate is due to the formation of Ettringite.

The attack of Sulphate on concrete is due to two principal actions, thereaction of either Sodium or Magnesium Sulphate with calcium

hydroxide Ca(OH)2, to form Gypsum, [Eq. (1) & (2)], and the reaction

of formed Gypsum with Calcium Aluminate hydrated to form Ettringite

[Eq. (3)].


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1. Na2SO4 + Ca(OH)2 + 2H2O CaSO4.2H2O + 2NaOH

2. MgSO4 + Ca(OH)2 + 2H2O CaSO4.2H2O + Mg(OH)2

3. 3CaO.Al2O3.12H2O + 3[CaSO4.2H2O] + 13H2O 3CaO.Al2O3.3CaSO4.31 H2O

4. MgSO4 + C-S-H CaSO4.2H2O + M-S-H

1. Na2SO4 + Ca(OH)2 + 2H2O CaSO4.2H2O + 2NaOH

2. MgSO4 + Ca(OH)2 + 2H2O CaSO4.2H2O + Mg(OH)2

3. 3CaO.Al2O3.12H2O + 3[CaSO4.2H2O] + 13H2O 3CaO.Al2O3.3CaSO4.31 H2O

4. MgSO4 + C-S-H CaSO4.2H2O + M-S-H


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Factors Affecting Sulphate Resistance

Parameters related to material properties such as: cement typeand its content in the mix, mineral admixture type, degree of

hydration and curing conditions.

Parameters related to hydrated concrete properties such as: pore

structure, permeability, diffusivity and mechanical properties.

Par ameter s r elated to exposur e conditi ons such as: types ofions associated with Sulphate, concentration of Sulphate,time of exposure, and temperature of exposure.


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Effect of SomeParameters on Sulphate

Attack

Type ofCement

CuringConditions

Types ofSulphateSulphate

Concentration

Presence ofchloride with

Sulphate

Temperatureof Exposur e


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Type of Cement:

Sulphate attack depend on the availability of Calcium hydroxideCa(OH)2 & C3A. Cement that has low C3A has good Sulphateresistance. Also high content of cement in the concrete increase its

Sulphate resistance.

Blending of mineral admixture such as: ground granulated blast furnaceslag (G G B F S), fly ash (F.A), Silica fume (S.F) ..Etc, in cementincrease the resistance to Sulphate attack.

The superior performance of blended cement over plain cementconcrete is attributed to the Pozzolanic reaction that consumes theCalcium hydroxide and to the dilution of Calcium Aluminate hydratesphase. Due to reduction in the quantity of plain cement in the totalbinder.


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Cur ing Conditions: Curing conditions of the concrete influence the hydration process.

For full hydration and strength development, longer and moistcuring is required. It is observed that there is a beneficial effect of

short initial air curing as apposed to continuous moist curing, onlong-term Sulphate resistance of plain and blended concrete. Thereasons for such behavior could be:

Initial dry curing reduces Ca(OH)2 concentration in the surface

zone. Initial dry curing do little carbonation in surface zone and reduce

the availability of Ca(OH)2 for Sulphate attack.


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Types of Sulphate: Deterioration rate of concrete under Sulphate attack also depends on

the types of Sulphate. Results of Sodium and Magnesium Sulphatesolution attack on mortar and concrete of plain cement type - I and

type - V, Fly ash, Silica-fume and ground granulated blast furnace slagblended cement are shown on table 1

Cement typeStrength reduction in % Expansion in %

In Na2SO4 In MgSO4 In Na2SO4

OPC 39 62 0.104 0.046

SRC 34 53 0.113 0.031

OPC +20% F.A 22 65 0.111 0.056

OPC +10% S.F 9 72 0.082 0.096

OPC +70% GGBFS 18 75 0.099 0.067


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Sulphate Concentr ation

Deterioration of concrete by Sulphate attack depends on concentration

of Sulphate in exposure solution. Higher concentration of Sulphate

leads to quick deterioration. Mortar cubes of plain and blended cementwere exposed to SO4

-- solution of varying concentrations (1.0, 1.5, 2.0,

2.5 & 4.0%), for up to 24 months. Degree of deterioration was

evaluated by strength reduction. It was found that the rate of

deterioration increased with increase in SO4-- concentration for both

plain and blended cement. The sample prepared with Fly Ash showedless deterioration compared to others, for all concentration and all times

of exposure , the highest strength reduction was in type I.


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Presence of chlor ide with Sulphate

At high concentration of chloride, the protective alkaline layer on steelreinforcement of concrete is broken, and in the presence of Oxygen & humidity,steel reinforcement gets corroded. The presence of chloride in Sulphate solutionaffects the deterioration of concrete under Sulphate attack. Study was made to seethis effect on plain and blended cement mortar by exposing the mortar to 2.10%

SO4-- solution & mixture of 2.10% SO4-- solution plus 15.70% Chloride solutionfor 365 days, the results showed that the deterioration was relatively more in pureSulphate solution. Increase in Sulphate resistance of cement in presence of chlorideattributed to;

Transformation of Aluminate hydrate phase into Chloro-Aluminate phase,there by reducing the quantity of Ettringite formed.

The rate of diffusion of chloride ions being much higher than that ofSulphate ions, allow he chloride to react with C3S to form Calcium ChloroAluminate hydrated as a result, quantity of C3S available for Sulphate ionsto react with and form Ettringite is reduced


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Temperature of Exposure

As a fact that increasing the temperature of the

Sulphate solution, accelerated the Sulphate attack inall types of mortars


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Mechanism of Sulphate Resistance

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