hydration of crystalline mgo and cao, corrosion

8/3/2019 Hydration of Crystalline MgO and CaO, Corrosion

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HYDRATION OF

CRYSTALLINE MgO and CaO

&

CORROSION prepared by Hikmet ERDİL



A review of Types of chemical

reactions responsible for concrete

deterioration.

part C shown in the figure indicates the corrosion of steel in concrete, hydration of crystalline MgO andCaO.



Deterioration of Concrete by

Chemical Reactions

chemical interactions between the aggressive agents in theenvironment and the constituents of the cement paste iscalled as the deterioration processes that caused by chemicalreactions.

Reactions Involving the Formation of Expansive Products

These chemical reactions can lead to harmful effects.Expansion may take place without any damage to concrete,

but the increasing of the internal stress manifests itself by closing of the expansion joints, deformations, anddisplacements in different parts of the structure, cracking,spalling, and pop-outs



We will deal with the two of the four phenomenaassociated with expansive chemical reactions:

hydration of free CaO and MgO

corrosion of steel in concrete.

(the other phenomena are sulfate attack, alkali-aggregateattack)



Hydration of Crystalline

MgO and CaO



Waters from the ground, lakes, and rivers contain smallamounts of chlorides, sulfates, and bicarbonates of calciumand magnesium. These so-called hard waters generally do not

attack the constituents of the portland cement paste. Pure water (from condensation of fog or water vapor) and

soft water from (rain or from melting of snow and ice)contain little or no calcium ions. When these waters come

into contact with portland cement paste they tend tohydrolyze or dissolve the calcium-containing products.

The contact solution attains chemical equilibrium, further

hydrolysis of the cement paste stops.

Hydrolysis of the cement paste

components



However, in the case of flowing water or seepage underpressure, dilution of the contact solution will take place, thusproviding the condition for continuous hydrolysis.

Calcium hydroxide is one of the constituent of hydratedportland cement pastes which, because of its relatively highsolubility in pure water (1230 mg/l), is most susceptible tohydrolysis.

CaO (lime) + H2O(water) → Ca(OH)2 (calcium hydroxide)

Magnesium hydroxide is generally low, and it rises with low temperature and large specific surface area

MgO(periclase) + H2O→ Mg(OH)2(magnesium hydroxide)



The expansive effect of high

MgO in cement was firstrecognized in 1884 when anumber of concrete bridgesand viaducts in France failed

two years after theconstruction with the cementcontained 16 to 30 percentMgO

About the same time, the

town hall of Kassel inGermany had to be rebuiltas a result of expansionand cracking attributed to

crystalline MgO in cement with the cementcontained 27 percentMgO,

History Cases

This led to restrictions on the maximum permissible MgO incement. ASTM Standard Specification for Portland Cement(ASTM C 150-83) requires that the MgO content in cementshall not exceed 6 percent.



Although expansion due to hydration of crystallineCaO has been known for a long time in the UnitedStates, the deleterious effect associated with the

phenomenon was recognized in the 1930s whencertain 2- to 5-year-old concrete pavementscracked. Initially suspected to be due to MgO, the

expansion and cracking were attributed later to thepresence of hard-burnt CaO in the cement used forthe construction of the pavements.

Laboratory tests showed that the cement pastesmade with a low-MgO portland cement, whichcontained 2.8 percent hard-burnt CaO, showed

considerable expansion



The phenomenon is unknown with modern concretebecause control of quality of Portland Cement clinker

have assured that the content of uncombined or free CaOin clinker exceeds 1 percent.

The crystalline MgO in a PC clinker that has beenexposed to 1400 to 1500°C is essentially inert to moistureat room temp. because the reactivity of crystalline MgOdrops sharply when it is heated above 900°C.

No cases of structural distress due to the presence of

crystalline MgO in modern Portland Cements arereported from countries, where raw material limitationscompel some cement producers to manufacture portlandcements containing more than 6 percent MgO.



Corrosion



Corrosion of Embedded Steel in

Concrete

Corrosion occurs when two different metals, ormetals in different environments, are electrically connected in a moist or damp

concrete.

The resulting rust occupies a greater

volume than the steel. This expansioncreates tensile stresses in the concrete,

which can eventually cause expantion, cracking,

and spalling.



Steel, is thermodynamically unstable under normalatmospheric conditions and will release energy andrevert back to its natural state — iron oxide, or rust. This process is called corrosion.

For corrosion to occur, three elements must bepresent:

1. There must be at least two metals (or two locationson a single metal) at different energy levels

2. an electrolyte

3. a metallic connection



Corrosion will occur when:

1. Steel reinforcement is in contact with an

aluminium conduit.2. Concrete pore water composition varies

between adjacent or along reinforcing bars.

3. Where there is a variation in alloy compositionbetween or along reinforcing bars.

4. Where there is a variation in residual or applied

stress along or between reinforcing bars.5. Where there are imposed stray electrical

currents.



Concrete acts as the electrolyte, and the metallic

connection is provided by wire ties, chairsupports, or the rebar itself.

Electrochemical process of steel

corrosion in concrete

Corrosion of reinforced concrete



Corrosion is an electrochemical processinvolving the flow of charges (electrons andions). At active sites on the bar, called anodes,

iron atoms lose electrons and move into thesurrounding concrete as ferrous ions. Thisprocess is called the anodic reaction , and is

represented as:2Fe → 2Fe2+ + 4e-



The electrons remain in the bar and flow to sitescalled cathodes, where they combine with water

and oxygen in the concrete. The reaction at thecathode is called a the cathodic reaction or

reduction reaction

A common reduction reaction is:2H2O + O2 + 4e- → 4OH-

To maintain electrical neutrality, the ferrous ions

migrate through the concrete pore water tothese cathodic sites where they combine to form

iron hydroxides or rust

2Fe2+ + 4OH- → 2Fe(OH)2



This initial precipitated hydroxide tends to reactfurther with oxygen to form higher oxides. Theincreases in volume as the reaction products

react further with dissolved oxygen leads tointernal stress within the concrete that may besufficient to cause cracking and spalling of the

concrete cover.



Volumetric changedepending on the oxidation state, the corrosion of

metallic iron can result in up to six times increasein the solid volume.



Absence of Chloride Ions in

Solution

the protective film on steel is reported to bestable as long as the pH of the solution staysabove 11.5.

Under some conditions (e.g., when concrete hashigh permeability and alkalies and most of thecalcium hydroxide have either been carbonated or

leached away), the pH of concrete in the vicinity of steel may have been reduced to less than 11.5. This would destroy the passivity of steel and set

the stage for the corrosion process.



Presence of Chloride Ions in

Solution depending on the Cl−/OH− ratio, it is reported

that the protective film is destroyed even at pH values considerably above 11.5.

when Cl−/OH− molar ratio is higher than 0.6, steelis no longer protected against corrosion probably because the iron-oxide film becomes either

permeable or unstable under these conditions. The chloride content to initiate corrosion is

reported to be in the range 0.6 to 0.9 kg Cl− per

cubic meter of concrete.



Corrosion Potential

The corrosion potential of the steel in reinforcedconcrete can be measured as the voltagedifference between the steel and a referenceelectrode in contact with the surface of theconcrete. Half-cell measurements may be maderelatively easily, using only a high impedance

voltmeter and a standard reference electrode,such as a copper-copper sulfate electrode.



System for measuring the halfcell

potential



The potential recorded in the half-cellmeasurement can be used to indicate the

probability of corrosion of the steelreinforcement



Results affected by

Degree of humidity in concrete. Themeasurement is very sensitive to the humidity existing in the concrete. More negative

potentials result for concrete with higher degreeof saturation.

Stray currents. The presence of stray currents

will significantly affect the measurements of thehalf-cell potential.



Oxygen content near the reinforcement. Thelack of oxygen near the reinforcement results inmore negative potentials as compared to moreaerated zones.

Microcracks. Localized corrosion can begenerated by microcracks, which also modify theconcrete resistivity, consequently affecting the

corrosion potential measurement.



Diagrammatic representation of the

cracking-corrosion-cracking cycle



Can corrosion be avoided in reinforced concrete?

-Yes if:

(a) Concrete is always dry, then there is no H2O toform rust. Also aggressive agents cannot easily diffuse into dry concrete.

(b) Concrete is always wet, then there is no oxygento form rust.

(c) Cathodic protection is used to convert all thereinforcement into a cathode using a battery. This

is not easy to implement because anodic mesh isexpensive, and this technology is not easy to installand maintain.



(d) A polymeric coating is applied to theconcrete member to keep out aggressive agents.

These are expensive and not easy to apply andmaintain.

(e) A polymeric coating is applied to the

reinforcing bars to protect them from moistureand aggressive agents. This is expensive andthere is some debate as to its long- termeffectiveness.

(f) Stainless steel or cladded stainless steel isused in lieu of conventional black bars. This ismuch more expensive than black bars.

C id i ?



Can we avoid corrosion?

- No, not entirely:

Concrete is not usually under water or continuously dry. Aggressive agents such as carbon dioxide, de-icing agents and/or sea water can diffuse into the bestof moist concrete, and corrosion will eventually result.

If corrosion cannot always be avoided andeconomical solutions are required, the effects of corrosion can be minimized by making a better

concrete. For example; Fly Ash added to a low w/cconcrete produces a much enhanced corrosionresisting structure with no significant increase in cost.



Corrosion damages



Corroded reinforcing steel in

concrete pile



Corroded reinforcing steel in

concrete deck

hydration of crystalline mgo and cao, corrosion

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