CONTINUATION…..

POZZOLANIC CEMENTS Pozzolanas, being a latent hydraulic material,

are always used in conjunction with Portland cement.

Like the high slag blastfurnace cement, pozzolanic cement has low 7-day strength (minimum of 12 MPa) but also low 28-day strength: minimum of 22.5 MPa.

SILICA FUME Silica fume is a relatively recent arrival

among cementitious materials. It was originally introduced as a pozzolana.

Silica fume is also referred to as microsilica or condensed silica fume, but the term silica fume has become generally accepted.

The specific gravity of silica fume is generally 2.20, but it is very slightly higher when the silica content is lower.

FILLERS Fillers can enhance the hydration of Portland

cement by acting as nucleation sites.

Fillers can be naturally occurring materials or processed inorganic mineral materials.

What is essential is that they have uniform properties, and especially fineness.

Because the action of fillers is predominantly physical, they have to be physically compatible with the cement in which they are included.

OTHER CEMENTS Anti-Bacterial Cement

It is a Portland cement interground with an anti-bacterial agent which prevents microbiological fermentation.

This bacterial action is encountered in concrete floors of food processing plants where the leaching out of cement by acids is followed by fermentation caused by bacteria in the presence of moisture.

OTHER CEMENTS Hydrophobic Cement

It deteriorates very little during prolonged storage under unfavorable conditions.

The hydrophobic properties are due to the formation of a water-repellent film around each particle of cement.

Hydrophobic cement is similar in appearance to ordinary Portland cement but has a characteristic musty smell.

OTHER CEMENTS Masonry Cement

It is used in mortar in brickwork, also made by integrinding Portland cement, limestone and an air-entraining agent, or alternatively Portland cement and hydrated lime, granulated slag or inert filler, and an air-entraining agent and other ingredients are usually also present.

Masonry cements make a more plastic mortar than ordinary Portland cement, they also have a greater water-retaining property and lead to lower shrinkage.

OTHER CEMENTS Expansive Cement

It has the property of expanding in its early life so as to counteract contraction induced by drying shrinkage.

OTHER CEMENTS Oil-Well Cement

It is used for grout or slurry to be pumped to depths of up to thousands of meters in the earth’s crust where temperature can exceed 150 °C (or 300 °F) and pressure can be 100 MPa (or 15 000 psi).

Essentially, oil-well cements have to have certain special features:

a. To have a particular fineness

b. To contain retarders or accelerators

c. To contain friction reducers

d. To contain lightweight additives

e. To lower the density of the grout or densifying additives

f. To increase the density of the grout

g. To contain pozzolana or silica fume

OTHER CEMENTS Natural Cement

This is the name given to cement obtained by calcining and grinding a so-called cement rock, which a clayey limestone is containing up to 25 per cent of argillaceous material.

Natural cements are rather variable in quality as adjustment of composition by blending is not possible.

WHICH CEMENT TO USE? First of all, no single cement is the best one

under all circumstances.

The choice depends on availability, on cost that important element in engineering decision-making and on the particular circumstances of equipment, skilled labor force, speed of construction and, of course, on the exigencies of the structure and its environment.

HIGH-ALUMINA CEMENT

This cement is very different in its composition, and also in some properties, from Portland cement and Portland blended cements so that its structural use is severely limited, but the concreting techniques are similar.

HIGH-ALUMINA CEMENT Manufacture

From the name of the cement, high-alumina it can be inferred that it contains a large proportion of alumina.

The one process of manufacturing of high-alumina cement is crushing bauxite into lumps not larger than 100 mm (or 4 in.).

Dust and small particles formed during this fragmentation are cemented into briquettes of similar size because dust would tend to damp the furnace.

HIGH-ALUMINA CEMENT Composition and Hydration

The main cementitious compounds are calcium aluminates of low basicity primarily CA and also C12A7. Other compounds are also present: C6A4.FeO.S and an isomorphous C6A4.MgO.S.

The hydration of CA, which has the highest rate of strength development, results in the formation of CAH10, a small quantity of C2AH8, and of alumina gel (Al2O3.aq).

HIGH-ALUMINA CEMENT Resistance to Chemical Attack

High-alumina cement was first developed to resist sulfate attack, and it is indeed highly satisfactory in this respect.

High-alumina cement is not attacked by CO2 dissolved in pure water.

It may be noted that, high-alumina cement stands up extremely well to sea water, this water should not be used as mixing water.

HIGH-ALUMINA CEMENT Physical Properties of High-Alumina Cement

A feature of high-alumina cement is its very high rate of strength development.

Concrete made with high-alumina cement, at a content of 400 kg/m3 (or 680 lb/yd3) and a water/cement ratio of 0.40, at 25 °C (77 °F), can reach a compressive strength (measured on cubes) of about 30 MPa (or 4500 psi) at 6 hours, and more than 40 MPa (or 6000 psi) at 24 hours.

 CONVERSION OF HIGH-ALUMINA CEMENT

The hydrate CAH10 is, however, chemically unstable both at higher and normal temperatures and becomes transformed into C3AH6 and alumina gel.

This change is known as conversion, because the symmetry of the crystal systems is pseudo-hexagonal for the decahydrate and cubic for the sesquihydrate, one can refer to it as the change from the hexagonal to cubic form.

REFRACTORY PROPERTIES OF HIGH-ALUMINA CEMENT

Between room temperature and about 500°C, high-alumina cement concrete loses strength to a greater extent but above about 1000 °C high-alumina cement gives excellent performance.

In this figure shows the behavior of high-alumina cement concrete made with four different aggregates at temperatures up to 1100 °C.

Strength Of High-alumina Cement Concretes Made With Different Aggregates As A

Function Of Temperature

REFRACTORY PROPERTIES OF HIGH-ALUMINA CEMENT

There is a gain in strength due to the development of the ceramic bond.

This bond is established by solid reactions between the cement and the fine aggregate, and increases with an increase in temperature and with the progress of the reactions.

Refractory concrete made with high-alumina cement has a good resistance to acid attack (e.g. acids in flue gases), the chemical resistance being in fact increased by firing at 900 to 1000 °C.