Gypsum Plasters (CaSO4.2H2O)

• It is a naturally occurring mineral

• It is the raw material for a no. of cementing materials such asplaster of Paris, keenes plaster & estrich plaster

• On heating, Gypsum undergoes partial or complete dehydrationto give a series of products

150°C 600°C 800°C

CaSO4.2H2O → CaSO4.1/2 H2O → CaSO4 ↔ CaO + SO3

Hemihydrate Anhydrite

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Plaster of Paris

• PoP is formulated as the hemihydrate (CaSO4.1/2H2O) butcommercial plaster is a mixture of the hemihydrate,anhydrite & unchanged gypsum

• It is manufactured by calcining Gypsum at 150°C followedby grinding the product to a fine powder

• It forms a plastic mass on mixing with water & sets quicklywith slight expansion

• Hardening is due to hydration of hemihydrate & anhydriteto gypsum

• Uses- for making accurate plaster casting moulds, indoorwall plastering

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Keene’s plaster & Estrich plaster

Keene’s plaster is the anhydrite which is lesssoluble in water as compared to thehemihydrate & sets slowly

– Zinc sulphite, potassium sulphite, alum, sod.Carbonate are used to quicken the setting process

– USES- for plastering exterior walls and in flooring

Estrich Plaster- It is obtained by heating gypsumabove 800°C. It sets and hardens very slowly

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Cement

Cement is a lime based product exhibiting adhesive &cohesive properties

It is used for binding together coarse aggregates such asbricks, building blocks and stones

Portland cement is the most commonly used cement inthe construction industry

In 1824, Joseph Aspdin from Leeds (UK) coined"Portland" cement because it produced a concrete thatresembled the color of the natural limestone quarried(extracted) on the Isle of Portland (US)

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Manufacturing of Portland cement

Manufactured from a mixture of calcareousmaterials rich in lime (limestone) and argillaceousmaterials rich in silica and alumina (clay)Three distinct operationsMixing of raw materials- Dry or wet conditionsBurningGrinding

The chemical reaction involves the combination of basicoxide (CaO) derived from lime stone with acidic oxides ofAluminium, ferric oxide, and silica (from clay) to form Ca-silicates, aluminates, aluminosilicates etc.

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Rich in silica & alumina

1400-1500°C

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1400-1500°C

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Chemical composition of cement

• Expressed in terms of the Oxides of Ca, Si, Al, Fe as it is a mixture of many compounds

• Oxide constituents exist in the form of 4 major phases-

• Dicalcium silicate, 2CaO.SiO2 (C2S)• Tricalcium silicate, 3CaO.SiO2 (C3S)• Tricalcium aluminate, 3CaO.Al2O3 (C3A)• Tetracalcium aluminoferrite, 4CaO.Al2O3.Fe2O3 (C4AF)

• Free oxides CaO, MgO, Na2O, K2O, TiO2, SO3, H2O, CO2 constitute the remaining 10%

90% by weight

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Chemical composition of cement

As per BIS requirement• The ratio of percentage of alumina to that of iron

oxide should not be less than 0.66

• Ratio of percentage of lime to those of alumina,iron oxide and silica (lime saturation factor, LSF)-should not be less than 0.66 & greater than 1.02

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Setting & Hardening of Cement• On mixing with half its wt of water (gauging), it forms a plastic

pasty mass which gradually looses its plasticity & ultimatelybecomes hard

Setting process occurs in 2 stages

• Initial Setting- recognized based on the ability of a weighedneedle to penetrate the mass to a certain depth (5mm)under a set of prescribed conditions

• Initial setting time depends on the quantity of water as well astemp.

• Usually 45 mins for Portland cement (sufficient time for mixing,transportation & placing between aggregates during construction)

• Final Setting-Occurs over a few hours (10 h for Portland)11

Hardening of Cement

• It is a slow process

• Takes a few days during which crystallizationoccurs & the strength of cement aggregateincreases

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Hydration & hydrolysis reactions

• Setting & Hardening of Cement is due to the hydration &hydrolysis reactions involving its constituents giving rise tohydrated calc. silicate (CSH) in the form of poorly crystallizedgels called tobermolite gels

• Individual components of C2S, C3S, C3A & C4AF undergohydration releasing different amounts of heat of hydration

• Initial setting is attributed to C3S, C3A & C4AF which reactrapidly forming calcium hydroxide saturating the gaugingwater.

• C3S is the active hydraulic component of Portland cement.

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Typical Oxide Composition of a General-Purpose Portland Cement

(Mindess and Young, 1981)

Types of CementDifferent types based on the composition, properties and uses.

• Natural Cement

(Portland cement)-prepared by calcining naturally occurring mixture ofcalcareous and argillaceous materials.

• White Portland cement-

doesn’t contain iron oxide as it is prepared from the material free fromiron oxide

• Pozzolana cement

(Naturally occurring, volcanic origin) 45% of silica, 10-20%alumina, <10%ferric oxide and oxides of Ca, Mg, Na, K & Ti

– Artificial Pozzolana-is made by burning clay or from fly ash (product ofcombustion)

– Pozzolana cement is usually mixed with Portland cement15

Types of Cement• Water Proof Cement- Obtained by grinding portland cement with

calcium stearate.

• Slag Cement- highly resistant to sea water, obtained from blastfurnace slag (a by-product of iron and steel-making) mixed withCa(OH)2 & CaSO4

• Super heated cement & high alumina cement- highly resistant tosea water but with a decreased rate of hardening

– manufactured from blast furnace slag

• Portland blast furnace cement- Portland+slag mixture

• Barium and Strontium cements- obtained by replacing Ca with Barium or Strontium salts

– It is used for heavy concrete to serve as radiation shields for X-rays and nuclear radiations

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