Engineering Materials 1
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Transcript of Engineering Materials 1
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Gypsum Plasters (CaSO4.2H2O)
It is a naturally occurring mineral
It is the raw material for a no. of cementing materials such as
plaster of Paris, keenes plaster & estrich plaster
On heating, Gypsum undergoes partial or complete dehydration
to give a series of products
150C 600C 800C
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 150C 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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Keenes plaster & Estrich plaster
Keenes plaster is the anhydrite which is less
soluble in water as compared to the
hemihydrate & 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 800C. 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 as bricks, building blocks and
stones
Portland cement is the most commonly used
cement in the construction industry
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Manufacturing of Portland cement
Three distinct operations
Mixing of raw materials- Dry or wet conditions
Burning
Grinding
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Chemical composition of cement
Usually 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 majorphases-
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 constitutethe 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), itforms a plastic pasty mass which gradually looses itsplasticity & ultimately becomes hard
The setting process occurs in 2 stages
Initial Setting- recognized based on the ability of aweighed needle to penetrate the mass to a certain depthunder a set of prescribed conditions
Initial setting time depends on the quantity of water as well as
temp. Usually 45 mins for Portland cement (sufficient time)
Final Setting-Occurs over a few hours (10 h for Portland)11
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Setting & Hardening of Cement
It is a slow process
Takes a few days during which crystallization
occurs & the strength of cement aggregate
increases
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Types of Cement Different types based on the composition, properties and uses.
Natural Cement (Portland cement)-prepared by calciningnaturally occurring mixture of calcareous and argillaceousmaterials.
White Portland cement- doesnt contain iron oxide as it is
prepared from the material free from iron oxide Pozzolana cement (Naturally occurring, volcanic origin) 45% of
silica, 10-20%alumina,
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Types of Cement
Water Proof Cement- Obtained by grinding portland cementwith calcium stearate.
Slag Cement- highly resistant to sea water, obtained fromblast furnace slag mixed with Ca(OH)2 & CaSO4
Super heated cement & high alumina cement- highlyresistant to sea 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 Cawith 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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