Engineering Materials 1

8/3/2019 Engineering Materials 1

1/15

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

1


2/15

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

2


3/15

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

3


4/15

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

4


5/15

Manufacturing of Portland cement

Three distinct operations

Mixing of raw materials- Dry or wet conditions

Burning

Grinding

5


6/15

6


7/15

7


8/15

8


9/15

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

9


10/15

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

10


11/15

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


12/15

Setting & Hardening of Cement

It is a slow process

Takes a few days during which crystallization

occurs & the strength of cement aggregate

increases

12


13/15

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,


14/15

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

14


15/15

15

Documents

Engineering Materials 1