Engineering Chemistry S Datta

418 ENGINEERING CHEMISTRY
These types of superconductors are used in superfast magnetically leviated (maglev)
train i.e., superfast floating train. This is an example of application ceramic superconductor
like YBa 2
Cu 3
O 7–y
.
CEMENT
The present day civilization very much depends upon lime, mortar and cement products, viz.
reinforced-concrete walls and girders (a chief beam), tunnels, dams and roads etc. The
tremendous application of these products is due to their:
• Convenience
• Cheapness
• Adaptability
• Strength
• Durability
Man had early discovered certain natural rocks. The rocks on calcination gave a product
that hardened on the addition of water. Cement was first used by the Egyptians in constructing
the pyramids. The Greek and Romans used volcanic tuff for cement and a number of these
structures are still standing. In 1824, an Englishman, Joseph Aspdin, patented an artificial
cement made by the calcination of an argillaceous lime stone (argil-clay). He called this ‘Portland’
as concrete made from it resembled a famous building stone obtained from the Portland island
near England. This was the start of the Portland cement industry as we know it today. The
hard clinkers (Brick type) resulting from the burning of argilaceous limestone is known by the
term Portland cement to distinguish it from natural or Pozzuolana and other cements.
The cement will set under water and therefore it is known by the name of Hydraulic
cement. A cement is a substance, which sets to a hard mass in presence of water. Two types of
cements are in use.
• Roman cement
• Portland cement.
Of these, Portland cement is now manufactured on a large scale. Portland cement is
made by mixing calcareous (lime-stone) and argilaceous materials in the proper ratios and
heating to the point of incipient fusion in a kiln and grinding the resulting clinker. The raw
materials should be so proportioned as to produce a cement in which the ratio of the amount of
CaO to the amount of (2.8 SiO 2
+ 1.2 Al 2
O 3
+ 0.65 Fe 2
O 3
) should not be more than 1.02 and not
less than 0.66 i.e.,
CaO
0.66 ≤
≤ 1.02
2.8 SiO + 1.2 Al O + 0.65 Fe O
2 2 3 2 3
The Portland cement is a mixture of the following compounds:
Compounds Formula Abbreviation
(i) Tricalcium aluminate 3 CaO . Al 2
O 3
C 3
A
or (CaO) 3
. Al 2
O 3
(ii) Tricalcium silicate 3 CaO . SiO 2
C 3
S
or (CaO) 3
. SiO 2
(iii) Dicalcium silicate (CaO 2
) . SiO 2
C 2
S
or 2CaO . SiO 2
(iv) Tetracalcium alumino ferrate (CaO) 4
Al 2
O 3
. Fe 2
O 3
C 4
AF
or 4CaO . Al 2
O 3
. Fe 2
O 3