What
is the History of Cement?
What is cement?
How it is manufactured?
Why is Gypsum added with clinker during cement production?
What is Setting Time?
Soundness (of cement)
Tri Calcium Silicate(C3S) & Di Calcium Silicate (C2S)
Tri Calcium aluminate (C3A)
Tetra Calcium Aluminoferrite (C4AF)
Magnesium Oxide ( MgO )
Free Lime ( CaO_free )
Litre weight
Fineness ( Blaine )
Gypsum
What is the History of Cement?
The origin of hydraulic cements goes back to ancient Greece(the period
following Mycenaean civilization, which ended in about 1200 BC, to the
death of Alexander the Great, in 323 BC) and Rome. The materials used
were lime and a volcanic ash that slowly reacted with it in the presence
of water to form a hard mass. This formed the cementing material of the
Roman mortars and concretes of 2,000 years ago.
What is cement?
In the general sense, cement is any material with adhesive properties.
The term cement is also commonly used to refer more specifically to
powdered materials which develop strong adhesive qualities when combined
with water. These materials are more properly known as hydraulic
cements.
ASTM defines Portland cement as "hydraulic cement (cement that not only
hardens by reacting with water but also forms a water-resistant product)
produced by pulverizing clinkers consisting essentially of hydraulic
calcium silicates, usually containing one or more of the forms of
calcium sulfate as an inter ground addition. Material that is produced
when a raw mixture of predetermined composition is heated to high
temperature. The low cost and widespread availability of the limestone,
shales, and other naturally occurring materials make Portland cement one
of the lowest-cost materials widely used over the last century
throughout the world. Concrete becomes one of the most versatile
construction materials available in the world.
Different types of Portland cement are manufactured to meet different
physical and chemical requirements for specific purposes, such as
durability and high-early strength.
How it is manufactured?
Proportioning involves determining the correct proportions of materials
to be combined to form the raw mix. After proportioning, the raw
materials for a dry process kiln are mixed to produce an aerated dry raw
meal.
There are four stages to this process — evaporation and preheating,
calcining, clinkering, and cooling.
Evaporation and preheating remove moisture and raise the temperature of
the raw mix preparatory to calcining.
Calcining takes place at 800-900˚C and breaks the calcium carbonate down
into calcium oxide and carbon dioxide which is evolved in the process.
Clinkering completes the calcination stage and fuses the calcined raw
mix into hard nodules resembling small grey pebbles.
Kiln temperatures in the burning zone range from 1350-1450˚C, and
retention
times in this zone are four to six seconds.
Clinkering is critical to the quality of cement and requires accurate
control of the energy input. Insufficient heat will cause the clinker to
be underburnt, containing unconverted lime, and reducing the hydration
(setting and hardening) properties of the resulting cement.
Finish milling is the grinding of clinker to produce a fine grey powder.
Gypsum (CaSO4) is blended with the ground clinker, along with other
materials, to produce finished Portland cement.
Why is Gypsum added with clinker during cement
production?
The gypsum controls the rate of hydration of the cement in the
cement-setting process. Amount corresponding to 1~3 % SO3 is added
during cement grinding to retard the setting time. It has favorable
affect on strength also. Large quantity leads to slow expansion of
cement.
What is Setting Time?
When cement is mixed with water, the cement starts stiffening and it
becomes unworkable. The process is known as setting of cement.
Soundness (of cement)
If cement expands during setting due to presence of excess free lime,
hydration of calcium Oxide, Magnesium Oxide. The cement cracks,
deteriorates and it shows stability of concrete when cement is used.
Tri Calcium Silicate(C3S) & Di Calcium Silicate
(C2S)
Compressive strength of up to 28 days increases with increase in C3S and
strength of 28 days to 6 months or later (ultimate strength) is linearly
related to C2S.
With increase in cement fineness C2S begins to produce its effects
earlier.
With high C3S content heat of hydration increases. C2S has relatively
low heat of hydration.
Tri Calcium aluminate (C3A)
C3A do not contribute to strength. Heat of hydration is high. Easily
attacked by Sulphate.
Increase in C3A reduces setting time. Very high C3A produce flash set as
in high alumina cement as
it readily reacts with water and liberate lot of heat. Due to its high
sulphate reactivity,C3A content is
kept minimum in Sulphate Resisting Cement (SRC).In low heat cement low
heat of hydration is
achieved by reducing C3A and C3S with corresponding increase in C2S.This
will result in reduction of early strength though not ultimate strength.
Tetra Calcium Aluminoferrite (C4AF)
C4AF do not affect strength or setting time. It has low heat of
hydration.
Magnesium Oxide ( MgO )
MgO content more than 4 % produce unsoundness due to expansion of
cement.
Free Lime ( CaO_free )
Insufficient burning and high content of lime in clinker result high
free lime in clinker which produce expansion of cement and affect
setting time.
Litre weight
Does not have any direct relation to the property of cement. For a given
kiln, it gives an indication of
burning and clinkerisation process.Excess burning results in high fuel
consumption and produce
high litre weight which is hard to grind.
Fineness ( Blaine )
Higher the fineness of cement increases the early strength of
cement,reduce setting time and
increase heat of hydration.
Gypsum
Amount corresponding to 1~3 % of SO3 is added during cement grinding to
retard the setting time. It has favorable affect on strength also. Large
quantity leads to slow expansion of cement.
Retardation of setting is not proportional to the amount of gypsum
added. |
