Important Notes on Cement for Civil Engineers

                                              CEMENT

Introduction

  • Popular as building material. 
  • Material with adhesive & cohesive properties. 
  • To bind the fine & coarse aggregate together. 
  • To fill voids in between fine & coarse aggregate particle form a compact mass.

Composition of Cement

  • Calcium Oxide = 60-65% 
  • Silica = 20-25% 
  • Aluminum Oxide = 4-8% 
  • Ferrous Oxide = 2-4% 
  • Magnesium Oxide = 1-3%

Types of Cement Mixture/Concrete

There are 3 types of mixture/concrete can be created using cement
that are:
 

1. Cement Grout  
       -A mixture of cement & water 
2. Cement Mortar
       -A mixture of cement & sand & water to form a paste. 
3. Cement concrete
       -A composite product which is obtained by mixing of cement,
        water, inert matrix of sand and gravel / crushed stone.

Portland Cement

Made by mixing substances containing Calcium Carbonate such as chalk/limestone, with substances containing silica , alumina and iron oxide such as clay/shale.
  • Clay/shale:
       Silica (silicon oxide) abbreviated S
       Ferrite (iron oxide) abbreviated F
       Alumina (aluminium oxide) abbreviated A

  • Limestone/chalk
      Calcium carbonate abbreviated C
 
  • Then the mixture is heated and became clinker. 
  • Clinker then grounded to powder. 
  • The hardening of Portland cement is a chemical process during which heat is evolved.

Q. Why is it called "Portland" cement?

Joseph Aspdin, an English Mason who patented the product in 1824, named it Portland cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel

Types of Cement

Depending upon our requirements i.e. using it at a suitable place, we use different types of cement.
  • Rapid Hardening or High early strength Cement 
  • Quick setting Cement 
  • High Alumina Cement 
  • Portland Slag Cement 
  • Low Heat Cement 
  • Air Entraining Cement 
  • White Cement 
  • Coloured Cement 
  • Portland Pozzolona Cement

Rapid Hardening Cement

This type cement gets the strength faster than OPC, However its
Initial and Final setting is same as those of OPC. It contains more
of Tri-Calcium Silicate and is more finely grounded. It gives out
more Heat while setting so it is as such unsuitable for massive
concrete. It is Used for the Structures which are Subjected to
loads early e.g. Roads, Bridges.

Quick Setting Cement

It sets faster than the Ordinary Portland Cement. Its Initial Setting
Time is 5 minutes and the Final Setting Time is not more 30
minutes. It is required for making concrete that is required to set
early as for lying under water or in running water. Initial setting
being very little there is always the danger of concrete having
undergone its initial setting. Thus this type of cement is used in
more special cases.

High Alumina Cement

It is manufactured by the burning of bauxite ore and lime stone in correct proportions and at high temperature. The resulting product is then ground finely. It develops Strength Rapidly. It is of black colour and resists well the attack of chemicals especially of sulphates and sea water. Its ultimate strength is much higher than OPC. Its initial setting takes more than 2 hours and the final set takes place immediately thereafter. Most of the heat it gives in the first 10 hrs as a result it can be conveniently used in freezing temperatures. At ordinary temperature it is used in thin layers.
 

Portland Slag Cement

It is obtained by mixing clinker, gypsum and granulated slag in a proper proportion. The Properties of this cement is very similar to that of OPC which are as under. It has lesser heat of hydration and has better resistance of soils, sulphates of alkali metals, alumina and iron. It has better resistance to acidic water. This type of cement is mostly used in Marine Works.
 

Low Heat Cement

The Heat Generated by cement while setting may cause the structure to crack in case of concrete. This Heat generation is controlled by keeping the percentage of Tri-Calcium Silicate and that of Tri-Calcium Aluminate low. Its initial setting and Final setting times are nearly the same as those of OPC. It is not very suitable for Ordinary structures because the use of cement will delayed time of drying. It will also need more curing.
 

Air Entraining Cement

It is the OPC mixed with some air entraining agents. The
common air entraining agents are oils, fats and fatty acids
etc. These materials have the property of entraining air in
the form of fine air bubbles. The bubbles render the
concrete to become more plastic, workable and more
resistant to freezing. However because of air entrainment
the strength of concrete reduces and as such the quantity
of air so entrained should not exceed 5%.
 

White Cement

It is the cement of pure white colour and having same
properties as those of Ordinary Portland Cement. Greyish
colour of cement is due to iron oxide. White cement
is manufactured from chalk and clay free from Iron Oxide.
Oil fuel and not the coal is used for the burning of this
cement.It is much more costly than ordinary cement.

Coloured Cement

Various coloured cement are prepared when required in special
cases. Suitable pigments are added with OPC to get red or
brown cement but for other colours 5-10% of desired pigments
are grounded with white cement. Pigments used should be
chemically inert and also durable so as they must not fade due to
the effect of lights sun or weather.

Portland Pozzolona Cement

Portland Pozzolona cement is produced by grinding together
Portland cement and Pozzolona. This cement has properties
similar to those of OPC and can therefore be used for all general
purpose. Portland Pozzolona cement produces less heat of
hydration and offers greater resistance to attack of aggressive
water or sulphates bearing than OPC. Portland Pozzolona
cement are particularly used in marine works. It takes a little
longer to gain strength. Ultimate Strength of this cement is more
than OPC.

HYDRATION

» It's a process of chemical reaction between
   cement and water.

» It results first in setting (the concrete become
   solid) and then hardening (increase of
   strength and stiffness).

» Heat is liberated during hydration process.

» Thus, during the hardening process, the
   concrete is being continually warmed by
   internal heat generated.

WHAT IS SETTING ?


» When cement is mixed with sufficient water, within 1 or 2
   hr after the mixing, the sticky paste losses its fluidity;
   within a few hours after mixing, noticeable stiffening
   commences.

» Setting can be divided to 2 stage that is:
   a) Initial Set                   b) Final Set

  • Initial set is when the paste begin to stiffen 
  • Final set is when the paste beginning to harden and able to sustain some loads. 
  • Initial Setting Time is the time lapse from the addition of water in the mix to the initial set. 
  • Initial Setting Time and Final Setting Time can be determine by using Vicat Apparatus in laboratory. 
  • They are measure at lab. As the time required for the cement paste to withstand a certain arbitrary pressure. 
  •  The time taken for a 1-mm diameter needle in the Vicat apparatus to penetrate a depth of 25mm into the cement past sample is the initial setting time. 
  • The final setting time is reached when in the modified Vicat apparatus only the needle penetrates the surface, while the attachment fails to do so. 
  • The rate of setting is also a measure of the rate of heat of hydration.

Test ofCement   

  • Consistency Test  

  • Compressive Strength Test

  • Tensile Strength Test




    Consistency Test

    It is used to determine the % of water required for
    preparing cement pastes for other tests.

    Procedure:
    i. Take 300g cement, add 30% or 90g of water
    ii. Mix water and cement on a non-porous surface. Mixing
        should be done.
     
    iii. Fill the mould of Vicat apparatus.

    The interval between the addition of water to the
    commencement of filling the mould is known as the time
    of gauging.


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