Different types of cement

Ordinary Portland cement
These are available in many grades , namely 33 grade , 43 grade, 53 grade etc If 28 day strength is not less than 33N/mm² then it  is called 33 grade cement. If 28 day strength is not less than 43N/mm² then it is called 43 grade cement. Use of higher grade cement offers many advantageous for making stronger concrete. Although they are little costlier than the low grade cement, they offer 10 to 20% saving in the cement consumption and also they offer many other hidden benefits. One of the most important benefits is the faster rate of development of the strength. Used for the ordinary works.
Rapid hardening cement
As the name indicate it develops the strength rapidly. This cement develops at the age of three days , the same strength as that expected of  Ordinary Portland cement at seven days. The rapid rate of development of the strength is due to the higher finess  and higher C₃S and lower C₂S. Used for the Road repair work, Early removal of the formwork, Cold weather concrete.
Sulphate resisting cement
Ordinary Portland cement is sucessible to the sulphate attack. Sulphate react with the free calcium hydroxide to form calcium sulphate and the hydrate of calcium aluminate to form calciumsulphoaluminates., the volume of which is approximately 227% of the volume of the original aluminates. Their expansion results in cracks. To remedy this the use of the cement with the low C₃A is recommended. Such cement with the low C₃A and content is known as the Sulphate resisting cement. Used for Marine condition, Foundation in soil infested with sulphates, Concrete used for the fabrication of pipes etc
 Quick setting cement
As the name indicates this type cement set quickly. This property is brought out by reducing the gypsum content at the time of the clinker grinding. This cement is required to mix, place and compacted very easly. Used for the underwater construction.
Low heat cement
Hydration of the cement is exothermic process which liberates high quantity of the heat. This will cause the formation of the cracks. A low heat evolution is brought by Reducing the C₃A and C₃S which are the compounds evolving the greater heat of hydration and increasing C₂S. Rate of evolution of heat of hydration will therefore will be less and evolution of heat will extend over a large period. Therefore Low heat cement rate of the development of the strength is very low. Used for the mass construction works
 Portland Pozzolona cement
Portland Pozzolona cement is manufactured by intergrinding OPC clinker with 10 to 25% of the Pozzolona material. Portland Pozzolona cement produces low heat of hydration and offer greater resistance to the attack of the aggressive water than OPC. Used for the mass construction works, marine and hydraulic works.
 Air entraining cement
This cement is manufactured by mixing small amount of the air entraining agent with the OPC clinker at the time of grinding. At the time of mixing this cement will produce air bubbles in the body of the concrete which will modify the properties of the plastic concrete with respect to the workability, segregation and bleeding
Coloured cement
Coloured cement consists of the Portland cement with the 5 to 10 % of the pigment. The cement and the pigment is grinded together.

General features of the main types of Portland cement

	Classification	Characteristics	Applications
Type I	General purpose Ordinary Portland cement	Fairly high C3S content for good early strength development	General construction (most buildings, bridges, pavements, precast units, etc)
Type II	Moderate sulfate resistance	Low C3A content (<8%)	Structures exposed to soil or water containing sulfate ions
Type III	High early strength    Rapid hardening cement	higher finess  and higher C3S and lower C2S Increased Lime content	Rapid construction, cold weather concreting
Type IV	Low heat of hydration (slow reacting)	Reducing content of C3S (<50%) and C3A and increasing C2S	Massive structures such as dams.  Now rare.
Type V	High sulfate resistance	Very low C3A content (<5%)	Structures exposed to high levels of sulfate ions
White	White color	No C4AF, low MgO	Decorative (otherwise has properties similar to Type I)

    

Important Notes on Setting Time Test of Cement

o    It is essential that cement set neither too rapidly nor too slowly. In the first case there might be insufficient time to transport and place the concrete before it becomes too rigid. In the second case too long a setting period tends to slow up the work unduly, also it might postpone the actual use of the structure because of inadequate strength at the desired age.
o    Setting should not be confused with hardening, which refers to the gain in mechanical strength after the certain degree of resistance to the penetration of a special attachment pressed into it.
o    Setting time is the time required for stiffening of cement paste to a defined consistency.
o    Indirectly related to the initial chemical reaction of cement with water to form aluminum-silicate compound.
o    Initial setting time is the time when the paste starts losing its plasticity.
o    Initial setting time test is important for transportation, placing and compaction of cement concrete.
o    Initial setting time duration is required to delay the process of hydration or hardening.
o    Final setting time is the time when the paste completely loses its plasticity.
o    It is the time taken for the cement paste or cement concrete to harden sufficiently and attain the shape of the mould in which it is cast.
o    Determination of final setting time period facilitates safe removal of scaffolding or form.
o    During this period of time primary chemical reaction of cement with water is almost completed.


Setting and Hardening of Cement
Setting  Process: The process of losing plasticity  with time  and becoming dense, which is formed by mixing the cement and water.   It happened in two stage initial setting and final setting.
Hardening Process: The process of forming hardening cement paste, which loses plasticity completely and its strength increases with time.
Factors such as fineness and types of cement, blend of gypsum, age, water content,  temperatureand humidity which influence setting and hardening of Portland cement.
Setting is important in concrete work to keep fresh concrete plastic for enough time which helps the processes of transporting, casting, and compaction. 
There are four stage occurs during setting of cement, First : when  mixing cement with water, a rapid heat evolution, lasting a few minutes,  This  heat evolution is probably due to the reaction of  cement compounds  then,  heat evolution ceases quickly.  Second:  this stage called “dormant period” and last (1-4) hours. Also cement particles start to form initial layer of hydration product. Bleeding and sedimentation appear in this stage. Third:  Next heat  evolution  is on account  of dissolution of weak gel which formed first on surfaces of C3S  crystals so water will reach these surfaces and starts to form new gel. This stage last about 6 hours. Forth:  in this stage cement begin to harden and gain strength.
False and flash Setting
False setting  is the rapid  development  of rigidity  in freshly mixed Portland cement paste, mortar, or  concrete  occurs after few minutes of mixing cement with water without the generation of much heat. It can be controlled by re- mixing without additional water or strength loss.
Causes of false setting are:
1.  Drying of gypsum: when gypsum (CaSO4.2H2O)  gridding with hot clinker it loss 75% of its water and formed (CaSO4.1/2 H2O) and if temperature of clinker increase gypsum loss all water in its composition and formed CaSO4. When water add to cement CaSO4 react causes rapid setting.
2.  Bad storage: alkalis in cement react with carbon dioxide forming  Alkali carbonate  which reacts with calcium hydroxide forming CaCO3 causing setting of cement paste.
3.  Activate effective C3A exposed to humidity: during bad storage water  adhere on cement particles surface and during mixing these active surface combined rapidly with water within minutes. 

Flash setting  is a  rapid development  of rigidity in  freshly mixed  Portland  –  cement  paste, mortar, or  concrete. Further  mixing can’t dispel  this rigidity,  and a large  amount of heat is produced in the  process. It happened  Due to rapid reaction of aluminates  –  when  insufficient sulfate present  so to prevent It  A small amount of gypsum (CaSO4·2H2O)  gridding  with the cement clinker retards the hydration reaction of tricalcium aluminate so that the calcium silicates can set firstly.
Soundness of Cement
It refers to the ability of a hardened cement paste to retain its volume after setting without delayed destructive expansion. This destructive expansion is caused by excessive amounts of free lime (CaO), (SO3) or magnesia (MgO).
The hydrating speed of the  over burnt  CaO or MgO is slow, CaO or MgO begins to hydrateafter cement hardening and causes the hardened cement to expand and crack.
When added too much, gypsum continues to react with calcium aluminate hydrate to form calcium sulfoaluminate hydrate, whose volume increase 1.5 times as big as gypsum and causes the hardened cement paste to crack. At this time, sulfoaluminate hydrate is called cement bacillus.

Hydration of cement
•  When Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical  reaction with water is called "hydration". Each one of these reactions occurs at a different time and rate. Together, the results of  these reactions determine how Portland cement hardens and gains strength.
•  Hydration starts as soon as the cement and water are mixed.
•  The rate of hydration and the heat liberated by the reaction of each compound is different.
•  Each compound produces different products when it hydrates.
•  Tricalcium silicate (C3S). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of C3S will exhibit higher early strength.
•  Tricalcium aluminate (C3A). Hydrates and hardens the quickest. Liberates  a large amount of heat almost immediately and contributes somewhat to early   strength. Gypsum is added to Portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause Portland cement to set almost immediately after adding water.
•  Dicalcium silicate (C2S). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.
•  Tetracalcium aluminoferrite (C4AF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to C4AF.