Expansive cement is a portland cement whose drying shrinkage is reduced by the addition of expanding agent and a stabilizer in controlled amount.
Expansive Portland Cement is basically introduced to reduce the shrinkage upon drying. There is no dedicated IS Standard that provides specifications for expansive cement.
To know how expansive cement works and where it is used, continue reading!
What is Expansive Cement?
Expansive cement falls under the category of hydraulic cement.
Portland cement undergoes shrinkage during the early stages of hydration after the setting of cement.
Now large, unrestrained expansions in cement paste can cause cracks in concrete. But, if the expansion is restrained, then its magnitude can be reduced. As a result, a prestress can be developed in concrete, which is beneficial.
Shrinkage Compensating Cement
- The magnitude of expansion is not large in shrinkage compensating cement.
- As a result, low prestress level is produced in this cement. It is only about 0.1-0.7 MPa, i.e. 15-100 psi
- The low prestress levels are just enough to counteract the tensile stresses developed from restrained drying shrinkage
- Use: Crack-free pavements and slabs
A restrained expansion is exhibited by this cement which induces compressive stresses.
Self Stressing Cement
- The magnitude of expansion is large in self stressing cement.
- As a result, high prestress level is produced in this cement. It is about 6.9 MPa, i.e. 1000 psi
- Use: In the production of chemically prestressed concrete elements.
Manufacture of Expansive Cement
Expansive cement has the following basic ingredients:
- Portland cement
- Expanding agent- A mixture of gypsum, bauxite, and chalk can give the expanding agent on burning. Calcium sulphate and calcium aluminate are formed by burning this mixture. These compounds can form calcium sulphoaluminate hydrate in presence of water. This calcium sulphoaluminate hydrate is also called ettringite.
- Stabilizer- Blast furnace slag can be used as stabilizer. It gradually consumes calcium sulphate and expansion ends with that.
Expansive cement expands upon hydration. Expansion in cement can be ensured by any one of the following phenomena:
- Formation of ettringite (calcium sulphoaluminate hydrate)
- Hydration of hard-burnt calcium hydroxide
Both the methods are tried and tested in manufacturing of the expansive cement.
Sulphoaluminate clinker is a modified portland cement clinker, which is added along with portland clinker in the expansive cement manufacturing.
Appropriate material and their controlled proportions should be taken to ensure the desired expansion.
For instance, sulphoaluminate clinker can consist of 8-20 parts, 100 parts of portland clinker, and 15 parts of stabilizer.
Types of Expansive Cement
Following types of expansive cement are available:
- Type K
- Type M
- Type S
- Type O
- Type E-1
Type K, M, and S cement are commonly used expansive cement types. Type O cement is modified in Japan.
Type K, M, and S cement are produced, but in United States, only Type K cement is commercially available.
ASTM C 854-04 has the specifications types of expanding cement under Type E-1. The classification is done based on the type of expansive agent used with portland cement and calcium sulphate.
1. Type K Expansive Cement
In type K expansive cement, grinding of sulphoaluminate clinker is done.
Sulphoaluminate clinker constituents are listed below:
- Principal cementitious compounds- C3S, C2S
- Considerable amount of- C4A3S, CS
It is essential to blend sulphoaluminate clinkers with ordinary portland cement clinkers. This is done to restrain the expansion of cement.
Stoichiometry Reaction-
C4AS + 8CS + 6CH + 90 H à 3 C6AS3H32
CH in the above reaction comes from the hydration of portland cement. Some free CaO is also present.
2. Type M Expansive Cement
Type M expansive cement is a mixture of the following:
- Portland cement
- Calcium aluminate cement (CA is the principal compound)
- Calcium sulphate
Stoichiometry Reaction-
CA + 3CS +2CH + 30 H → C6AS3H32
CH in the above reaction comes from the hydration of portland cement.
3. Type S Expansive Cement
Type S expansive cement has the following composition:
- Portland cement (very high C3A is present, about 20 %)
- Calcium sulphate (in large proportion)
Stoichiometry Reaction-
C3A + 3CS + 32 H à C6AS3H32
4. Type O Expansive Cemenet
The expansion occurs from the hard-burnt CaO in this type of cement.
Type O cement was originally developed in Japan by Onoda Cement Company.
Heat of Hydration for Expansive Cement
The properties of expansive cement are almost similar to that of OPC.
However, the resistance to sulphate attack of Type K is more in case of expansive cement. To achieve sulphate resistance, Type K cement shrinkage compensating cement can be blended with ASTM Type II or Type V portland cement. The sulphate resistance comes from the little reactive alumina or monosulphate produced after hydration.
On the other hand, Type M and Type S have compounds vulnerable to sulphate attack.
Advantages of Expansive Cement
- Shrinkage of cement upon hydration can be avoided
- A small expansion after hydration proves beneficial
- Type K expansive cement has increased resistance to sulphate attack
Disadvantages of Expansive Cement
- Usage of expansive cement requires skill and experience
- Accurate proportioning of materials taken properly is essential
Use of Expansive Cement
Expansion cement uses are
- It is used for grouting anchor bolts
- It is used for grouting machine foundations
- It is used for grouting pretressed concrete ducts
- It can be used to prevent water leakage
- It can be used to reduce the shrinkage cracks in concrete slabs, pavements, and other structure
Most common use of expansive cement is for grouting. If shrinkage occurs in grouting, then it goes against the purpose for which it is done. Hence, expansive cement is the suitable choice for this operation.
Application of Expansive Cement
- Grouting
- Prevention of drying shrinakge cracks
- Water retaining structures
- Repair of damaged concrete surface
Storage Criteria of Expanding Cement is the same as that of OPC. You can find them HERE.
Key Take Away
Definition: Expansive cement is cement in which expanding agent and a stabilizer are added in controlled proportions so that cement does not suffer notable change in volume upon drying.
Thus, cement that undergoes no overall volume change upon drying is the expansive cement.
Components: Expansive agent and stabilizer are the most important component of expanding cement besides usual components present in cement.
Shrinkage Compensating Cement: Steel reinforcement restrains the expansion inducing compressive stresses in concrete. These stresses can balance the tensile stresses induced during drying shrinkage.
Self Stressing Cement: Residual compressive stresses up to 7 MPa are present in concrete after the occurrence of drying shrinkage.
Employing expansive cement does not mean shrinkage is prevented in the cement. Instead, restrained early expansion is developed to counterbalance the drying shrinkage and subsequently prevent the shrinkage cracks.
FAQ
What is expansive cement used for?
Expansive cement is used for the following purpose:
- It can be used to reduce the shrinkage cracks in concrete slabs, pavements, and other structure
- It is used for grouting anchor bolts
- It is used for grouting machine foundations
- It is used for grouting pretressed concrete ducts
- It can be used to prevent water leakage
What is the meaning of expansive cement?
Expansive cement means a cement type that does not undergo considerable volume change upon drying. Most of the cement undergoes shrinkage owing to drying shrinkage. This is balanced by expansion caused by expanding agent.
Tensile stresses are produced during drying shrinkage. Compressive stresses induced by the expansive cement offset them.
What causes expansion in concrete?
Expanding agents, when added to cement causes its expansion upon hydration. But, this expansion needs to be restrained, hence stabilizer is also added. Unrestrained expansion can again lead to cracking in concrete.
Sulphoaluminate clinker can be used as expansive agent.