ADMIXTURES IN CEMENT
Admixture is defined as a mineral or chemical sub- stance that is added to cement mortar or concrete to increase its strength, durability, resistance against wear, water proofing capability, etc.
TYPES OF ADMIXTURES IN CEMENT
1. Bonding mixtures
2. Plasticizers
3. Super Plasticizers
4. Antifungal admixtures
5. Colouring admixtures
6. Damp proofing and water proofing admixtures
7. Retarders
1 BONDING MIXTURES: This type of admixture is used for patching in old concrete works. It increases the strength and bond between old and new concrete. Natural and artificial rubber etc., are mixed to the cement concrete or cement mortar.
2. PLASTICIZERS: A plasticizer is defined as an admixture added to wet concrete mix to impart ade- quate workability properties. It is added to the fresh concrete to reduce the quantity of water to be used and thereby increase the strength of concrete.
3. SUPER PLASTICIZERS: Super plasticizers pro- duce extreme workability. They achieve reduction in water content without loss of work ability.
4. ANTIFUNGAL ADMIXTURES: It is mixed to the concrete to protect the surfaces from bacteria, fungi, Phenol, Dieldrin, etc., are used as antifungal admix- tures.
5. COLOURING ADMLXTURES: Natural and syn- thetic materials are used for producing various colours. Colouring pigments are added to cement in dry condition. It does not affect the strength and set- ting time of cement. To avoid the cracks on concrete surface, polypropylene fibres are added to concrete
while adding colouring pigments. Metallic oxide and mineral pigments are generally used as colouring admixtures.
6. DAMP PROOFING AND WATER PROOFING ADMIXTURES: Presence of hygroscopic moisture on a surface is called dampness. Damp proofing admixtures are added to cement concrete to reduce the water absorbing power of concrete. Aluminium and Zinc Sulphate, Aluminium and Calcium Chloride are used as water proofing admixtures. They increase the strength of concrete.
7. RETARDERS: Retarders are added to concrete to increase the setting time of cement and also to reduce the water cement ratio. These are important types of admixtures used in the ready mix concrete industry for the purpose of retaining slump loss. Calcium Sulphate is the commonly used retarder. Other admixtures used as retarders are lignosul- phonic acids and their salts and hydroxylates of car- boxylic acid and their salts.
