Cement is a broad category of binding material having a plastic consistency, adhesive properties, and a hydraulic setting action. There is a wide range of cement available in the Indian market. Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC) are the most commonly used types of cement, which is a powder made from crushing the clinker formed from burning lime and clay. It sets quickly when the powder is mixed with water. When the paste is mixed with aggregates (sand or gravel), concrete is formed. The components of cement includes Lime (60-65% ), Silica(17-25%), Alumina (3-8%), Magnesia (0.5-4%), Iron oxide : (0.5-6%), Calcium Sulfate (0.1-0.5%), Sulfur Trioxide(1-2%), Alkaline(0-1%).
List Of Contents :
1. Technical Terms To Know When Buying Cement
Hydration Of Cement : When Portland cement is mixed with water, its molecules combine with the water molecules to form crystals. This results in the interlocking of these crystals with the other aggregate added to the mix. The cement has an early initial setting time of not more than 45 minutes and a final setting time of not more than ten hours. Continued reaction leads to further hardening, where the early stages are rapid but get slower over time.
Setting Time : The time taken for the thickening of cement paste to a defined consistency is called setting time. Cement should neither set too rapidly nor too slowly. If it sets too fast, time for transportation becomes insufficient, and it ends up being placed and compacted before it becomes rigid enough. Whereas, if the setting time is too low, it will slow down the work and postpone the actual use of the structure due to its inadequate strength.
Initial Setting Time : Initial setting time is the time when the cement paste starts to lose its plasticity. It affects the transportation, placing, and compacting of cement concrete. Initial setting duration is meant to delay the process of hydration or hardening.
Final Setting Time: Final setting time is the time when the cement paste completely loses its plasticity and no longer reacts with water. It determines the time taken for the cement to harden and take the shape of the mould. It also determines the right time to remove the scaffolding safely without collapsing.
Hardening Time : Hardening is different from setting. Hardening refers to the gain in mechanical strength after a certain degree of resistance.
2. Types Of Cement Commonly Used in India
Ordinary Portland Cement (OPC): Ordinary Portland Cement is the hydraulic binding substance made by blending Portland cement clinker, 6% to 15% blended materials, and a certain amount of gypsum. It is also referred to as "ordinary cement." It has a moderate rate of strength gain, drying shrinkage, and resistance to cracking.
Portland Pozzolana Cement (PPC): When OPC cement is produced in a certain ratio with pozzolanic minerals, an integrated cement called Portland Pozzolana cement is created. A pozzolanic substance is one that combines reactive silica and aluminium, which can be in either natural or artificial form. The rate of development of strength is lower than that of OPC. The advantages of using this cement are its increased workability, water tightness, greater resistance to cracking, and greater resistance against sulphate action. It also has a lower cost than OPC.
Sulphate Resistant Cement (SRC): SRCs reduce the amount of sulphate salts in the cement by reducing Tricalcium Aluminate up to 5% and Calcium Aluminate (2C3A) and Tetracalcium aluminoferrite (C4AF) up to 25%. When the presence of sulphates is greater than the prescribed limit, SRC should be used. To procure this cement, special orders have to be placed with the cement manufacturer.
3. Other Types Of Cement Available In The Indian Market
Portland Slag Cement (PSC):
It is used in the construction of high-rise structures and in harsh climates. Buildings and bridges can have a lifespan of 100 years due to the use of high-strength concrete. Additionally, marine applications and infrastructure for wastewater treatment are built using this cement.
White Cement:
For projects meant to retain water, such as dams, bridge abutments, retaining walls, slabs, and piers, this type of Portland cement is applied.
Low heat Portland cement:
This cement type is utilised in water-retaining structures such as dams, bridge abutments, retaining walls, slabs, and piers, among others. The cement is used as it does not form cracks and shrink.
Rapid hardening cement:
This type of cement is used for repair and to speed up the construction process. It sets more quickly and gains strength pretty fast.
Quick Setting Cement (QSC):
When compared to typical cement systems, quick setting cement (QSC) generates a rapid compressive strength and considerably slows down the waiting on cement (WOC) time.
4. Difference Between PPC and OPC Cement
Pozzolana cement attains strength slowly, and Ordinary Portland Cement attains strength very quickly. But both types of cement attain the same strength at the end of 90 days. It is better for plastering and also resists sulphate attack. It emits less heat, produces fewer cracks, and improves impermeability and strength.
5. Drawbacks Of Portland Cement
Although Portland cement is advantageous for its great resistance to shrinkage and cracking, its short curing period, and its early initial setting period, it is criticised on the following grounds:
Irreversible: Once the cement settles, it cannot be reversed back to its original state. Thus, it leads to high waste and pollution.
Unnecessary Strength: Portland cement is suitable only for reinforced cement concrete (RCC) structures.
Impermeable: The non-porous nature of cement leads to frost damage to stone.
Shrinkage: Despite its resistance, shrinkage can still happen and cause cracks and moisture penetration.
Natural Colour: The shade of cement in its natural state lacks colour harmony and is rather unpleasant, but white-coloured coment is also available.
Plasticity: It cracks quickly due to its early settlement time. It also lacks thermal plasticity, i.e., its plastic nature reduces with heat.
Sulphate Attack: Sulfate attack is caused by the reaction of sulphate ions with calcium hydroxide and calcium aluminate hydrate to form gypsum and ettringite. It can cause concrete to expand, fracture, lose strength, and disintegrate.
6. Different Grades Of Cement
There are different grades of cement available on the market, where grades 33, 43, and 53 are the most common. The grade number indicates the minimum compressive strength of cement and mortar in N/mm2 at 28 days, under standard test conditions. 43-grade cement is most commonly available.
7. Conclusion:
Focusing on the cement's brand image rather than the type suitable for the job is one of the major mistakes independent construction companies are making. Thus, it is critical to understand the various types of cement and use them appropriately to avoid compromising structural stability or using high-rise cement for small houses.
Also, the indiscriminate usage of cement has resulted in a slew of major issues throughout the years. The benefits of lime, on the other hand, are becoming increasingly apparent as a consequence of recent study findings from numerous nations. Lime is a time-tested material with numerous qualities.
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