What are different grades of Cement?

Cement was first discovered by an English brick layer named Joseph Aspdin in 1824. He called it Portland cement for the reason that the cement he discovered resembled the limestone found in Portland. The approximate composition of Portland cement is given below

• Lime (CaO)             60 – 70%
• Silica (SiO₂)             20 – 25%
• Alumina (Al₂O₃)         5 – 10%
• Ferric oxide (Fe₂O₃)         2 – 3 %

The raw materials used for the manufacture of Portland cement are limestone (provides CaO) and clay (provides SiO2 , Al2O3 and Fe2O3) which are finely powdered and then mixed in the ratio 3 : 1 by mass. The mixture is again ground to a fine powder and water is added. The finely ground powder called slurry is heated to 1773 K in a rotary kiln. On heating, lime, silica, alumina and ferric oxide react together and produces a mixture of dicalcium silicate, tricalcium silicate and tricalcium aluminate called clinker. The clinker is cooled and a small amount of gypsum (2 – 5%) is added to it, to delay the setting time of cement. The mixture of clinker and gypsum is then ground to a fine powder which is called cement. It is stored in tall structures called silos. The cement is then packed in water-proof bags and sold in markets.

When cement is mixed with water, it becomes hard over a period of time. This is called setting of cement. Gypsum is often added to Portland cement to prevent early hardening or “flash setting”, allowing a longer working time.

What are Different Grades of Cement?

The grade 43 and 53 in cement mainly corresponds to the average compressive strength attained after 28 days ( 6724 hours) in mega pascals (Mpa) of at least three mortar cubes ( area of face 50 cm squared) composed of one part cement, 3 parts of standard s and ( conforming to IS 650:1966) by mass and P/4 ( P is the percentage of water required to produce a paste of standard consistency as per IS standard) + 3 percentage ( of combined mass of cement plus sand) of water , prepared, stored and tested in the manner described in methods of physical test for hydraulic cement.

• 721 hr not less than 23 MPa for 43 grade, 27 MPa for 53 grade
  
• 1682 hrs not less than 33MPa for 43 grade, 37MPa for 53 grade
  
• 6724 hrs not less than 43MPa for 43 grade, 53 MPa for 53 grade