http://www.gharexpert.com/articles/Bricks-887/Specification-Brick_0.aspx
Also see about fly ash bricks
Dear Abhishek Fly ash bricks are generally used for making curtain and partition walls of the houses/ buildings. Fly ash bricks are used in domestic building in the category of low or middle-income groups mostly have single storied dwelling units. Use of fly ash bricks in this type of construction is meant mainly to achieve economy and make profits. PWD-DELHI Issues Circular for Use of Fly ash Brick“The PWD-DELHI department Suggested Experimental Use of Fly ash bricks in Site office, Temporary Accommodation/ Boundary walls”