Brick Clay suitable clay for the manufacturing of bricks or coarse pottery. Often an impure clay containing iron, calcium, magnesium and other ingredients is referred to as brick clay. The chief chemical composition of brick clays is silica, alumina, iron oxide, magnesia, lime and alkalis. Either more or less than the required amount of any of these constituents may cause serious damage to the products. The presence of some harmful constituents like iron concretions, stone particles, soluble salts and limestone has a detrimental effect on brick.
In Bangladesh the mineralogical, chemical and engineering properties of the Pleistocene and Holocene brick clays of Dhaka, Narayanganj and Narsingdi districts are well documented. Quartz, illite, chlorite and kaolinite are the major components of the Holocene samples whereas quartz, illite and kaolinite are the major constituents of the Pleistocene clays. Both potash and plagioclase feldspars are present in all the samples as minor constituents. Montmorillonite is not detected in any of the samples.
The bulk chemistry of the Holocene and Pleistocene samples has been found satisfactory for manufacturing good quality bricks. However, the Holocene clay is slightly enriched with silica (60.72 to 63.49%), lime (1.11 to 1.49%) and magnesia (2.16 to 2.87%) while the Pleistocene Madhupur clay is rich in alumina (19.95 to 21.07%) and iron oxide (8.12 to 9.82%). Though the madhupur clay is rich in alumina, the concentration of alumina noted is considerably lower than that of the recommended value for an ideal brick clay. Harmful constituents like soluble salts, organic matter and vegetation are found at low concentrations. But considerable amounts of iron concretions are detected in the Pleistocene samples.
The Holocene samples are composed dominantly of silt-sized particles (53.24 to 84.31%) whereas the percentage of clay-sized particles (43.43 to 49.85%) in the Pleistocene samples are greater than that of the silt-sized particles (34.75 to 43.25%). The obtained liquid limit, plastic limit and plasticity index values suggest that the Holocene samples have intermediate plasticity and the Pleistocene samples are in the range of intermediate to high plasticity. The Pleistocene Madhupur clay shows slightly greater swelling potential than the Holocene clay. As far as the activity of the clay is concerned, all the samples are 'inactive'; to 'normal'; and are quite suitable for brick making. Linear shrinkage values of the samples indicate that the Holocene ones are 'non-critical'; to 'marginal'; while the Pleistocene ones are 'marginal';. The volumetric shrinkage values, in all the samples, are within the range recommended for typical brick clays. Recently clays of the dupi tila formation are also used in brickfields for the manufacture of good quality bricks. [Sifatul Quader Chowdhury] [Chowdhury, Sifatul Quader Professor of Geology, Dhaka University]