ABSTRACT
Portland cement is made from clay and limestone or chalk. The raw materials are generally quarried by means of electrically-driven navvies and sent from quarry to works by trucks, conveyor belts or buckets, or they are sometimes piped in the form of slurry. Cement works should not be placed too near to residential areas. The rotary kilns are kept revolving continuously day and night, and the noise is disturbing at close quarters. Abrasive paper and cloth, including the domestic sandpaper, is made from a number of different materials, such as silicon carbide and fused alumina, both of which are electric furnace products imported mainly from Canada and the United States in crude form. Grinding wheels are made from two principal types of abrasive, silicon carbide and fused alumina. After crushing and grading into different sizes the abrasive grain is variously bonded with ceramic materials, synthetic resins, rubber, silicate, or shellac, depending on the properties required.
