CHEMISTRY OF INDUSTRIAL SUBSTANCES 101 



between bricks or stones and slowly hardens or "sets" by 

 losing water and absorbing carbon dioxide. The object of 

 the process is to make the mortar porous and to facilitate 

 the change of the hydroxide into the carbonate. 



187. Cement. Cement is either a natural or artificial 

 mixture of limestone, clay, sand, and iron oxide. Limestone 

 is an impure form of calcium carbonate mixed with silica 

 (sand) and clay. When the limestone contains about 10% 

 silica and clay it has the desired proportion to form a good 

 mixture that hardens under water, as well as when exposed 

 to the air. It is then good material for making cement and 

 is called hydraulic lime. Portland cement is made by heat- 

 ing the powdered combination into a clinkered mass and 

 then grinding it. A mixture of cement, sand, water, and 

 crushed stone is called concrete. 



188. Bleaching. A better appearance may be given to 

 cotton and many other fabrics by passing them through 

 bleaching solutions. The most effective bleaching agent is 

 bleaching powder, a white powder made by passing chlorine 

 gas (made by heating common salt and sulphuric acid) into 

 oxide of lime (CaO). Other bleaching agents, such as 

 sodium sulphite (Na^SOs) and sodium peroxide (Na 2 O 2 ), 

 are sometimes used. The lime in the bleaching powder holds 

 the chlorine gas. The cloth to be bleached is placed in a 

 mixture of bleaching powder and water. The chlorine gas 

 from the bleaching powder acts on the water forming hydro- 

 chloric acid and oxygen. The oxygen combines with the 

 coloring matter and destroys it, thus leaving a white-surfaced 

 fabric. Bleaching is a distinct chemical action and may 

 weaken the fabric. 



