INTRODUCTION. 9 



the resulting powder (natural cement) be mixed with water, it will 

 harden rapidly. This hardening or setting will also take place under 

 water. 



3. Portland cement is produced by burning a finely ground artificial 

 mixture containing essentially lime, silica, alumina, and iron oxide, in 

 certain definite proportions. Usually this combination is made by 

 mixing limestone, chalk, or marl with clay or shale, in which case about 

 three times as much of lime carbonate as of clayey materials should be 

 present in the mixture. The burning of this mixture takes place at a 

 high temperature, approaching 3000 F., and must therefore be carried 

 on in kilns of special design and lining. During the burning, com- 

 bination of the lime with the silica, alumina, and iron oxide takes place. 

 The product of the burning is a semifused mass called "clinker", and 

 consists of silicates, aluminates, and ferrites of lime, each being present 

 in certain fairly definite proportions. This "clinker" must be finely 

 ground. After such grinding the resulting powder, which is then Port- 

 land cement, will harden under water. 



Portland cement is blue to gray in color, with a specific gravity of 

 3.0 to 3.2. It sets more slowly than natural cements, but soon attains 

 a higher tensile strength. 



4. Puzzolan cements. Certain natural and . artificial products, 

 such as some volcanic ashes and blast-furnace slags, will show feeble 

 hydraulic properties if finely pulverized. Their hydraulicity is very 

 markedly increased if their powder, instead of being used alone, is 

 mixed with powdered slaked lime. Such mixtures of slaked lime with 

 a relatively feeble hydraulic agent are known as Puzzolan Cements. 

 The puzzolan cements are therefore simply mechanical mixtures of 

 the two ingredients, as the mixture is not burned at any stage of the 

 process. 



Puzzolan cements are usually light bluish in color, and of lower 

 specific gravity and less tensile strength than Portland cement. They 

 are better adapted to use under water than to use in air, as is explained 

 later in this volume 



Subgroup II b. Oxychloride cements. In 1853 the chemist Sorel 

 discovered the fact that zinc chloride mixed with zinc oxide united 

 with it to form a very hard cement. Later it was found that a solution 

 of magnesium chloride will unite, in the same manner, with magnesia. 

 The product is, in both cases, an oxychloride of zinc or magnesium as 

 the case may be. 



It is obvious that cementing materials of this character cannot 

 well be placed on the market as structural cements, but the property 



