INTRODUCTION. 5 



clayey materials and fragments of quartz and limestone, often exercise 

 an appreciable effect upon the properties of the plaster resulting from 

 burning such impure gypsum. On burning gypsum at a relatively 

 low temperature (350-400 F.) much of its water of combination is 

 driven off, leaving a partially dehydrated calcium sulphate. This, 

 when ground, is plaster of Paris, or if it either naturally or artificially 

 contains certain impurities, it is called "cement plaster". When either 

 plaster of Paris or cement plaster is mixed with water, the percentage 

 of water which was driven off during calcination is reabsorbed, and 

 the mixture hardens, having again become a hydrous sulphate of lime. 

 The processes involved in the manufacture and setting of the dead- 

 burned plasters and hard-finish plasters are slightly more complicated, 

 but the reactions involved are of the same general type. 



Subgroup I b. Carbonate cementing materials : limes and magnesia. 

 The cementing materials falling in the present subgroup are oxides 

 derived from natural carbonates by the application of heat. On exposure, 

 under proper conditions, to any source of carbon dioxide, the cement- 

 ing material recarbonates and "sets". In practice the carbon dioxide 

 required for setting is obtained simply by exposure of the mortar to 

 the air. In consequence the set of these carbonate cements, as com- 

 monly used, is very slow (owing to the small amount of carbon dioxide 

 which can be taken up from ordinary air) ; and, what is more important 

 from an engineering point of view, none of the mortar in the interior 

 of a wall ever acquires hardness, as only the exposed portions have an 

 .opportunity to absorb carbon dioxide. From the examination of old 

 mortars it has been thought probable that a certain amount of chemical 

 action takes place between the sand and the lime, resulting in the forma- 

 tion of lime silicates ; but this effect is slight and of little engineering 

 importance compared with the hardening which occurs in consequence 

 of the reabsorption of carbon dioxide from the air. 



Limestone is the natural raw material whose calcination furnishes 

 most of the cementing materials of this group.* If the limestone be 

 an almost pure calcium carbonate, it will, on calcination, yield calcium 

 oxide, or "quicklime". If, however, the limestone should contain 

 any appreciable percentage of magnesium carbonate, the product will 

 be a mixture of the oxides of calcium and magnesium, commercially 

 known as a magnesian lime. A brief sketch of the mineralogic re- 

 lationships of the various kinds of limestone, in connection with the 



* The subject of magnesia as a cementing material, being too complicated for 

 brief discussion, will not be taken up here. See Chapters XI and XII. 



