MANUFACTURE AND PROPERTIES OF LIME-SAND BRICKS. 133 



sand of the mortar, though occasionally the statement is made that 

 with increasing age a certain amount of chemical action does take place, 

 resulting in the formation of a certain percentage of lime silicate. This, 

 however, is more than doubtful. 



In making lime-sand brick by modern processes, it is claimed by 

 its advocates that the lime and sand of the brick do combine to form 

 lime silicates, the combination being in this case brought about 

 through the action of steam under high pressure. It is also claimed, 

 though rather by suggestion than by direct statement, that these lime 

 silicates make up a considerable proportion of the entire mass of the 

 brick. 



To the present writer the first claim does not seem to be justified. 

 No proofs have been presented that, in the course of lime-sand brick 

 manufacture, any chemical combination takes place between the lime 

 and the sand. It is undoubtedly true that the treatment with steam 

 under pressure increases in some unexplained way the chemical activity 

 of the slaked lime; but further than that we cannot go at present. 



The second point, regarding the percentage of lime silicate which 

 would be formed if chemical combination of lime and silica took place, 

 can be readily settled. The lime and silica might unite in any one of 

 three proportions, forming respectively the calcic silicate (CaO.SiC^), 

 the dicalcic silicate (2CaO.SiO 2 ), or the tricalcic silicate (3CaO.Si0 2 ). 

 The last of these is the one which is formed during the processes of 

 manufacture of Portland cement, and is the hydraulic silicate par ex- 

 cellence. In most sand-lime brick literature a similarity between that 

 product and Portland cement is suggested, even if not definitely claimed. 

 In the little table below the percentage composition of these three lime 

 silicates is presented. It will be seen that the tricalcic silicate contains 

 73.59 per cent lime to 26.41 per cent silica, while the proportion of 

 lime decreases until it reaches 51.84 per cent in the unicalcic silicate. 



TABLE 47. 

 PERCENTAGE COMPOSITION OF VARIOUS LIME SILICATES. 



The bearing of these facts upon lime-sand brick manufacture is obvious 

 enough. 



