6 CEMENTS, LIMES, AND PLASTERS. 



chemistry of lime-burning, will be of service at this point of the dis- 

 cussion. 



Pure limestone has the composition of the mineral calcite, whose 

 formula is CaC0 3 , corresponding to the composition : calcium oxide 56% , 

 carbon dioxide 44%. In the magnesian limestones part of this calcium 

 carbonate is replaced by magnesium carbonate, the resulting rock there- 

 fore having a formula of the type XCaC0 3 ,YMgC0 3 . This replacement 

 may reach the point at which the rock has the composition of the mineral 

 dolomite an equal mixture of the two carbonates, with the formula 

 CaCO 3 ,MgC0 3 , corresponding to the composition: calcium oxide 30.44%, 

 magnesium oxide 21.73%, carbon dioxide 47.83%. Limestones may 

 therefore occur with any intermediate amount of magnesium carbonate, 

 and the lime which they produce on calcination will carry correspond- 

 ing percentages of magnesium oxide, from 0% tq 41.65%. Commer- 

 cially those limes which carry less than 10% of magnesium oxide are, 

 for building purposes, marketable as "pure limes"; while those carry- 

 ing more than that percentage will show sufficiently different properties 

 to necessitate being marketed as "magnesian limes". 



Aside from the question of magnesia, a limestone may contain a 

 greater or lesser amount of real impurities. Of these the most impor- 

 tant are silica (SiO 2 ), alumina (A1 2 O 3 ), and iron oxide (Fe 2 3 ). These 

 impurities, if present in sufficient quantity, will materially affect the 

 properties of the lime produced, as will be noted later under the heads 

 of Hydraulic Limes and Natural Cements. 



The limes may be divided into two classes: 



(1) High-calcium limes; 



(2) Magnesian limes. 



High-calcium limes. On heating a relatively pure carbonate of lime 

 to a sufficiently high degree, its carbon dioxide is driven off, leaving 

 calcium oxide (CaO), or "quicklime". Under ordinary conditions, 

 the expulsion of the carbon dioxide is not perfectly effected until a 

 temperature of 925 C. is reached. The process is greatly facilitated 

 by blowing air through the kiln, or by the injection of steam. On 

 treating quicklime with water, "slaking" occurs, heat being given off, 

 and the hydrated calcium oxide (CaH 2 02) being formed. The hydrated 

 oxide (slaked lime) will, upon exposure to the atmosphere, slowly reabsorb 

 sufficient carbon dioxide to reassume its original composition as lime car- 

 bonate. As this reabsorption can take place only at points where the 

 mortar is exposed to the air, the material in the middle of thick walls 

 never becomes recarbonated. In order to counteract the shrinkage which 

 would otherwise take place during the drying of the mortar, sand is 



