20 CEMENT MATEBIALS AND INDUSTRY. fCLL. ,43. 
(2) The powder has hydraulic properties, i. e., if properly pre- 
Tat:lltr„r^fro m Portland cements in the following 
itSS are not made from carefully prepared and lineiy 
j f;«,.;al mixtures but from natural rocK. 
ground artificial mixtures, Duv temperature than 
whiie' b ands of natural cements vary greatly - imposition 
JbrftouZ «— -Portland cement >s produced by J^^L* 
finely -round artificial mixture containing essential y lime ^ 
therefore be carried on in kilns of special design and lining. JU*m 
the burning, combination of the lime with silica, »l™' a " d £j 
oxide takes place. The product of the burning is a semifused ma, 
idtl£r ," which P eonsists of silicates, aluminates and en t 
of lime in certain fairly definite proportions. This pinker mu 
finely ground. After such grinding, the powder (Poitland cemet 
Wi pirn er C — '-The cementing materials include, under t, 
J£ m made by mixing powdered ^J^J^^ 
ash or a blast-furnace slag. The product is, theietoie am p. 
m echanical mixture of two ingredients, as the ^f^f^ 
at any stage of the process. After mixing, the m.xtu e ^ hn 
ground. The resulting powder (puzzolan cement) will set un 
" Puzzolan cements are usually light bluish, and of lower *ef| 
grayity and less tensile strength than Portland cemen .They ■} 
better adapted to use under water than in air, as is explainec 
later pages. 
