eckel.]  AMERICAN    CEMENT    INDUSTRY.  497 
Natural  cements  differ  from  Portland  cements  in  the  following 
important  particulars: 
(1)  Natural  cements  are  not  made  from  carefully  prepared  and 
finely  ground  artificial  mixtures,  but  from  a  natural  rock. 
(ii)  Natural  cements  are  burned  at  a  lower  temperature  than  Port- 
land, the  mass  in  the  kiln  never  being  heated  high  enough  to  even 
approach  the  fusing  or  clinkering  point. 
(3)  Natural  cements,  after  burning  and  grinding,  are  usually  yel- 
low to  brown  in  color  and  light  in  weight,  having  a  specific  gravity 
of  '2.7  to  3.1,  while  Portland  cement  is  commonly  blue  to  gray  in 
Golor  and  heavier,  its  specific  gravity  ranging  from  >\  to  3.2. 
(4)  Natural  cements  set  more  rapidly  than  Portland  cement,  but 
do  not  attain  so  high  a  tensile  strength. 
(5)  Portland  cement  is  a  definite  product,  its  percentages  of  lime, 
silica,  alumina,  and  iron  oxide  varying  only  between  narrow  limits, 
while  brands  of  natural  cements  vary  greatly  in  composition. 
Portland  cement. — 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  com- 
bination is  made  by  mixing  limestone  or  marl  with  clay  or  shale,  in 
which  case  the  mixture  should  contain  about  three  parts  of  lime  car- 
bonate to  one  part  of  the  clayey  materials.  The  burning  takes  place 
at  a  high  temperature,  approaching  3,000°  P.,  and  must  therefore  be 
carried  on  in  kilns  of  special  design  and  lining.  During  the  burn- 
ing, combination  of  the  lime  with  silica,  alumina,  and  iron  oxide 
takes  place.  The  product  of  the  burning  is  a  semifused  mass  called 
"  clinker,"  which  consists  of  silicates,  aluminates,  and  ferrites  of  lime 
in  certain  fairly  definite  proportions.  This  clinker  must  be  finely 
ground.  After  such  grinding,  the  powder  (Portland  cement)  will 
set  under  water. 
Puzzolan  cements. — The  cementing  materials  included  under  this 
name  are  made  by  mixing  powdered  slaked  lime  with  either  a  vol- 
canic ash  or  a  blast-furnace  slag.  The  product  is,  therefore,  simply 
a  mechanical  mixture  of  two  ingredients,  as  the  mixture  is  not  burned 
at  any  stage  of  the  process.  The  mixture  is  finely  ground  after 
mixing.  The  resulting  powder  (Puzzolan  cement)  will  set  under 
water. 
Puzzolan  cements  are  usually  light  bluish,  and  of  lower  specific 
gravity  and  le^s  tensile  strength  than  Portland  cement.  They  are 
>ettcr  adapted  to  use  under  water  than  in  air. 
Cement   production^   by   classes. — The   cement   production   of   the 
Jnited  States  for  the  years  1900-1903,  inclusive,  is  given  in  the  table 
ollowing,  the  figures  being  those  in  the  annual  volume  on  Mineral 
Resources,  published  by  the  United  States  Geological  Survey : 
Bull.  260—05  m 32 
