THE  AMERICAN  CEMENT  INDUSTRY. 
By  Edwin  O.  Eckel. 
In  the  present  paper  the  figures  relative  to  cement  production  of 
the  United  States  published  annually  by  the  Survey  have  been  used 
as  a  basis  for  discussion.  These  figures,  when  rearranged  and  prop- 
erly interpreted,  bring  out  certain  facts  of  interest  in  regard  to  the 
conditions  and  development  of  the  cement  industry. 
CLASSIFICATION   OF  CEMENTS. 
Commercial  and  engineering  considerations  have  led  to  the  separa- 
tion of  the  hydraulic  cements  into  three  groups,  which  differ  both  in 
their  methods  of  manufacture  and  in  their  properties.  The  facts  on 
which  this  classification  is  based  may  be  briefly  stated  as  in  the  fol- 
lowing paragraphs: 
Natural  cements. — Natural  cements  are  produced  by  burning  a  nat- 
urally impure  limestone,  containing  from  15.  to  40  per  cent  of  silica, 
alumina,  and  iron  oxide,  at  a  comparatively  low  temperature,  usually 
about  that  of  ordinary  lime  burning.  The  operation  can  therefore 
be  carried  on  in  a  kiln  closely  resembling  an  ordinary  limekiln. 
During  the  burning  the  carbon  dioxide  of  the  limestone  is  almost 
entirely  driven  off,  and  the  lime  combines  with  the  silica,  alumina, 
and  iron  oxide,  forming  a  mass  containing  silicates,  aluminates,  and 
ferrites  of  lime.  If  the  original  limestone  contained  much  mag- 
nesium carbonate  the  burned  rock  will  also  contain  a  corresponding 
amount  of  magnesia. 
The  burned  mass  will  not  slack  if  water  be  added  after  burning. 
It  is  necessary,  therefore,  to  grind  it  finely.  After  grinding,  if  the 
resulting  powder  (natural  cement)  be  mixed  with  water  it  will 
harden  rapidly.  This  hardening,  or  setting,  Avill  also  take  place 
under  water.  Natural  cements  differ  from  ordinary  limes  in  two 
noticeable  ways : 
(1)  The  burned  mass  does  not  slack  on  the  addition  of  water. 
(2)  The  powder  has  hydraulic  properties — i.  e.,  if  properly  pre- 
pared, it  will  set  under  water. 
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