﻿400 
  NEW 
  YORK 
  STATE 
  MUSEfUM 
  

  

  4 
  Magnesia 
  cement 
  with 
  a 
  large 
  amount 
  of 
  magnesia, 
  namely 
  

   over 
  20$, 
  less 
  alumina 
  and 
  iron 
  and 
  less 
  undecomposed 
  silicates 
  

   than 
  in 
  the 
  preceding 
  class. 
  

  

  5 
  Magnesia 
  cement 
  efficient 
  in 
  alumina 
  and 
  iron 
  oxids 
  as 
  well 
  

   as 
  in 
  combined 
  silica. 
  

  

  6 
  Magnesia 
  cements 
  thoroughly 
  burned, 
  made 
  from 
  rock 
  hav- 
  

   ing 
  a 
  smaller 
  amount 
  of 
  silicates 
  than 
  those 
  of 
  class 
  4, 
  with 
  

   only 
  a 
  medium 
  per 
  cent 
  of 
  magnesia 
  and 
  little 
  uncombined 
  sili- 
  

   cates. 
  : 
  

  

  Cements 
  of 
  the 
  first 
  class 
  set 
  and 
  acquire 
  strength 
  rapidly 
  and 
  

   increase 
  in 
  this 
  direction 
  for 
  a 
  long 
  period 
  but 
  the 
  final 
  result 
  is 
  

   a 
  more 
  brittle 
  mortar 
  than 
  is 
  obtained 
  with 
  the 
  magnesia 
  brands. 
  

   This 
  class 
  includes 
  the 
  lime 
  cements 
  of 
  the 
  Potomac 
  valley. 
  

   According 
  to 
  Richardson 
  the 
  second 
  class 
  has 
  not 
  as 
  variable 
  a 
  

   relation 
  of 
  silicates 
  to 
  lime, 
  and 
  consequently 
  the 
  cements 
  are 
  apt 
  

   to 
  be 
  fiery 
  and 
  not 
  as 
  satisfactory. 
  They 
  are 
  shown 
  to 
  improve 
  

   by 
  the 
  addition 
  of 
  portland 
  cement, 
  after 
  which 
  they 
  can 
  be 
  

   used 
  quite 
  successfully. 
  This 
  class 
  includes 
  those 
  of 
  the 
  Lehigh 
  

   valley. 
  The 
  third 
  class 
  is 
  represented 
  by 
  the 
  best 
  Rosendale 
  

   brands, 
  which 
  set 
  and 
  acquire 
  strength 
  slowly, 
  but 
  which 
  con- 
  

   tinue 
  to 
  develop 
  it 
  for 
  a 
  long 
  time 
  and 
  eventually 
  are 
  very 
  strong 
  

   and 
  tough. 
  The 
  fourth 
  class 
  includes 
  cements 
  like 
  those 
  of 
  

   western 
  New 
  York 
  which 
  have 
  been, 
  while 
  containing 
  an 
  unusual 
  

   amount 
  of 
  magnesia, 
  burned 
  so 
  hard 
  that 
  little 
  of 
  the 
  silicates 
  

   remain 
  undecomposed 
  and 
  uncombined 
  with 
  the 
  lime 
  and 
  mag- 
  

   nesia, 
  and 
  in 
  consequence 
  are 
  apt 
  to 
  stand 
  a 
  long 
  time 
  after 
  use, 
  

   unless 
  carefully 
  hydrated. 
  The 
  fifth 
  class 
  is 
  one 
  in 
  which 
  the 
  

   cement 
  is 
  essentially 
  a 
  lightly 
  burned, 
  highly 
  magnesian 
  material 
  

   in 
  the 
  preparation 
  of 
  which 
  the 
  heat 
  has 
  not 
  been 
  sufficiently 
  

   high 
  or 
  prolonged 
  to 
  bring 
  the 
  greater 
  portion 
  of 
  the 
  silica 
  in 
  

   composition 
  with 
  the 
  lime 
  or 
  magnesia, 
  in 
  this 
  respect 
  being 
  in 
  

   contrast 
  to 
  the 
  preceding 
  class. 
  The 
  hydraulic 
  principle 
  and 
  

   strength 
  are 
  therefore 
  largely 
  due 
  to 
  the 
  magnesia 
  and 
  car- 
  

   bonates 
  rather 
  than 
  to 
  the 
  silicates 
  and 
  aluminates. 
  Examples 
  

   of 
  this 
  are 
  those 
  cements 
  made 
  at 
  La 
  Salle, 
  111. 
  The 
  last 
  class 
  

   fortunately 
  is 
  a 
  cement 
  in 
  which 
  there 
  is 
  rather 
  less 
  mag- 
  

  

  