38 NEW YORK STATE MUSEUM 



sideration is to find a suitable combination of limestone and clay. 

 Marl may take the place of hard limestone, but is not so desirable 

 from a manufacturing standpoint on account of the large amount 

 of moisture present. Shale of suitable composition is as good as 

 soft clay. Gypsum performs a subsidiary function in the cement 

 mix, acting simply as a retarder for the high-lime, quick-setting 

 cements, and is added in such small proportions that the existence 

 of a nearby supply is not of great moment. All of the elements 

 necessary for making portland cement, with the exception of coal, 

 are provided by the local resources. The soft coal fields of Penn- 

 sylvania are within easy shipping distance of most parts of New 

 York State. 



Clay, as noted in the special part of this report devoted to that 

 subject, covers wide areas, and is further supplemented by extensive 

 shale deposits which occur in the series of stratified formations. 

 There is practically no considerable section in which argillaceous 

 materials suitable for cement mixture are not present. High- 

 calcium limestones on the other hand have a more limited develop- 

 ment, so that their distribution and consideration of their local 

 availability may be regarded as of fundamental importance in the 

 location of plants. 



It is to be noted that the term high-calcium is not used strictly 

 in a quantitative sense. Limestones may contain considerable per- 

 centages of impurities in the form of silica, alumina, iron and 

 alkalies and still be very serviceable for cement. Silica and alumina 

 are desirable in that they take the place of so much clay, and iron 

 is allowable, if not present as sulphide. It is well to remark, 

 however, that in the case of impure limestones there should be a 

 degree of uniformity in the composition that will admit of the easy 

 attainment of the proper mix. A marked variation in this respect 

 will necessitate very careful oversight, if not excessive labor, to 

 maintain the proper proportions between the different ingredients. 



Many limestones contain magnesia as a substitute for the lime, 

 in which case they are called magnesian limestones if the percentage 

 is low, or dolomites if the magnesia attains proportions of 15 per 

 cent or so. The proportion of magnesia is determinable with 

 certainty only by analysis. For portland cement it is important 

 to ascertain the amount of magnesia in the average rock, as the 

 presence of more than 4 per cent in the finished product will exclude 

 the use of the cement under standard American specifications. 



In view of these limitations as to quality, the selection of a 

 limestone supply must be made with some care, giving due regard 



