LIMESTONE AND DOLOMITE 



359 



Figure 43. — Production and use of limestone and dolomite in the United States, 1950- 



"Minerals Yearbook." 



Data from U.S. Bureau of Mines 



and crushing and screening of rock are noisy and 

 dusty operations and, hence, are commonly curtailed 

 or banned in populous areas. 



CONSTRUCTION STONE 



Nearly two-thirds of all limestone produced in 

 the United States in 1969 was used in construction, 

 chiefly as aggregate, fill, and road metal. The stone 

 for these purposes must be clean, hard, strong, and 

 free of soft or friable material ; it should break into 

 irregular but nearly equidimensional fragments; 

 road metal should also be resistant to abrasion and 

 have low porosity. Concrete aggregates should be 



low in reactive materials such as alkalis, soluble 

 sulfides, and certain types of silica, particularly 

 chert, that weaken concrete or even cause it to dis- 

 integrate. Many rocks are interchangeable with 

 limestone for construction purposes. 



PORTLAND CEMENT 



Portland cement manufacture consumed about 15 

 percent of limestone production in the United States 

 in 1969. Portland cement is a complex mixture of 

 calcium silicate, calcium aluminate, and calcium 

 aluminoferrite made by fusing a finely ground mix- 

 ture of limestone and aluminous and siliceous ma- 



