56 : ANNUAL REPORT 
purer. limestone. So also with blast furnace slags, which are often a 
most excellent material for cement manufacture. Both the natural cement 
rock and the slags have been discussed in previous paragraphs and there 
remains now the discussion of the clays and calcareous material with 
special reference to the Ohio resources. 
Fig. 4. Clay residue of Rosendale cement, seen under the microscope. 
THE CLAYS. 
Clay may be defined as a complex derivative rock, generally of a 
soft and earthy, nature, in which a mass of mineral debris of variable 
composition and amount is bonded and held together by a matrix of 
kaolinite, or allied hydrous silicates of alumina. The distinguishing 
characteristics of clays as a class are, first, plasticity when mixed with the 
right quantity of water, and second, the property of hardening by heat, 
to form strong and durable silicates not easily destroyed by weather or 
other natural agencies. This definition is not rigidly exact, as some 
clays are not plastic, and others do not harden to make durable silicates 
by heat, but these are minor exceptions to the group as a whole. 
In order to fully comprehend the nature of clay and to understand the 
source of its advantages and disabilities as an ingredient in cement 
mixtures, it is necessary to devote some space to a discussion of its origin 
and classification. 
Origin of Clay. —As before stated, clay is a complex rock and not 
a single mineral. The characteristic or essential part of it, however, is 
the mineral kaolinite, which is derived from the weathering of feldspar 
and many other igneous aluminous silicate minerals. 
Considering the typical feldspar, orthoclase, we find that it possesses 
the formula: K,O, Al,O;, 6SiO, and the following percentage compo- 
sition : 
