CHAPTER XIII 



CEMENT— CEMENT CONCKETE — REINFORCED CONCRETE— EEINFORCED 

 CONCRETE FENCING 



Portland Cement. — Portland cement is made from a mixture of calcium 

 carbonate and silicate of alumina. 



Materials which contain calcium carbonate and silicate of alumina 

 are mixed in relative quantities, such that these two substances form 

 suitable proportions of the mixture. 



Calcium carbonate occurs in the forms of lime-stone and chalk. 



Silicate of alumina, more or less free from impurities, occurs as clay 

 and shale. 



The process of manufacture of Portland cement includes the following 

 operations : — 



1. Pulverising and mixing the ingredients. 



2. Roasting or calcining. 



3. Grinding to a fine powder. 



When limestone or chalk (CaC0 3 ) is calcined, lime (CaO) is produced. 

 The presence of clay, or more correctly of the silica (Si0 2 ) contained in 

 the clay, renders the cement " hydraulic," i.e. capable of hardening under 

 water. 



The name " Portland cement " was conferred by its inventor Joseph 

 Aspdin of Leeds, who gave the cement that name on account of the 

 resemblance of artificial stone made with it to the Portland stone, 

 largely used in England in his time. The date of Aspdin's invention 

 was 1824. 



Cement Concrete. — Cement concrete is an artificial stone, made by 

 mixing cement with an aggregate, which commonly consists of broken 

 stone and sand. The sand fills the voids between the stones, while the 

 cement, besides binding the whole together, fills the voids between the 

 separate grains of sand. 



Sand. — The sand should be clean, i.e. free from loam or clay, and 



