Polytechnic Association. 677 



Hancock, Maryland, etc. To this group belong, also, tlie so-called 

 Poman cements. 



3. With the name Portland cements we designate those artificial 

 and natural hydraulic mortars wltich are so strongly burned that 

 softening has taken place, and which, in their burned state, contain 

 no free lime. They possess, furthermore, a specific weight of over 

 3. As mentioned above, it was Aspdin who first manufactured such 

 a cement, but the name given by him for his product has now become 

 a collective name, it being used for all mortars here defined. 



Neat cement, that is, cement without any admixture, is but seldom 

 used. The ordinary admixture is sand, a material employed for that 

 purpose from time immemorial. As regards the quantity which may 

 be admixed to hydraulic cements in general, it is almost impossible 

 to lay down strict rules; in many cases it may, therefore, . be advis- 

 able to resort to an experiment to ascertain what mixture answers 

 best for the object to be attained before carrying out works on a large 

 scale. With reference to Portland cement, however, we would state 

 that it may be mixed with three to four times its quantity of sand, but 

 that this must be considered the extreme limit whenever it is intended 

 for solid structures. Not every kind of sand is fit for the preparation of 

 mortar. Scarcely more than five per cent of clay are, for instance, 

 sufficient to reduce the strength of the solidified cement considerably. 

 Such sand ought, in all cases, to be carefully washed before used. With 

 regard to the shape of the grain, pit sand is preferable to river or sea 

 sand, since its surface is largest in proportion to its quantity. Sea 

 sand and river sand, though very clean in most cases, consist of spheri- 

 cal bodies, which, as well known, offer the smallest surface to the 

 quantity. Sand assuming the form of leaflets is preferable to any 

 other. From a chemical point of view an admixture of feldspar or 

 feldspathic minerals (gneiss, granite, syenite, diorite, etc.) might be 

 recommended, inasmuch as where free lime is present the potash or 

 soda in the feldspar will be replaced, a silicate of lime being formed, 

 which, in combination with the silicate of alumina in the feldspar, 

 possesses great cementing qualities. 



In preparing cement for use it is always best to mix the dry sand 

 as uniformly as possible with the cement before adding any water to 

 it. Since the strength of a mortar depends directly upon the quan- 

 tity of water present during the moment of setting, the quantity of 

 water must not exceed what is barely sufficient to convert the mortar 

 into a stiff and plastic paste. This end is best attained by mixing 



