LIME AND CEMENT INDUSTRIES 745 



of at least 16 kilo. q. c. m. (16 kilograms per square centimeter, 

 227 pounds per square inch). The compressive strength shall 

 be at least 160 kilo. q. c. m. (2270 pounds per square inch). 



Cement which shows a higher tensile or compressive strength 

 in many cases of a greater addition of sand, from this point of 

 view, as well as on account of its greater strength for the same 

 amount of sand, is entitled to a correspondingly higher price. 



For slow setting cements the strength after 20 days is less in 

 general than the one above specified, therefore, in giving the 

 results of tests, the time of setting shall also be given. 



The tests shall be made in the following manner. 



To determine the time of setting cement, a slow setting neat 

 cement shall be mixed three minutes, and a quick setting neat 

 cement shall be mixed one minute with water to a stiff paste. A 

 cake about 1.5 cm (.59 inch) thick, with thin edges, shall be 

 formed of this paste on a plate of glass. The consistency of the 

 cement paste for this cake shall be such that, when brought with 

 a trowel on the plate, the paste will only begin to run toward 

 the edges of the same after the paste has been repeatedly jarred. 

 As a rule 27/^ to 30^ water will suffice to give the necessary con- 

 sistency to the paste. As soon as the cake is sufficiently hard- 

 ened, so that it will resist a slight pressure of the finger, nail, the 

 cement is to be considered as having set. 



For the exact determination of the time of setting, and for 

 determining the beginning of the time of setting, which latter m 

 of importance in the case of quick setting cements, since they 

 must be worked up before they begin to set, a standard needle 

 300 grams (10 ounces) in weight and 1 square mm (.00155 

 square inch) in cross-section, is used. A metal ring 4 cm (1.575 

 inch) in hight and 8 cm (3.15 inches) clear diameter (inside) is 

 placed on a glass plate, filled with cement paste of the above 

 consistency and brought under the needle. The moment at which 

 the needle is no longer capable of completely penetrating the 

 cement cakes is considered the beginning of the time of setting. 

 The time elapsing between this and the moment when the stand- 

 ard needle no longer leaves an appreciable impression on the 

 hardened cake is considered the time of setting. 



For making the heat test (3) a stiff paste of neat cement and 

 water is made, and from this cakes 8 cm (3.15 inches) to 10 cm 

 (3.94 inches) in diameter and 1 cm (.394 inch) thick are formed 

 on a smooth, impermeable plate, covered mth blotting paper. 

 Two of these cakes, which are to be protected against dryings 



