MATERIALS. 



129 



The briquettes, twenty-four hours after mixing, should 

 be placed in water, and they should be kept there until 

 required for testing. 



The following diagram (Fig. 20) gives a fair idea of the 

 manner in which air-slaked Portland cement gains strength 

 with age. It will be seen that after the lapse of seven or eight 

 s g months from the time of mixing, it nearly attains its 

 maximum strength, the gain after that period being very 

 small. Indeed, when seven or eight months old briquettes 

 often show a slight falling off in strength, as indicated by 

 the dotted line. 

 ll 



1.700 

 1.600 

 1,500 

 1.400 

 1.300 

 1.200 

 1,100 

 1,000 



eoo 



800 

 700 

 600 

 000 

 4OO 

 300 

 200 

 IOO 



FIG. 20. Diagram showing, approximately, the rate at which briquettes of Portland cement (aerated) 

 gain tensile strength with age. 



It has been suggested, in explanation of this phenomenon, 

 that, seven or eight months after mixing, the elasticity of Port- 

 land cement begins to diminish by reason of continued crystalli- 

 zation producing brittleness; but this explanation is not alto- 

 gether satisfactory, seeing that after this diminution in strength 

 has been observed the briquettes often gradually gain strength 

 again. In briquettes made with " hot " cement, it is probable 

 that this loss of strength which is often very marked and con- 

 tinuous is attributable to gradual expansion and disintegration. 

 The following may be taken as a fair average analysis of a 

 good Portland cement : 



Silica ... ... ... ... ... ... 21-8 



Alumina and oxide of iron ... ... ... ... 12-0 



Lime ... ... ... ... ... ... 613 



Magnesia ... ... ... ... ... ... \-\ 



Sulphuric acid ... ... ... ... ... \~\ 



Carbonic acid ... ... ... ... ... 09 



Potash and soda ... ... ... ... ... Q'9 



Insol uble residue (sand, etc.) ... ... ... ... 09 



100-0 

 K 



