670 CEMENTS, LIMES, AND PLASTERS. 



the relative proportions in which these ingredients have been mixed. 

 Slag cements do not stain masonry; and an imported cement of closely 

 related origin (Meier's Pozzuolan) has long been in favor in this country 

 for architectural uses, because of this non-staining property. 



Rapidity of set. Normally slag cements are slower setting than 

 Portlands. Whether this property is a disadvantage or not will depend 

 on the use to which the cement is to be applied. As before mentioned 

 the rapidity of set increases naturally with the amount of alumina in 

 the slag. Set can be artificially hastened by the addition of puzzo- 

 lanic material to the cement; burned clay, active forms of silica, slags 

 high in alumina, etc., are additions which are both effective and cheap. 

 The treatment of the cement during manufacture with alkalies to accel- 

 erate the set has already been discussed. 



Strength. While slag cements fall below high-grade Portlands in 

 tensile strength, good American slag cements develop sufficient strength 

 to pass the usual specifications for Portlands. Tested neat they do 

 not approximate so closely to the Portlands as they do if tested in 2:1 

 or 3:1 mortars. Part of this property may be due to the fact that they 

 are in general ground finer than Portlands, especially than foreign 

 Portlands. Prof. W. K. Hatt recently made a large series of tests on 

 American slag cements, and reported that there was no noticeable 

 deficiency in strength of briquettes kept in air as compared with those 

 kept in water. Other investigators have arrived at opposite conclu- 

 sions ; and it is probable that these conflicting results arise from differ- 

 ences in the chemical composition of the various brands tested. 



Resistance to mechanical wear. Slag cements are notably deficient 

 in this property, and are therefore not available for use for the surface 

 of pavement, floors, etc., where this quality must be highly developed; 

 they seem to be well fitted, however, for pavement foundations, or 

 indeed for any work which will not be exposed to dry air, and in which 

 a high strength is not necessary. 



Ratio of tensile to compressive strength. This ratio, which is of 

 importance (as noted in the discussion of Portland cements) seems to 

 be much lower for slag cements than for Portlands. In the case tabu- 

 lated below, the results of tests show the ratio for slag cement to aver- 

 age 5.3:1, in place of the 10:1 ratio, which is a fair average for Port- 

 land cements. 



,. compressive strength ,. 



The average value for ^ n ^- for the whole series 



tensile strength 



is 5.3. 



