PACINI, METAMORPHISM OF PORTLAND CEMENT 209 



The glass cylinders containing the cement were then stored in a damp 

 closet for 28 days, when the cylinders were broken out, and two speci- 

 mens, each exactly one diameter high, cut from each cylinder. These 

 were put into water for a few hours, so that they might be in the moist, 

 state when crushed. The cylinders were kept in the damp closet instead 

 of being stored under water, to avoid leaching out the salts contained in 

 the mixing water, thus obtaining the maximum effect of the dissolved 

 salts. 



It will be noted that there is a decided loss of strength in all but one 

 case (number 16). This particular case may be explained by the prob- 

 able formation of an oxychloride, by the magnesium chloride and the 

 magnesium hydroxide liberated by the action of the magnesium sulphate 

 upon the calcium hydroxide of the cement. The oxychloride formed 

 from these two materials has a tensile strength far superior to that of 

 Portland cement itself, and its presence probably counteracted the de- 

 structive action of the salts upon the cement. It is probable, however, 

 that, at longer periods, this increase would disappear and become a de- 

 crease. Otherwise, the presence of saline matter dissolved in the mixing 

 water seems to have a decided deleterious effect upon the strength of 

 cement. This point is of marked importance in construction, inasmuch 

 as the problem of mixing water is often solved by using the water nearest 

 at hand, without inquiry into its qualities. 



It is the custom to specify that the water used in mixing concrete shall 

 be free from oil, acid, strong alkalies or vegetable matter (77) ; but such 

 a specification does not cover the case in point, and the presence of large 

 quantities of dissolved salts in water used for construction is easily over- 

 looked. In concrete construction, it is of the utmost importance that the 

 water which may be used in mixing be additionally subjected to such 

 tests as will reveal either its mineral content or its action when mixed 

 with cement and possible subsequent attack thereon. 



The action of sodium chloride appears to be nearly directly propor- 

 tional to the amount employed. This salt is used in mixing water for 

 construction carried on in cold weather, in order to prevent freezing of 

 the deposited concrete. Its effect upon the strength of cement, if used in 

 excessive quantities, is, as has been shown above, likely to become a seri- 

 ous matter. Under the conditions of construction which generally pre- 

 vail, however, much of the salt may be leached out of the mass. The 

 results above represent a condition of maximum attack. 



Dieckmann (25) recommends the use of from 1 to 2.5 per cent of salt 

 for concrete to be laid in cold weather, but states that percentages larger 

 than this cause a marked decrease in the strength. 



