PACINI, METAMORPHISM OF PORTLAND CEMENT 213 



As was found in the case of the grouts last mentioned, waters of this 

 general concentration do not appear to affect the strength of cement 

 mortars with which they are gaged, and the probabilities are that no 

 serious effects will result from this cause alone. 



QUALITY OF WATEE THAT MAT SUBSEQUENTLY COME INTO CONTACT 

 WITH THE SYSTEM 



Theoretical considerations. — The action of dissolved salts in water 

 that comes into contact with concrete, where such action is deleterious 

 to the concrete, has been carefully studied by a large number of investi- 

 gators (68, 81, 96, 112). Of the salts which have been found injurious, 

 magnesium sulphate and magnesium chloride seem to have the greatest 

 effects. What concentration of dissolved salts is necessary in order that 

 disintegrating effects shall manifest themselves cannot be definitely 

 stated. This is a field problem and is subject to wide variations under 

 different conditions. 



A water containing relatively little dissolved material, acting under 

 favorable conditions of porosity, pressure and wide temperature changes 

 upon one concrete, may accomplish failure of the structure; while 

 another water, of high saline content, meeting a dense, impervious con- 

 crete, not forced through the mass by pressure and under conditions of 

 small temperature change, may have practically no action. Manifestly, 

 unless these varying conditions are taken into account, it is unscientific 

 to draw any conclusions regarding the attack of different waters or the 

 resistivity of different cements. 



It may be laid down as a basic principle, however, that the denser a 

 concrete, other conditions being equal, the greater its resistance to the 

 attack of saline waters (10, 41, 57). The alkali waters of the Western 

 states have given a great deal of trouble in concrete construction. Most 

 experimenters conclude that their action upon concrete is in the main 

 mechanical and due to the disruptive force of crystallizing or efflorescing 

 salts deposited in the pores by intermittent submergence and drying 

 out (30, 38, 49, 56). 



Of course, as has been pointed out, action of this sort is not confined 

 to concrete, and any material of construction possessing porosity is 

 liable to a similar disintegration. The remedy, therefore, is to prevent 

 the penetration of the saline solutions by the employment of courses of 

 permanent, impenetrable materials, preferably asphaltic layers. 



Where the attack is not mechanical but chemical, this remedy is also 

 applicable. Unfortunately, there are examples of construction which 

 are exceptions, and, in these, some change in the chemical or mechanical 



