PACIXI, METAMORPHISM OF PORTLAND CEMENT 133 



In general, the consideration is worthy of attention whether concrete 

 structures which are under stress are not more liable to chemical disin- 

 tegration than those which are in repose, or whether a single structure is 

 not more liable to this action in its strained parts than in those not so 

 affected. We have data to show that strained iron is more liable to corro- 

 sion than unstrained, and it has been asserted that strained minerals are 

 more acted upon by underground solutions (104). 



A number of protective measures against the action of saline waters 

 upon concrete have been suggested and tried, but none has been so strik- 

 ingly effective as to achieve universal recognition. The simplest remedy 

 suggested is to make the concrete for such uses denser and more imper- 

 vious by the employment of a greater proportion of cement, yet this may 

 not always be practicable. When concrete is exposed to the gases result- 

 ing from the decomposition of sewage, it is suggested that even such a 

 proceeding may be of no avail (29). 



Previous air-hardening of the concrete before laying under sea water 

 is acquiring more widespread use and is highly recommended (87). 

 The cause of its protective action is attributed to the carbonation of the 

 calcium hydroxide (48). 



Variations in the fineness of grinding and in the chemical composition 

 of the cement used in concrete for sea-water construction have been pro- 

 posed. The French specifications for sea-water cements call for a finer 

 grinding than that which is required for ordinary construction. Much 

 has been claimed regarding the resistance to disintegration offered by the 

 so-called "iron ore" cement, which contains a minimum of alumina, this 

 being almost entirely replaced by iron. 



Having material in suspension. The peculiar nature of the series of 

 compounds forming and formed from cement, in that they are all of 

 relatively low solubility, tends, as has been before observed, to retard the 

 reactions which may occur. Mechanical agitation, by promoting diffu- 

 sion and by transporting the reacting materials to their possible spheres 

 of action, will accelerate these reactions. The motion of water, per se, 

 can and does produce this effect, and when the water is armed with sus- 

 pended material, its activity in this direction is greatly enhanced. 



Where water has immediate access only to the outer surface of a mass 

 of set cement and its pressure is low, the effect is a slow corrasion of the 

 dense surface skin and ultimate removal thereof, rendering the interior 

 gradually more accessible. Ordinarily, this process is a slow one, al- 

 though under certain conditions, as in coast protection works where the 

 velocity of the water is high and the suspended material coarse and 

 plentiful, the destructive effects are more to be reckoned with. 



