PACINI, METAM0RPHI8M OF PORTLAND CEMENT 181 



of the action is physical or chemical is not quite settled. There are not 

 lacking investigators who assert that the destructive action is mostly 

 physical and is due, among other causes, to intermittent submergence 

 and consequent deposition, by evaporation of crystals in the pores of the 

 structure, which, either by their pressure of formation or by expansion 

 during efflorescence, have a disruptive effect similar to that of frost (98). 



There are those who hold that the action is entirely physical, and is 

 due to this factor and the effects of frost (91, 102), although probably 

 the latter is seldom the case in sea water, owing to its low freezing point 

 (50). The effect of direct sunshine has been found deleterious when 

 alternating with that of tidal action (20). Undoubtedly, all of these 

 factors contribute to the total effect, and there is as well a marked 

 chemical action. 



The chemical effects of sea water upon cement are capable of various 

 interpretations. They are summarized as the formation of complexes by 

 the action of the dissolved sulphates and chlorides in the water upon the 

 calcium silicates and aluminates of the cement (74). It has been stated 

 that sodium chloride solutions have the power of dissolving calcium sili- 

 cate with the formation of an unknown salt (58, 70), and also that the 

 sodium chloride enters into combination in the mass, the chlorine ion 

 entering into the combination calcium chloro-aluminate, and the sodium 

 ion combining with lime, silica and alumina, to form compounds of the 

 nature of the zeolites. 



Working with strong solutions of the individual salts of sea water, it 

 has been found that the chief harmful constituent is magnesium sulphate, 

 and it has been suggested that this salt reacts with the lime of the cement 

 to form calcium sulphate and magnesium hydroxide. The calcium sul- 

 phate further reacts with calcium aluminate to form a calcium sulpho- 

 aluminate, which by swelling causes the disruption of the mass. The 

 magnesium hydroxide formed has been regarded as a restraining agent, 

 by virtue of its filling up the pores of the cement and preventing further 

 ingress of sea water (70). Again, the disruption has been directly at- 

 tributed to the increase of volume caused by the formation of this mag- 

 nesium hydroxide (46). It has been calculated that, apart from the 

 formation of hypothetical sulpho-aluminates, a molecularly equivalent 

 amount of calcium sulphate replacing the calcium hydroxide of the ce- 

 ment occupies 2.08 times as much space and is, therefore, the cause of 

 the disintegration (13). 



Alkali and deep rock waters. — Burke and Pinckney (13) have formu- 

 lated a working theory of the action of the various salts common to all 

 natural waters. They attribute the disruptive action to more rapid re- 



