602 CEMENTS, LIMES, AND PLASTERS. 



Effects of exposure to sea-water. Portland cement is not entirely 

 satisfactory in its resistance to exposure to salt water, though in part 

 this is often due to the use of porous mixtures which permit access of the 

 water to the interior of the block of cement or concrete. The use of 

 richer mixtures, or at least of a richer mixture for the surface of the 

 block, will do away with many of the difficulties encountered. Aside 

 from this, two methods of improvements have been advocated. One 

 is to make the cement more resistant of itself by making it of such 

 a chemical composition as will show the maximum resistance to the 

 effects of salt water. This is the method of Le Chatelier, discussed 

 below. The second method is, to add to the cement, trass, slag, or 

 other puzzolanic material, in order that the lime liberated by the cement 

 during hardening may be taken up and combined with the trass. 



Le Chatelier considers that the aluminous compounds present in 

 Portland cement are the direct cause of its destruction by sea-water. 

 His theory, to account for this disintegration, is as follows: Free lime, 

 liberated during the hardening of the cement, reacts with the mag- 

 nesium sulphate always present in sea-water, to form calcium sulphate. 

 This in turn reacts with the calcium aluminate of the cement to form 

 a sulphaluminate of lime, which swells considerably on hydration and 

 thus disintegrates the cement mass. The extent of the disintegration 

 varies directly with the percentage of alumina present in the cement. 

 Cements containing 1 or 2 per cent of alumina are, for example, practi- 

 cally unaffected by sea-water; while in cement containing as high as 7 

 or 8 per cent of alumina the swelling and consequent disintegration 

 are very rapid. 



If the alumina of a cement be replaced by an oxide not reacting 

 with calcium sulphate, the stability of the cement in sea-water is 

 greatly improved. Le Chatelier has demonstrated this by preparing 

 cements in which the alumina was replaced by oxides of iron, chro- 

 mium, cobalt, etc. All of these were more resistant than an alumina 

 cement to the disintegrating effect of lime sulphate. The best effects 

 were obtained when iron oxide was used, a cement corresponding in 

 composition to 5Si02,Fe20 3 ,17CaO being found to be not only stable in 

 presence of sea-water, but to possess excellent mechanical properties. 



DevaFs researches * on the effect of direct addition of calcium sul- 

 phate to various cements confirm the above theory. Each of the finely 

 ground cements tested was completely hydrated by mixing with 50 per 

 cent of water, and storing the mixture under water for three months out of 



* Abstract in Joura. Soc. Chem. Industry, vol. 21, p. 971-972. 



