The theory of the action of sea water upon cements is not fully understood. It is known that some cement structures exposed to the worst conditions have given most satisfactory results, while others have failed in greater or less degree. It may be said at once, however, that many of the most eminent and conservative engineers consider that the failures that have occurred in the use of Portland cement in sea water are due to improper specifications, proportions and manipulation, rather than to any defect in Portland cements as a class.
496. It is thought the following represents, in the main, the most generally accepted theory of the chemical action. In the setting of cements that are rich in lime, the whole of the lime is not engaged in stable compounds, and when placed in the sea the sulphate of magnesia of the sea water is able to combine with the lime, forming calcic sulphate, the magnesia being precipitated. The discovery of magnesia in decomposed mortars led, at first, to the supposition that the cause of failure was the presence of magnesia in the cement when used. If the water level about the structure changes frequently, as is usual, or if the wall is at times subjected to a greater head on one side than on the other as in tide docks, the percolation of water through the wall is stimulated, and the sulphate of lime may then be washed out if the mortar is quite pervious, and more will be formed from a fresh supply of sea water attacking the lime of the cement, until the latter is destroyed. If, however, the sulphate of lime is not washed out, it may crystallize and thus cause swelling of the mortar.
497. It would appear from the above that for successful use in sea water the hydraulic index of the cement should be high; that is, that the lime should be comparatively low in order that the lime compounds may be more stable. For this reason it is not impossible that some of our natural cements, which are so much more nearly uniform than the Roman cements of Europe that have been condemned for this reason, may give fairly good results in sea water. The fact that the mortars of natural cement are more permeable than those of Portland, is, however, a serious defect.
Following a similar reasoning, Dr. Wm. Michaelis has advanced the theory that if trass, or other pozzolanas of proper composition, be mixed with Portland cement subsequent to the burning, the hydrate of lime which separates from the cement in hardening will at once combine with the pozzolanas, forming a stable compound. This view, however, has been vigorously opposed by the Society of German Portland Cement Manufacturers, as well as by many engineers, especially of France, and the discussion is not yet at an end.
M. Candlot1 says that, from the experiments of various engineers, "we have arrived at this conclusion, that the only remedy to adopt against decomposition is to prevent the sea water from penetrating the mortar. We are led thus to dismiss the chemical reactions of sea water on mortars and to consider their action from a purely physical standpoint".
498. To resist the attacks of sea water the mortar should not only be impervious, but also as little porous as possible. The cement should be finely ground and should not contain free lime. The content of magnesia and of sulphuric anhydride should be as low as possible, the latter not exceeding one and five-tenths per cent. The proportion of lime should not be too high, and above all, special pains should be taken with the manufacture to insure proper comminution and mixing of the raw materials, and uniform burning. The addition of sulphate of lime to regulate the setting is believed to be injurious for cements to be used in sea water; even two or three per cent, is said to cause rapid disintegration, and in the specifications for recent extensive works in dock construction, the addition of gypsum or other foreign matter was entirely prohibited.
1 "Le Ciment," September, 1896, quoted by F. H. Lewis, M. Am. Soc. c. E. Trans. A. S. C. E., Vol. xxxvii, p. 523.
Although slag cements have given good results in the sea for a short time, it is considered that they will not, in general, resist the action of sea water for long periods.
499. Sand or aggregate containing argillaceous or soft calcareous matter should be avoided for works in the sea. Two instances of failure of sea walls in which shells were used as the aggregate are mentioned by Col. Wm. M. Black,1 and although the failures are not definitely traced to the calcareous matter in the concrete, the fact that experiments have shown that calcareous sands do not withstand the action of sea water, makes it probable that this was an important cause of the failure.
Fine sands that give porous mortars, though not easily permeable, are to be strictly avoided. Coarse sands giving permeable, though not porous, mortars are better, but still leave much to be desired as to immunity from decomposition. The best sands are those containing various grades of sizes of particles from coarse to fine, as mortars made with such sands are not only compact, but practically impermeable.
500. Since the mortar and concrete should be made as compact as possible, the precautions mentioned under the head of water-proof mortar and concrete should be taken in the preparation of mortars and concretes for use in the sea. That is, the proportion of cement should exceed the voids in the sand and the mortar should exceed the voids in the aggregate.
M. Alexandre has found that the mortars mixed to the ordinary consistency are attacked least by sea water. When specimens are merely immersed in the water, those mixed dry suffer the most, but some tests indicate that if mortars are submitted to the filtration of water soon after made, those mixed wet are most easily decomposed. As to whether fresh or salt water should be employed in mixing mortars to be used in sea water, although Mr. Eliot C. Clarke, M. Paul Alexandre and many others have investigated this subject, the conclusions are not definite and it is probable that either may be used as convenient.
1 Trans. A. S. C. E., Vol. xxx, p. 601.