Magnetic Iron Ore as a Material for Concrete Blocks

Abstract

THE account of harbour works in NATURE of October 24 (p. 639) causes me once more to draw attention to the great advantage which would be gained by the use of magnetic iron ore as a material for concrete blocks. If magnetite is used instead of ordinary rock in the shape of fragments, and magnetic sand or ilmenite sand instead of common sea sand, concrete blocks can be obtained which have all the strength of the ordinary concrete blocks and which weigh, when immersed in water, exactly twice as much as the ordinary blocks. Such an increase in weight makes the magnetic blocks far superior as regards resistance to the waves. Work constructed with magnetic blocks will stand when other work will be destroyed. This superior effect of magnetic blocks is quite independent of the size of the blocks. The artificial increase of the size of ordinary concrete blocks is mentioned as a means of increasing the power of resistance, but there are certain to be some objections to this method, and if the great masses are ruptured after the rusting away of the cases, portions may give way. It is therefore better and more convenient to use the superior composition. As regards the expense, it may be mentioned that to obtain a good effect it is only necessary to use magnetic blocks for the most exposed spots of a dam, and more in the nature of a surface coating. There are immense natural deposits of magnetite, also of titanium ore, which latter is not of value for steel making, and it would surely be possible to obtain the necessary quantities in Scandinavia, or if for harbours in the East, then there would be inexhaustible supplies in southern India not too far from the coast. It has been argued that the iron ore would decay on exposure to sea water and that it would injure the cement. This may be true for inferior iron ores, but not for rich, pure magnetite and ilmenite, as I have proved by direct experiments. I have exposed fragments of magnetite to the action of filtered sea water in clean glass jars where every trace of decomposition would have been detected, but though I continued the test for a year the specimens stood the test very well. Moreover, I made sample blocks with Portland cement and subjected them to crushing tests, which showed them to be perfectly satisfactory as regards strength.