the extraction of water from lunar soils becomes operational, steel industry technology can provide a simpler foaming method. Blast furnace slag as produced is thoroughly anhydrous. Water injected into the molten slag will dissolve rapidly, however, as appreciable water must dissolve before the partial pressure of water reaches 1 atm. On subsequent cooling, anhydrous silicate phases form, increasing the concentration of water in the residual melt, until finally this vapor pressure exceeds 1 atm and the material foams. Much space construction will involve the use of very large, low-mass units, but rigidity will certainly be a common prerequisite, to maintain alignment of various parts. The most logical material for rigid construction in space, i.e., that which requires a minimum investment in materials transport costs, from Earth or Moon, would appear to be a foamed silicate glass made of lunar soil. Considerable discussion at the workshop centered on the seemingly difficult problem of how blocks of such foam could be combined to form long rigid columns on which large two- or three-dimensional structures might be based. Numerous procedures, including even “welding” of the blocks together by various exotic sources of heat has been suggested, but the shrinkage of such foams on surface melting prior to “welding” would present serious difficulties. I believe that at least one rather simple solution to this problem exists. Rigidity in a structure is normally achieved through a combination of elements, some in compressive loading and others in tensile loading. From the literature and the workshop, it appears that glass fibers (8) might provide an effective basis for most elements in tension, and foamed glass for elements in compression. I suggest that thin glass fiber elements be run through holes in a stack of foam blocks, with tension adjusted to hold the blocks together, in analogy to the use of steel cables in prestressed concrete beams. Such a combination beam would be limited probably by the compressive strength of the foam, which could be controlled, in turn, by the size and amount of bubbles. Silicate melts, both in metallurgy and geology, commonly show a nonuniform bubble size and distribution near to a quenching surface. A normally low-density foam may have only a few small bubbles at its contacts with a heat sink, such as a slag pot or a cold rock. Thus, it should be possible, by adjusting the heat flow during nucleation and growth of the bubbles in the lunar foam blocks, to obtain a block that has a dense, strong rind and a lightweight core and that hence is considerably improved in rigidity per unit weight. Nature evolved that optimum structure in the bones of animals millions of years ago, and we need only reproduce it. REFERENCES I. D.R. Criswell and R.D. Waldron, Commercial Prospects for Extraterrestrial Materials, J. Contemporary Business 7, 153-169, 1978. 2. G.K. O'Neill. The Colonization of Space, Physics Today 27, 32-40, 1974. 3. D.R. Criswell, Extraterrestrial Materials Processing and Construction, Final Report, NSR 09-051-001, Lunar and Planetary Institute, Houston, 1978. 4. J. Billingham, W. Gilbreath, and B. O’Leary, eds., Space Resources and Space Settlements, NASA SP 428, NASA Ames Research Center, Houston, 1979. 5. R. Williams, D. McKay, D. Goles, and T. Bunch, Mining and Beneficiation of Lunar Ores, in Space Resources and Space Settlements, J. Billingham, W. Gilbreath, and B. O’Leary, eds., NASA SP428, pp. 243-255, NASA Ames Research Center, Houston, 1979. 6. E. Roedder, Silicate Liquid Immiscibility in Magmas and in the System K2O-FeO-AI2O:I-SiO2: An Example of Serendipity, Geochim. Cosmochim. Acta 42, 1597-1617, 1978. 7. J.F. Schairer and N.L. Bowen, Melting Relations in the Systems Na2O-Al2O3-SiO2 and K2O- AI2O3-SiO2, Am. J. Sci. 245, 193-204, 1947.
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