Nature produces porous organic materials such as wood in which the geometry is quite regular and the micro-units are of a regular shape and uniform arrangement. Wood as a structural material has been discussed in CBD 85, CBD 86 and CBD 88. Figure 1e is a micrograph of a section of wood cell at a right angle to the grain. Total porosity is greater than in any of the previous examples, but the mechanical properties of strength and modulus are higher than would be expected for such a porous system. This is because the micro-units are assembled in the manner of a composite, where the wood cells in the shape of tubes are bonded along their length with other wood cells by the cementing action of the lignin.
Wood is perhaps the best example of a natural composite.
Because the micro-units are usually very small, this internal surface can be very large. A pound of hydrated cement in concrete, which has a layered or folded sheet structure and interconnected space, will have a total surface area of about 5.5 acres. The significance of this will be discussed in a future Digest. Originally published July 1970.