This post continues thoughts about design, last posted here.
Here is a description of how cement is made from the Portland Cement Association:
In its simplest form, concrete is a mixture of paste and aggregates, or rocks. The paste, composed of portland cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.
The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is not cost-effective and can more easily crack.
The design in this case is the proportion of ingredients in the mixture. It might happen that the ingredients formed naturally but they would be in the correct proportion only by design. That is, the particular application entails a goal, which the design meets.
Certainly concrete can and does happen naturally in aggregate rock formations. But it does not meet a need without a design. And that doesn’t happen naturally. Roads built with concrete only happen because engineers and construction crews built them. There’s nothing natural about that.