I have long been a "fan" of unbound macadams, which are granular materials that exhibit a surprisingly high strength when compacted, simply because of the careful grading of the various sizes of material used. In contrast, bound materials gain their strength mainly from being held together with a binder such as cement (as in concrete) or bitumen (as in bitumen macadam), although some of their strength still comes from the interlocking of the aggregate.
It comes from the 1970s when I wrote specifications for civil engineering projects such as airports and roads. I did an extensive literature survey of a vast range of bound and unbound materials, and developed a library of standard specifications which could be incorporated into the contract documents for each specific project. Coupled with extensive site experience on road and airport projects, I became a specialist in construction materials, something which I thoroughly enjoyed, and which paid my mortgage for 30+ years.