Carboniferous stones come in a wide variety of forms: compact, crystalline, crynoidal, oolitic, pisolite, reef, and metamorphosed (marble), all with quite different properties from a constructional point of view.

The core of the Giza pyramids is made from oolitic limestone quarried nearby on the plateau. The rock comprises oolites (the shells or fossilised remains of small sea creatures) bound together by clay. The quality varies considerably across the plateau with some areas being so marly that the rock is crumbly and easily eroded by rainfall. The outer casing stones were made from fine white Tura limestone from across the Nile.

I think its unwise to claim any association between Silbury and the pyramids based on the materials used, since in both cases the builders used the most readily available materials at their respective sites.

Furthermore, the statistical significance in the agreement of a single number like five is virtually nil. It's probably more significant that it happens to be the number of fingers and thumbs on one hand.