Glass cullet is what all your recycled bottles become: pieces of glass smaller than 19mm but larger than .075 mm, $300-500/ton depending on colour, composition (borosylicate lab glass for example) and destination – glass production, landscape material or aggregate.
Concretes made with green glass cullet aggregate have been found to be stronger, attributed to better bonding with the cement – this reported in the Magazine of Concrete Research in 2004. And while it seems that glass cullet concrete is used for lots of rough applications such as roadbeds and fill, as an aggregate in concrete it increases the strength and insulation value, glass having better thermal qualities that other aggregates.
So, what does it look like? This is a decorative application, if used as normal aggregate it would be invisible.
However, milled glass has been used as a partial replacement for cement, where the glass undergoes 'pozzolanic reactions with cement hydrates, forming secondary calcium silicate hydrate', producing 'significant gains in strength and durability of recycled aggregate concrete'. [This from a really interesting paper by Roz-Ud-Din Nassar and Parviz Soroushian here] Milled glass in the cement allows a greater range of waste material to be used as aggregate.
It is very interesting that such an ancient building material, formed through a series of chemical reactions, is so complex, certainly not to be taken for granted that we know all there is to know about it after many centuries of use.
Glass in all its lovely varieties – Gabbert Cullet in Williamstown West Virginia: