You've got to use energy to store energy, and the slab making process has taken quite a bit of energy to produce. While a lot of the materials were sourced locally (sand, aggregate and second-hand bricks), some materials like the cement, reinforcing steel and plastic are quite energy hungry to produce and travel great distances. By using these effectively, one can justify the use of such energy intensive materials. The slab will be used to catch and store energy, hopefully much more than was used to create it.
A concrete floor can be pretty unattractive on its own. Investigating flooring options is a real eye opener; it's pretty expensive whatever you choose. For the slab to be most effective in its passive solar job of storing the sun's energy, it's best to leave it exposed, rather than cover it. I decided to look into getting the slab ground back, liking the natural patterns in polished stone, but got put off by the cost of a professional job. After asking some questions about what is involved, I decided to hire a grinder and give it a go myself.
By grinding back the top layer, the aggregate (both the small and the larger in the concrete) gets exposed, adding a whole new dimension to the floor. The slab will be polished when construction is complete, and then sealed.