Generally I understand the need to simplify the process of refining ore into metal for most games, but since Eco is so in-depth I'd really like to see the process expanded on and made more accurate. I especially don't like the current system, where smelting ore produces tailings whether you use the bloomery or the blast furnace (either instance is incorrect, as smelting the ore does not produce tailings)
My suggestion is as follows:
Expand the mining and refining process to follow a more realistic approach used in the real world, laid out in these steps below.
Finding/Uncovering Veins of Ore -> Mining Ore Body -> Crushing Ore (Produces Oxides and Tailings) -> Smelting Oxides (Produces Metal and Slag)
This would require a new workbench to be implemented, maybe at 2 tech levels. One at the base level with the bloomery, a basic Rock Crusher, which would slowly crush ore into Oxides (the valuable mineral portion of ore) and tailings (the uneconomical/useless portion of the ore). Tailings would not have to be changed, although I feel like increasing the amount/spread of their pollution should be done.
The workbench would be called a "Stamp Mill" and should be built of wood and stone, and require mechanical energy to function either from a windmill or waterwheel.
The 2nd tier, which would be at the same level as the bloomery and also be used for making concrete, would be an Electric Rock Crusher. It would require electricity to run, and crush ore faster, as well as being able to perform two other functions. It would take rock and crush it into "Calcium Silicate" and it would take Slag and crush it into "Aggregate" This structure would of course still produce tailings from ore.
Then you would take the Oxides obtained by crushing and smelt them into metal ingots at the Bloomery or Blast Furnace. Both would produce slag, although the bloomery would produce less usable metal and more slag, and the blast furnace would produce more usable metal and less slag with a byproduct of air pollution. Slag would be used as above at the Rock Crusher to make Aggregate.
This would facilitate the small change to cement and concrete.
"Concrete" as it is in game would change from "Concrete" to "Cement" and be produced at the Cement Kiln by baking Calcium Silicate from the Rock Crusher. Cement would retain its status as a non-building ingredient material.
A new recipe would be added "Concrete" to replace the one changed to "Cement". It would require Cement and Aggregate to create, and would replace "Reinforced Concrete" as the Tier 3 building material.
Finally, "Reinforced Concrete" would be changed to a Tier 4 building material and would require Cement, Aggregate, and Rebar to be produced.