There were two different ideas on concrete. First one was better...

There were two different ideas on concrete.
First one was better properties which has not been talked much since.
Second one was rheology modifier which is still current.

As per DFS the feed to produce the HRM is run of mine ore, same as feed for other products. I agree that sand content in this product could be higher than in others but it still needs to be kaolin.
Also most probably fine particle size distribution not required so in theory it could be part of the "waste stream" from wet refining process.
It would still need to be produced in a format allowing exact dosing and mixing and we have never heard anything on how this can be achieved.

Use of overburden for concrete additive is a HC creation.

We have heard about potential use of sand separated in wet refining plant but I would guess that demand for sand is rather insignificant in the economic transport range distance from GWKP.