I was a young engineer when the Feds first made it more expensive to landfill clinkers (fly ash). The coal industry immediately looked for other places they could dispose of it without paying a penalty. One of them was to use it in place of cement in concrete.
When we started doing that (following the recommendations of the concrete industry), I immediately started having unexplained concrete failures. The recommendations of the concrete industry changed, but still included fly ash. Rinse and repeat. This went on for about 10-15 years.
Finally, I started refusing to allow fly ash to be used in my projects. The cost was slightly higher (a very small increase on the total cost), however, the unexplained failures stopped.