Sorry, I forgot to tell you the specs for these cassons is 25 mpa. I would like to use a 20mpa footing mix with 40 to 50% slag . Iam I asking for trouble? Thanks
If your spec is 25mpa and you are using a 20mpa mix design with slag cement, yes, you are asking for trouble. You will not likely achieve anywhere near the required design strength in the time indicated. Ultimately you might, but that is questionable as well.
Use a 25mpa mix with slag cement if you want, with the understanding that your strength gain will be slower and might not achieve the ultimate in the time frame (28 days, typically).
If acceptance of your slag concrete is based on testing cylinders at 28 days, I would suggest using a higher strength, say 32 MPa. That way you should get 25 at 28 days and avoid trouble. But that should be confirmed with testing of the mix design, and approved by the engineer prior to construction.
Are there sulphates or chlorides? The W:C ratio may be too high and 20 MPa doesn't have much resistance to deterioration. Maybe not a good idea for foundations.
