Technically speaking, its hard to say what you will actually get, concrete is a composite mix of differing sieve size aggregate, binder(cementitious material=Portland), water and other admixtures depending on the actual mix design. To attain a certain compressive strength, the mix design must have the correct amount of these materials to reach a specified compressive strength. It has to be mixed uniformly and with the same materials. The same materials meaning each of those is tested, gradation, specific gravity, moisture and or what have you. Change sources, you have to test the mix design again to prove it out to meet the specifications called for. Given the sieve size of portland, which to me is fine dust,finer than sand and you replace say the smaller aggregate(sand)that the design mix calls for, with the questionable portland,it could go 2 ways. One would be it increases the strength, because the material still retains its qualities, (you already have the required portland) and it will hydrate. Add to much the water cement ratio is off, and it could effect it adversely (portland is still good) material could easily fail due to proportion, water/cement ratio. I can't say if excess portland would make it more like hydraulic cement and give you minutes to place before it starts to hydrate and is unworkable to place. If the questionable portland replaces a larger sieve size aggregate, say like sand, (and is now inert from age or moisture,does not bind etc.) which is called for, that mix being tested and proved, you would in fact have to test it with that(your questionable portland) material in measured quantities that is used in place or addition to what is being used to create the mix,different than was tested and proven, if that makes any sense. In summary, DO NOT fool with a proven mix design without testing it first, before placing in say a structural component or even a slab for that matter if in fact a load will be imposed on it.
The other point being that each amount of aggregate of a certain sieve size is usually required from the smallest to the largest in measured quantities, so in effect if you place a bunch of powder in that mix to represent/replace the sand and its much smaller in sieve size and you overwhelm the mix with it, (say it seems like a good way to get rid of it etc.) it would be my opinion that the results may or may not add up to what you need in compressive strength, without testing you will not know. ASTM reference standards on this are actually fairly easy to understand as it applies to a mix design. The safe thing is to test the mix and know it proves out.
I suppose the point of writing this here is to make sure anyone reading does not get the idea that its okay to randomly change, add or substitute materials to a concrete mix design, one that has been proven, and expect the altered or modified material to perform the same as what was tested and proven to be a specific compressive strength. Its also hard to say how it would mix in, if too much, does it concentrate in areas, violating the compressive strength, cause failure say on a slab surface where it prematurely fails, spalls and exposes larger aggregate below etc. There is a myriad of things to consider when doing this. However, if its really a concrete fill for something non critical, non load bearing, not a column, beam, load bearing slab/deck, precast structure/pipe even a pole base for like a traffic light with a moment arm, no cause for concern, but use that mix without testing you very well can create an unsafe condition, premature failure or poor performance. Might not be the clearest way to describe it here, also a bit redundant after re-reading but hopefully the point gets across about the potential problems with untested, altered or modified mix designs.
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