Put a couple packets of sugar in about two quarts of leftover bag concrete once just to see how it went. Initially it acted like it had plasticizer and got loose and wet again. A day or two later it was acting like it had just set and a month later I could still crumble the edges off with my thumb.

Problems with using this for sabotage. Any serious project will have an engineer checking every truck and it will not pass unless the person adding water at the plant was in on it and did some very good calculations. Then the saved samples would immediately give them away in the investigation. During and after the pour there are too many people watching. I would have to say putting brown sugar and salt in the footing trenches the night before it’s poured or spraying (big puddles will get drained) a solution on top of the plastic to look like rain or dew.