Adding too much gypsum gives you the condition of "false set" wherein the ettringite stiffens the concrete. You also get this condition if you use anhydrous gypsum, the water causes the gypsum to rehydrate and stiffens the concrete mixture. The end result is that you need to remix the concrete, but in the case of too much gypsum, you're probably going to want to throw away the batch anyway. I did my master's thesis on sulfate attack in concrete, looking at a specific attack that was induced by the application of calcium magnesium acetate deicing solutions.
I think it's also important to note that lightweight concrete is primarily made from lighweight aggregates. While you can make concrete with less of the ferro aluminate phase (C4AF), thus decreasing the specific gravity of the material, most of the weight decrease comes from using slightly less cement in the batch and lighter aggregates. Remember that the specific gravity of cement is 3.15, while the specific gravity of most aggregates is about 2.67.
What I am adding is Anhydrite
http://en.wikipedia.org/wiki/Anhydrite
Gypsum is Anhydrite plus water.
I am getting a condition similar to false set, but I was trying to follow as best as I could the conditions of the concrete in the World Trade Center.
Here is a quote from Dr. Greening.
I recall reading somewhere that a lot of gypsum was added to the WTC concrete, and I mean a lot! I beleive some gypsum is allowed in lightweight concrete, but this was implying that the amount of gypsum was way out of spec. So that's one thing to consider.
Since the aggregate is small crushed limestone, and that is what is most interesting in the debate, some of it probably formed Gypsum which is why I am having the problems that I have with false set.
Thank Almond you have been very helpfully, if gypsum was formed in the concrete it can store sulfuric acid the same as the drywall did, and as the limited samples that I prepared of the concrete did.
Thank you, your incite has been most helpful.
When heated to 350c Gypsum breaks down into anhydrite and water releasing the trapped water and sulfuric acid.
That can happen in wall board or in concrete with high Gypsum.
I am just trying now to figure out the maximum acid that a content that a four in slab of concrete can store, and stay together.
Any incite anyone has will be helpful.