Magnesium Oxide Activation?

Hi, I picked up some Magnesium Oxide powder after reading about it here. It actually gave my bowel movement this morning after taking what must have been 1400mg. It did not fizz in the glass when I added water, which I thought it would. I'm looking for the oxigenating effects on my colon, although I'm positive this is why I had a bm after constipation for weeks. Does it need to be activated? I have severe reaction to all citrus, so I purposely bought the straight powder.
Thanks!!
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