Both, Type II White Portland Cement and Type II Gray Portland Cement
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Yep, thats suppose to be the stuff that "sets" quickest and easiest to get the pH down. I have heard hit or miss. Some people never had a problem with it, and others can let it sit for months and still raise pH.
back when I was thinking about building and selling rock, I found a guy that had a 1 month cycle. He made them and set them on a basketball court in the sun for 1 week to cure. then 1 week in a outside tank. and the last 2 weeks were in an outdoor tank plumbed into his huge growout system. by the time he went to ship they were already getting coraline growth!!! he just had a slow rotation of rock moving at all times