SeaScope Magazine Reprint, it states that, "One of the theories why calcium hydroxide might produce better results than calcium chloride has been that the high pH and high calcium in a saturated calcium hydroxide solution cause precipitation of phosphate from the freshwater, eliminating this algae fertilizer from the solution." Take a look at the results of the test they did using calcium hydroxide to see what the rate of reduction of phosphates were. It is interesting that, when allowing the mix to sit, after two days there was only a 50% drop in phosphates, but after five days there was a 90% drop. The conclusion is that the drop in phosphates, when using calcium hydroxide, is slow and if the mix is used right away, any phosphates remaining in the mix will be pumped into your tank.
Source: http://saltaquarium.about.com/cs/phosphate.../a/aa060398.htm