If the only reason for using the sealant is to keep the pH from going high, then it is not needed. The high pH comes from the migration of calcium hydroxide, a chemical byproduct of the hydration process of the cement in the concrete. The hydroxide has a pH of 14 and will increase the pH of the water, if there is inadequate buffering with the water to prevent the pH shift. The typical buffer that you will find being used in ponds is baking soda, which has a pH of about 8.3 and will keep the pH from falling below that level. It will also split to make carbonate and hydrogen ions. The hydrogen ions will react with the hydroxide to make water. The carbonates ions are also high pH, but can be removed by calcium within the water column. The calcium hydroxide has one calcium and two hydroxide ions, so the reactions will deplete the calcium leaving a surplus of the carbonates. Adding calcium chloride, swimming pool hardness increaser or some deicers, will provide the calcium ions needed to react with the carbonates, Calcium carbonate is a mineral like limestone and unless the pond were to go acid will remain as a solid precipitate. With all of the hydroxide reacted and the carbonates reacted there is nothing affecting the pH. Over time the migration of the calcium hydroxide will diminish until effectively stopping.
Maintain a KH level of at least 150 with the baking soda, and watch the GH to see what the minimum is in the first week or so of the pond having water and add enough calcium chloride to raise the GH to 100ppm above that minimum. The intent is to have a means of measuring the GH as calcium, where the test measures the total of calcium, magnesium, iron, and a number of other elements.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard