I dont know if aquariumfertilizers sells it powdered or not. Where i used to work it came in 50 lb bags and it waS a coarse mix of rock, dust and chips. That powder is used to mix up with mineralized topsoil, and people use Dolomite in mts precisely because it dissolves so slowly.
Just buy the gh booster... I add a quarter teaspoon when I chang my water. Add your amount in a glass half full of water, swirl it around in the cup/glass to get it all suspended, And pour it in your tank. It clouds the water for like 5 minutes then the water is clear again after it has fully dissolved.
Be sure to mix it with water first. Don't just dump it n there, or else you'll get lumps and they will take a while to dissolve. The lumps are similar to those calcium fish feeders that people use when they go on vacation.