When it clumps like that you can break it up with a hammer and chisel or just let it dissolve. I once had a 45 lb bucket of Proteam Supreme Plus that had become a solid lump in the container. I had to break it up into chunks with a hammer and then put them in the pool. They all dissolved in about a day. I never noticed borax to be exothermic on dissolving but calcium chloride certainly is!