Solid aluminum powder
If you had a sealed container of aluminum powder that became exposed to air, your prospects for turning your current material back into aluminum powder are grim.  

Your block probably appears like something that became clumpy when exposed to humidity (table salt, sugar, etc.), and that you aught to be able to just break it apart with a little force to get the powdery stuff again.  

Unfortunately, aluminum reacts with the oxygen in air to form a very hard oxide layer, which is likely what is now binding the formerly pure aluminum powder together. In other words, you don't just have aluminum "sticking" to aluminum, but you've formed strong chemical bonds, actually creating a new compound altogether (aluminum oxide) There is likely no simple solution for returning your 150 pound blocks of mixed aluminum / aluminum oxide back into pure aluminum powder.