Alginate powder contains both inorganic and organic compounds. The organic compounds are derived from plant materials and WILL deteriorate over time. Two factors will accelerate this deterioration- heat and moisture.
In a closed bag at room temperature, alginate has a shelf life of at least 3-5 years. We have tested alginate that is 9 years old and it performed just fine. The two things that happen is that the setting time will get longer and longer- and the strength of the set alginate will not be as high.
High temperatures will degrade the organics as will the moisture from keeping the bag unsealed. Alginate is "hygroscopic"and will actually suck moisture right out of the air.
Accu-Cast alginates are "double bagged" when they arrive at your door. The outer bag is heat sealed to prevent any leakage and the inner bag is twist tied. If you close the bag with the twist tie and store it at room temperature, it will last a long time. If its really important that the alginate last longer than 5 years, store it in the refrigerator.