Methods for treatment a heavy metal-contaminated solid, semi-solid, liquid or gaseous matrix with magnesium oxide, magnesium sulfite, magnesium sulfate, magnesium chloride, magnesium hydroxide, and mixtures of any two or more of these. Methods for treatment of a solid or semi-solid waste containing unacceptable levels of leachable aluminum and reactive metallic aluminum-containing waste with magnesium oxide, magnesium sulfite, magnesium sulfate, magnesium hydroxide, magnesium chloride, and mixtures of any two or more of these.