▁For ▁example , ▁fly ▁ash ▁in ▁its ▁natural ▁state ▁is ▁typically , ▁but ▁not ▁always , ▁more ▁" po zz olan ic " ▁than ▁vol can ic ▁ash . ▁ ▁An ▁important ▁consideration ▁in ▁choosing ▁a ▁concrete ▁material ▁is ▁its ▁strength - development ▁within ▁a ▁specified ▁time ▁period , ▁ ▁which ▁must ▁match ▁or ▁exceed ▁a ▁project ' s ▁specific ations . ▁According ▁to ▁Ron in , ▁the ▁current ▁upper - limit ▁for ▁using ▁E MC ▁made ▁from ▁natural ▁po zz ol ans ▁is ▁ 6 0 % ▁for ▁practical ▁large - scale ▁usage . ▁ ▁Less ▁no x ious ▁em issions ▁and ▁low ▁le ach ability ▁of ▁E MC s ▁The ▁E MC ▁activation ▁of ▁fly ▁ash ▁is ▁a ▁mechanical ▁process , ▁and ▁does ▁not ▁involve ▁he ating ▁or ▁burning . ▁ ▁Le ach ability ▁tests ▁were ▁performed ▁by ▁L T U ▁in ▁ 2 0 0 1 ▁in ▁Sweden ▁on ▁beh alf ▁of ▁a ▁Swedish ▁power ▁production ▁company .    Th ese ▁tests ▁confirmed ▁that ▁E MC ▁made ▁from ▁fly ▁ash ▁" show ed ▁a ▁low ▁surface ▁specific ▁le ach ability " ▁with ▁respect ▁to ▁" all ▁environment ally ▁relevant ▁met als ." ▁ ▁E MC s ▁using ▁California ▁vol can ic ▁ash ▁ ▁Ener get ically ▁modified ▁c ements ▁have ▁been ▁used ▁in ▁large ▁infrastr ucture ▁projects ▁in ▁the ▁United ▁States . > ▁E MC s ▁made ▁by ▁replacing ▁at ▁least ▁ 5 0 % ▁of ▁the ▁Port land ▁c ement ▁with ▁have ▁yield ed ▁consistent ▁field ▁results ▁in ▁high