ness ▁ 7 . 0 ▁to ▁ 7 . 5 ▁on ▁Moh s ▁scale );    or ▁in ▁combination ▁with ▁sil ica , ▁to ▁form ▁str ät ling ite ▁( Ca 2 Al 2 Si O 7 · 8 H 2 O ▁or ▁C 2 ASH 8 ▁in ▁c ement ▁chem ist ▁notation ), ▁which ▁ge olog ically ▁can ▁form ▁as ▁x en ol ith s ▁in ▁bas alt ▁as ▁met am orph osed ▁lim estone . ▁Poz z olan ic ▁c ement ▁chem istry ▁( al ong ▁with ▁high - al um ini ate ▁c ement ▁chem istry ) ▁is ▁complex ▁and ▁per ▁se ▁is ▁not ▁const rained ▁by ▁the ▁for ego ing ▁path ways . ▁For ▁example , ▁str ät ling ite ▁can ▁be ▁formed ▁in ▁a ▁number ▁of ▁ways , ▁including ▁per ▁the ▁following ▁equation ▁which ▁can ▁add ▁to ▁a ▁concrete ' s ▁strength : ▁ ▁C 2 A H 8    +    2 C SH    +    A H 3    +    3 H      →      C 2 ASH 8      ( cement ▁chem ist ▁notation ) ▁▁ ▁The ▁role ▁of ▁po zz ol ans ▁in ▁a ▁concrete ' s ▁chem istry ▁is ▁not ▁fully ▁understood . ▁ ▁For ▁example , ▁str ät ling ite ▁is ▁met ast able , ▁which ▁in ▁a ▁high ▁temperature ▁and ▁water - content ▁environment ▁( that ▁can ▁be ▁generated ▁during ▁the ▁early ▁cur ing ▁stages ▁of ▁concrete ) ▁may ▁of ▁itself ▁yield ▁stable ▁calci um ▁al umin ium ▁gar