me able ▁and ▁with ▁a ▁poor ▁resistance ▁to ▁acid ▁attack , ▁which ▁causes ▁it ▁to ▁release ▁carbon ▁dio x ide . ▁ ▁Poz z olan ic ▁con cre tes , ▁including ▁E MC s , ▁however , ▁continue ▁to ▁consume ▁the ▁soft ▁and ▁por ous ▁Port land ite ▁as ▁the ▁hyd r ation ▁process ▁continues , ▁turning ▁it ▁into ▁additional ▁hard ened ▁concrete ▁as ▁calci um ▁sil icate ▁hyd rate ▁( C - S - H ) ▁rather ▁than ▁calci um ▁carbon ate . ▁This ▁results ▁in ▁a ▁dens er , ▁less ▁per me able ▁and ▁more ▁dur able ▁concrete . ▁This ▁reaction ▁is ▁an ▁acid - base ▁reaction ▁between ▁Port land ite ▁and ▁sil ic ic ▁acid ▁( H 4 Si O 4 ) ▁that ▁may ▁be ▁represented ▁as ▁follows : ▁ ▁Ca ( OH ) 2 ▁+ ▁H 4 Si O 4 ▁-> ▁Ca ^ 2 + ▁+ ▁H 2 Si O 4 ^ 2 - ▁+ ▁▁ 2 H 2 O ▁-> ▁Ca H 2 Si O 4 ▁* ▁ 2 H 2 O   ▁Further , ▁many ▁po zz ol ans ▁contain ▁al uminate ▁( Al ( OH ) 4 − ) ▁that ▁will ▁react ▁with ▁Port land ite ▁and ▁water ▁to ▁form : ▁ ▁calci um ▁al uminate ▁hyd r ates , ▁such ▁as ▁calci um ▁al umin ium ▁gar net ▁( h yd rog ross ular : ▁C 4 A H 1 3 ▁or ▁C 3 A H 6 ▁in ▁c ement ▁chem ist ▁notation , ▁hard