Acids dissolved in water attack concrete by destroying hydrated and not hydrated cement compounds (more details on concrete technology
Due to the transformation of easily soluble calcium hydroxide into more stable CSH* or CSAH** phases, acids have less possibility to attack concrete when metakaolin is used. This is especially true for concretes with a low w/b ratio***.
Portland cement concrete usually does not have good resistance to strong acids.
Metakaolin protects the concrete structure more efficiently because of the small pore size
formed during the curing phase.
General information on acid-reaction
* CSH - calcium silicate hydrate
** CSAH - calcium silicate aluminum hydrate
*** w/b ratio - water to binder ratio
Pictures by temcon, Germany
Pictures on the right show the high porosity of unmodified mortars (above)
compared the high density of modified one (below)