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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 on wikipedia.

* CSH - calcium silicate hydrate
** CSAH - calcium silicate aluminum hydrate
*** w/b ratio - water to binder ratio
Pictures on the right show the high porosity of unmodified mortars (above)
compared the high density of modified one (below)
Pictures by temcon, Germany

Newchem GmbH, Flamminggasse 79, Baden, Austria –