Abstract: The present invention is directed to a process for sealing and/or priming concrete by applying to concrete an aqueous polyisocyanate dispersion which has an isocyanate content of at least 0.5% by weight, based on the weight of the polyisocyanate, and is prepared bya) dispersing in water a polyisocyanate which has an isocyanate content of at least 12% by weight andb) optionally reacting the dispersed polyisocyanate withi) a primary or secondary monoamine containing at least one hydroxyl group in an amount sufficient to provide an equivalent ratio of amino groups to isocyanate groups of 0:1.0 to 0.7:1.0 and/orii) a polyamine having primary and/or secondary amino groups and a molecular weight of 400 or less in an amount sufficient to provide an equivalent ratio of amino groups to isocyanate groups of 0:1.0 to 0.4:1.