42nd Annual Meeting – Architecture Session, 31 May, "Lime-Metakaolin Grouts for Conservation" by Norman Weiss

This technical talk discussed a new method and material for architectural conservation. Norman Weiss began by addressing the problems of lime grouts, and the nature of metakaolin. The problem of lime grouts is the anaerobic nature of the area the lime is being applied to, as the lime requires carbon dioxide from air for the setting phase of the lime cycle. Metakaolin is a class N pozzelan, a ‘thermally activated clay’, where dehydroxylation is accompanied by a loss of crystal structure. Metakaolin is between fumed silica (0.3 µm) and Portland cement (5 µm) in terms of particle size. Metakaolin is very quickly and specifically dehydroxylated between 500 and 530 degrees Celsius, a process which Weiss noted you “don’t have to finesse”.
Weiss continued by addressing the purpose of pozzelans within architectural conservation. The material must densify and fill gaps, must strengthen in order to create bonds, must reduce the amount of cement needed, and must reduce the amount of ‘bleed water’ in order to be the most desirable pozzelan possible. Metakaolin has the potential to achieve all of these goals, if used in a specific way.
This material has been previously looked at somewhat, the first study having been completed in 1993, and the commercial introduction of the material in 1994. Weiss also noted that there has been a lot of research into metakaolin as a grout in Portugal.
Metakaolin has a high water demand, and is not great for a grout, as it does not set well; in order to function, it requires a superplasticizer. Weiss and his colleagues have experimented with the reaction between lime and metakaolinite, which forms Straetlingite. What seemed to be the most important part of this talk was the discussion of the new methodology that Weiss and his colleagues have developed and patented using this material. The wall is mechanically stabilized, and the void is filled through a tube. This method gives the material the time necessary for the slow-strengthening material to achieve the necessary strength.
It will be interesting to see in the future the results of further study and case studies of completed projects using this material and methodology.