AS a conservator for historic buildings, practicing in New York City for the last 10 years, I read with great interest the article ?Good glue to keep heritage structures alive.? (Inquirer, 6/14/09) I think it is great news that architects and manufacturers are working together to develop suitable mortars for our heritage buildings like the Dingras Cathedral.

I understand from the article that they are trying to develop a suitable replacement for the original lime mortar of Dingras Cathedral. In particular, they are looking primarily at strength?that the new mortar ?bind strongly? as the original, so that it may also be used on ?most old churches in the country.?

I?d like to point out that while mortar strength is important, it is not the only quality that should be considered. Other properties, such as vapor permeability and flexibility are equally important. If the new mortar is stronger, less vapor permeable, or less flexible than the original, then it will cause the brick to powder or crack?prematurely decaying Dingras Cathedral. If the same mix is proposed for use on other old churches in the country, then it will cause the premature deterioration of the adobe or other brick as well.

Also note that mortars based on a known brand of cement are typically stronger, less vapor permeable, and more rigid than lime and, therefore, should be used with caution on historic buildings. As for the new cement product having ?the same ?pozzolanic? qualities as lime,? it may be pointed out that lime by itself is not inherently pozzolanic. A pozzolan is an additive to make lime hydraulic, with one of the benefits being an increase in strength. So, perhaps, another question to ask is, were pozzolans ever used in the historic mortar mix? Lime picks up strength as it cures, and may take decades to fully cure. The traditional mix of lime and sand alone, without pozzolans, could account for the very strong mortar that survives today. Adding pozzolans to the proposed new cement without verification of its existence, or in excess of quantities used originally, could make it too strong for the brick, leading to premature decay of the masonry as described above.
If the goal is to try and match the original mortar of Dingras Cathedral in the hope of identifying a suitable formulation, perhaps some questions to be considered are :1) what properties does the original have that make it so long-lasting and 2) how do we replicate these properties using modern materials?

I am glad the United Architects of the Philippines Vigan and Holcim Philippines will study the new mortar, but I think it would also be a good idea to analyze the old, original lime mortar. The analysis would point us to the properties that caused it to last through centuries. Once we identify these properties, then we can evaluate the proposed Holcim Wallright cement against these same properties.

In the end, if we can?t be certain of what constituents or properties the original mortar had, then how can we hope to make something just like it?

?TINA PATERNO,
tpaterno@hotmail.com