1915 was in the transitional period, it could be either, but if it was portland mortar, it would have deteriorated long before now (or would still be OK). That is to say, if it was portland mortar laid in 1915 that was defective, it would have deteriorated long ago.
If it was lime, it was probably killed by later repointing with a portland mortar, which also would spall the brick.
Using no more than 10% portland in the mix is normally all that is allowed for a lime mortar, but the newest research I have seen cuts that back to less than 1% for tuckpointing mortar. So, adding lime to a Type N portland mortar is probably not a good idea.
<edit> ..not a good idea for use as a tuckpoint over lime mortar. That is a good mix for tuckpointing portland mortar, even old commons.
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Last edited by Tscarborough; 12-01-2012 at 12:19 AM.