Is it possible to determine the composition of the mortar in an existing masonry wall?
Yes. Many laboratories can determine mortar composition. Look for one with expertise in both modern and historic mortar systems. The lab staff must include both cement chemists and experienced petrographers, because mortar petrography is needed for proper analysis. For example, it can be difficult to distinguish masonry cement mortars from carbonated portland cement/lime mortars through chemical analysis alone. An intact sample or several sections amounting to a 3x3x3/8-inch volume is needed for proper petrographic and chemical analysis. Mortar protrusions from the back of a wall generally do not make good samples because they often are highly carbonated.