Architecture Specialty Group Morning Session: Mortars

The first trio of papers presented during the Architecture Specialty Group’s morning session all covered research on mortars. Brad Shotwell and Joshua Freedland, both of Wiss, Janney, Elstner Associates, asked the question “Evaluation of Historic Mortars: Is Petrography Ever Enough?” Mr. Shotwell presented a methodology for analysis of historic mortars beginning with petrographic examination. Petrography can be both qualitative and quantitative, incorporating techniques such as chemical spot tests to identify specific components and modal analysis to determine relative amounts of binder and aggregate. Several case studies were used to demonstrate the answer to the question posed in the paper’s title: it depends on the questions being asked and the goals of the project. Depending on the research goals and on the nature of the mortar being examined, ASTM C1324: “Standard Test Method for Examination and Analysis of Hardened Masonry Mortar,” which is weighted toward chemical analysis, may not be completely relevant. The approach to mortar analysis recommended by Mr. Shotwell is to begin with petrography and to supplement it with chemical analysis as needed.

John Walsh of Highbridge Materials Consulting presented “The Mortars and Concretes of Fort Jefferson: A Critical Examination of Effective Analytical Techniques for Unique Construction Materials.” The goal of the analysis was to identify the components of the mortar used at Fort Jefferson, constructed between 1846 and 1876. Mr. Walsh’s approach to mortar analysis, starting with qualitative petrographic examination and applying supplemental instrumental analysis, reinforced the first presentation of the day. As demonstrated by Mr. Walsh and Magdalena Malaj in the Fort Jefferson case study, petrography is also important to anticipate anomalies or interference that may arise during other analytical work.

In the third paper of the morning, chemical engineer Nora Perez presented her research on the mucilage additive to historic mortars in “The Application of Opuntia sp. Mucilage in the Pre-Hispanic Age, Today.” Polysaccharide extracts from the mucilage cactus has been used traditionally as an additive to Pre-Hispanic mortars to increase lime solubility, improve mechanical resistance and delay setting time. In a testing program conducted by Ms. Perez, the mechanical and physical properties of lime mortars prepared with different concentrations of mucilage additive were evaluated. Mucilage extract used as an additive to lime mortars does improve the physical properties of the mortars, makes injection mortars easier to apply and promotes the formation of a compact crystalline lattice. Biogrowth does not appear to be a concern with mortars having the mucilage additive, as the extract breaks down over time.