Laboratory Testing of Historic Mortars: Part 1 - A Close Look to a 200-year-old Historic Clay-Lime Mortar

A large stone barn built in 1820s in the Valley Forge historic national park in Pennsylvania used some of the early historic lime mortars in the US masonry construction during the time when natural cement was just produced in the US and about to gain its momentum for next 70 years, and Portland cement, though formulated in Europe, didn’t enter the US market for at least 50 years. A brown original fist-size bedding mortar fragment was examined, which showed many interesting microstructures of a clay-mixed historic lime mortar. The fragment was examined by a plethora of analytical techniques encompassing optical and scanning electron microscopy and microanalysis, acid digestion, chemical (gravimetric) analysis, XRD, XRF, thermal studies (TGA, DSC, DTG), FTIR, and ion chromatography. Such a comprehensive investigation provided detailed information about the mortar type, composition, and grain size distribution of masonry sand, original non-hydraulic dolomitic lime plus clay-based binder compositions and resultant microstructure of paste, origin of lime lumps, along with evidence of any distress and alterations from two centuries of exposures to a moist outdoor environment. Based on all these studies, the overall condition, extent of deterioration/alterations, and sand and binder proportions of the original mortar were assessed by reverse engineering, from which a suitable replacement mortar was recommended for long-term restoration.