How do Natural Hydraulic Lime Mortars Compare to Common Type-O Mortars Containing Portland Cement? by Randy Ruth

by Randy Ruth

For well over 30 years it has been common practice to prescribe the use of a Type-O masonry mortar for use on the conservation of masonry buildings. The most common formula of a Type-O mortar is 1:2:9, a blend of 1 part white Portland cement, 2 parts Type-S hydrated lime (most often dolomitic lime) and 9 parts sand by volume. In recent years there has been an increasing amount of research on similar mix designs and how they compare to Natural Hydraulic Lime (NHL) based mortars used in Europe to repoint historic masonry buildings. Just recently, a professor and two graduates of Columbia University published a research study on the same topic here in the United States. The results of this study can be found in the Association for Preservation Trades International bulletin Vol. XLIII. Their innovative approach to allow a real comparison between different mortar types used in conservation reveals some interesting results.
Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Striling, Scotland

Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Striling, Scotland

The innovative approach the research team had developed with their curing protocol of various binder types, established a relatively level field of comparison for various mortars in regards to the way each mortar uniquely cures. With this testing detail established, the 11 commonly used repointing mortars were tested at various stages on their splitting tensile strength, compressive strength, water absorption and water-vapor transmission.

The tests reveal that for a splitting tensile strength the NHL/sand mortars are most comparable to traditional pure lime/sand mortars made of High calcium lime and dolomitic lime, while Type-O mortars were more than twice the splitting strength of NHL mortars. Although anecdotal, the cases where historic pure lime mortar has been used to repoint soft brickwork and has eroded causing a need for repointing, Type-O mortars could be consequently be too rigid for use a repointing mortar.

Compressive strength data shows that although NHL mortars have higher values than that of high-calcium lime mortars (with the exception of NHL 2 being less in strength than dolomitic lime) when compared to Type-O mortars their values are nearly half.

When mortars were measured for their water absorption, initially all NHL mortars significantly out-performed other mix designs. The most comparable mix was that of the Type-O mortars that ended up with similar characteristics to NHL mortars and still outperformed both pure lime mortars.

Water-vapor transmission results indicate that all NHL mortars process water vapor at much higher rates than dolomitic and both Type-O mortars, with values comparable to High-calcium lime mortars.

Although the results from the laboratory study are not entirely representative of values that are obtained in field work, they do represent what specifiers and conservation masons are attempting to achieve in real world situations. By eliminating variables, that can give anecdotal results and margins of error, the data suggests that NHL\’s are indeed appropriate when specifying a historic repointing mortar or a new construction mortar used for masonry mortar, plaster or stucco applications.

In conjunction with this study and the new ASTM C1713Standard Specification for Mortars for the Repair of Historic Masonry, Natural Hydraulic Lime clearly proves its self as an alternative to mortar mix designs that have been used widely in the United States for many decades and have shown, for one reason or another premature failure.

LimeWorks.us

Phone: 245-536-6706

LimeWorks.us (FaceBook : Google+ : LinkedIn : Twitter : YouTube)

 

Yes, We Can Save The Frank Furness 19th Street Baptist Church

by Sean Maxwell 2012

IMG_2128-1024x682

This beautiful church was designed by Frank Furness and is located at the corner of 19th and Titan in South Philadelphia.  It almost went the way of many Furness structures; due to neglect and time the church leadership is under pressure to begin repairs or plan for it to be demolished due to unsafe conditions sited by the License and Inspections Department of the city. Under the leadership of Reverend Vincent J. Smith and in partnership with the Greater Philadelphia Preservation Alliance and some volunteers, stabilization efforts have finally begun.

As the morning sun crept over the skyline on the last Saturday of April, Randy Ruth, mason and the laboratory technician from  LimeWorks.us volunteered to lead a workshop with a team of a few other volunteers to complete some small but crucial repairs on the north facing wall of the 19th Street Baptist Church.

The church has been in need of serious repairs for a number of years as observed and documented in a 2000 University of Pennsylvania report, Thesis by Molly Anne Sheehan, which show that the conditions of the church and the exterior Serpentine Stonework have continued to deteriorate steadily.

Under Randy’s supervision the team of volunteers used a lime mortar donated by LimeWorks.us for doing some in-kind repairs to stabilize the failing back-up walls behind the Serpentine stone. This lime and sand mortar reflects what would have been used originally when the church was constructed in the late 1800s. Ecologic® Mortar contains NO harmful Portland Cement and allows the building to breathe and process water as it did originally.  At some point in time, a hard and brittle Portland Cement stucco was placed over a significant part of the building, trapping moisture and leading to further deterioration of the soft, sedimentary serpentine stone and contributing to mold issues that the church basement still has to deal with today.

This was the first step in saving the most colorful building ever designed by Furness… A true master plan is needed for the next step in partnership with strong fundraising efforts to truly bring this sacred place back to its original glory.

Other stories about the 19th Street Baptist Church…

HiddenCityPhila.org

Plan Philly

Gallery photos  ©SeanKmaxweLL.com

LimeWorks.us
Phone: 215-536-6706

LimeWorks.us (FaceBook : Google+ : LinkedIn : Twitter : YouTube)