"My hope is built on nothing less than Jesus' blood and righteousness" - the words from the spiritual hymn The Solid Rock written by Edward Mote (1797-1874). The music, added by William Bradbury (1816-1868), gave the hymn more intense effectiveness as any melodious song can do when complementing written lyrics. However, Bradbury didn't add his tune until 29 years after the lyrics were first written, which brings me to the realization that certain great thoughts and truths may lie dormant until someone amplifies their simple, yet profound, message.
    The message for me, as it relates to the simple truth about historic brick and stone mortar, was profoundly amplified by M. William Revie of Construction Materials Consultants located in Stirling, Scotland. When I saw his thin section photography of popular mortar mixes, I felt I saw the proverbial "picture worth a thousand words." My kids' faces are pasted on each one. More about this later.		  
 
1 part Portland cement to 3 parts sharp sand. A Shrinkage crack is visible in blue.
1 part Natural Hydraulic Lime (NHL) #3.5 to 3 parts sharp sand. This mix contains the NHL that we sell and is accepted worldwide by knowledgeable conservators as an excellent replacement binder to reproduce the properties of a correctly burned lime putty at a fraction of the cost. Consistently measured values for the lime enables engineers to design appropriate replacement mortars that can be specified with confidence.
Left. Needle like structure of Portland cement.
Right: Hexagonal crystal structure of lime.

    During September 2002, I attended the annual gathering meeting for The Building Limes Forum held in Kilkenny, Ireland. It was there that Mr. Revie showed the slides for these photos which depict the open pore structure these mortar ratios have by the presence of a blue-dyed epoxy resin first injected. The blue in the photos clearly shows that traditional lime putty and Natural Hydraulic Lime mortars have superior open pore structures. An open pore structure allows a high level of liquid and vapor permeability, which should be maintained when mortar is used to cement historic brick and stone together. Water can be absorbed quickly through an open pore structure and also evaporates back out very easily.

   When rainwater gets into a wall through absorptive brick and stone that has been repointed using Portland cement mortar or through its shrinkage cracks, water quickly wicks to the original lime and sand bedding mortar. Then the water must egress back out through the face of historic brick and stone, thereby accelerating their deterioration. If it can't evaporate out, trapped water may migrate into interior spaces, possibly causing interior damage.
    My children loved the sweaters and leprechaun trinkets that I brought back for them. However, when I showed the mortar photos by William Revie and explained the damaging effects Portland cement repairs made to historic buildings during the last 50 years in contrast to the thousands of years of success builders have had using lime mortars, their reactions were similar to those pictured.
 
  Portland cement, consisting of a needlelike crystal structure, has the same expansion and contraction coefficient as steel. Lime has a hexagonal crystal structure which allows flexibility in the lime mortar. Lime's expansion and contraction coefficient is approximately one-half of its final p.s.i. strength since the plates can shift between each other rather than cracking.
    The Pennsylvania Historical and Museum Commission asked me to lecture about this and other masonry related information at their maintenance conference in October 2002. 1 received a nice letter of appreciation and compliments from PHMC on my amplification of
1 part lime putty to 3 parts sharp sand. The open pore structure is depicted by the blue coloring. This mix is representative of the binder and aggregate ratios for mortar used in most buildings constructed prior to the invention and widespread use of Portland cement. 1 part Natural Hydraulic Lime (NHL) #5 to 3 parts sharp sand. This NHL is more dense and suitable for copings and marine environments with high salt content in wind-driven rain. The 3.5 material is used for most straight forward above grade repointing projects.
 
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