Analysis of a sample from a 1930s Melbourne monument reveals the fascinating stories that can be discovered through mortar testing for heritage conservation projects.
Article by Mark Milevski
At Stone Initiatives, we receive a wide range of mortar types for analysis, ranging from early nineteenth-century mortars made entirely from termite mounds, to lime:sand mortars, through to twenty-first-century cement:lime:sand mortars. And it’s not just mortars that come through our laboratory doors – renders, precast moulding, shelter coats, plasters, bedding, pointing – we’ve seen them all!
Recently we were tasked with analysing a precast concrete moulding sample from a significant Melbourne memorial, to determine its composition and formulate a replacement mix recipe. While the sample at first appeared to be a typical cement-based mortar, deeper investigations revealed a more complex story.
Investigations
Initial inspection of the sample presented what looked like a cementitious mortar. It had a hard, rough exterior and it wasn’t easily broken apart by hand. The aggregate on the surface looked angular and relatively coarse. The binder was an ordinary grey colour, similar to that of a typical general-purpose cement.
We pulverised a sample of the mortar and tested it with X-ray diffraction (XRD) to determine its mineralogy. The XRD testing revealed plagioclase feldspar, potassium feldspar, calcite, quartz, biotite and chlorite. Of the minerals detected, only one stood out as non-typical, chlorite. Chlorite is a mineral rarely ever found in building sands and aggregates; it is most commonly found in metamorphic and igneous rocks. This was our first clue that there was more to discover than a typical cementitious mortar.
A portion of the sample then underwent chemical analysis to determine the proportion of various chemical compounds that make up a mortar, while another portion of the sample was subjected to a controlled dissolve to release the aggregate from the binder to aid in examination and sieve sizing of the aggregate.
Findings
After microscopic examination, it was evident that the aggregate was not a typical “brickies sand” from a local sand yard. In fact, this aggregate couldn’t have been sourced from any sand yard – but rather, a stone quarry.
As seen in the microscope image capture above, the aggregate consists of black biotite, white feldspars and translucent quartz, the ingredients that make up a granite. It appears that the aggregate used in the precast concrete is crushed granite.
This begs the question: Why not just use cheap, readily available sand? The answer can be found in the surrounding masonry.
The monument’s exterior is predominantly made of a local white granite-type stone. The composition of the crushed granite aggregate found in the mortar sample is consistent with the granite ashlar within the structure. This tells us that back in 1934 when the project was built, it is likely that a particular technique for blending mortar with the surrounding masonry was used – this involves using crushed masonry (of the same type as the surrounding masonry) as aggregate within the mortar mix to achieve a uniform, monolithic appearance.
The binder component was determined to be ordinary Portland cement, typical of what was used widely at the time of its construction.
Using the XRD results, chemical analysis results and aggregate analysis, Stone Initiatives was able to provide the client with an original mix ratio (cement:lime:aggregate) and detailed aggregate description, including particle size distribution, texture, colour and mineralogy. This type of information can help our clients to create informed conservation plans and sensitively restore heritage structures.
The findings in this job demonstrate the value of obtaining comprehensive analysis by experienced technicians, not only in relation to test results and mortar matching guidance, but also in regards to the stories that can unfold alongside these results. Our heritage buildings have much to share with us.
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