Lime Mortar Weathering Tests
What happens when lime mortar remains out in the weather?
Over the years, several times I accidentally left mortar samples sitting both directly out in the weather and outside under cover. Eventually, I began to realize that in a few months' time, mortar left in direct weather consistently became much harder and stronger than an identical sample left protected from the rain, I began purposefully set-ting mortar in the rain to see the effects of the weathering.
To test out my observations, I proposed an experiment to Joe Ebbling and Mark Degeneva at Mississippi Lime. I proposed that we make a precisely controlled batch of mortar at Mississippi Lime, both a standard lime putty as well as a batch from frozen lime. We would then set groups of frozen and standard mortar samples both out in the weather and under cover.
It was interesting to note that the frozen lime mortar samples would not come out of the rings we put them in. Most things shrink slightly when water evaporates, but we had to cut the rings off these samples. The particle size of frozen mortar is such that it holds on to water. It doesn't shrink-- it expands.
We set batches of frozen and unfrozen mortar outside and inside and let them sit for two months. This chart details the process of the experiment. "July batch putty" refers to the standard lime putty, and "GBV putty" refers to the frozen lime putty.
It is useful to see the data relating the volumetric mixes of the samples with the weight of the different components dropping as the water leaves but increasing as the mortar fully carbonates.
The bottom of the chart shows the pieces that were sampled at one month and two months. There is definitely a discrepancy between the Leco test and XRD, but this it typical. The Leco numbers are probably more accurate.
These tests verified what I had observed for years: that lime mortar made from very high quality lime in fact carbonates more fully when exposed to the weather. On this chart, you can see that in both cases, the weathered mortar cured quicker. The frozen mortar, the GBV, was completely cured at two months. The protected mortar lagged a little bit behind.
Different factors most likely contribute to this, and possibly include CO2 dissolved in rainwater, repeated wetting and drying cycles, and redistribution with the dissolving of carbonates moving with the water and recarbonating in different crystal structures.
There is more work to be done here. It will be interesting to have more tests run on these to better understand what's going on.