What Happens When Lime Putty Freezes?
Is it unusable? Can it be reprocessed? What happens during reprocessing?
Nearly everyone who has worked with lime putty for awhile has had the experience of having a putty freeze. The standard conclusion is to assume that it has been destroyed. The very firm large-curd cottage cheese appearance of frozen putty is certainly startling, but although the appearance has changed, the lime is chemically unchanged - freezing does not remove water and allow carbonation - so it is still lime putty and thus should still be usable... after re-processing to return it to a paste form.
When we had our entire batch on a project in Georgia freeze, we had to tackle this question. To process the curds back into a usable paste, we laboriously hoed and chopped for hours, then finally ended pushing it through a screen. Obviously spending hours to reprocess one 5-gallon pail was not a practical approach. After locating a vertical shaft mortar mixer from Imer-USA that simulates the action of a dough machine, we were able to remix large quantities efficiently, especially after adjusting the rubber pads on the front of each paddle to a finer tolerance.
Curious how the frozen putty would compare to unfrozen, I made some 1:3 lime:sand mortar samples and set them on a brick to cure overnight. The next morning I was surprised to see these samples had cured faster and harder than any mortar I had ever made. In subsequent mortar batches I noticed the lime was carbonating on the trowel within minutes.
How do these putties compare in laboratory tests?
Although they were initially skeptical of my findings, the chemists as Mississippi Lime Company agreed to test the frozen putty. I was surprised by what appeared to be contradictory results from the first round of tests until I learned they had only had the "cottage-cheese curd" frozen lime in a blender for a few minutes before testing. We then had them run a larger battery of tests on lime we had re-processed in the Imer to a consistently smooth paste (a one hour run time in a 15-gallon batch Mortarman 120 mixer).
What the Test Measures:
- BET is a measurement of surface area, which affects the rate and depth of carbonation since lime putty cures via reaction with carbon dioxide in the air. Greater surface area increases the carbonation rate. The reading of 30m2/g is almost triple the surface area of the best bagged hydrates on the market.
- The Microtrac tests the range of particle sizes and indicates a mean. Not unlike surface area, smaller particle sizes contribute to more opportunities for reactivity with atmospheric CO2. Yet a range of sizes is good for lime just as it is for aggregates in ensuring a better packing or adhesion.
- Plasticity refers to working characteristics for the plasterer. The higher the number, the nicer the trowelability of the lime. The Emley plasticimeter gave a much higher plasticity reading for the frozen, although both of these limes have uncommonly high plasticity. By comparison, a plasticity reading of 200 is considered very good for S-type limes to be added to portland cement.
- The 10% viscosity is a Brookfield test with readings in centapois. It measures resistance, or drag. The smaller the particle, the greater the drag.
- The 20% decantation is a test of the settling rate of calcium hydroxide dispersed in water. Smaller particles take longer to settle.
- A bench test, "CO2 Air Dry," measured the rate at which each putty carbonated sitting out in the air. The frozen lime carbonated about 15% faster.
What was happening with the frozen lime?
At first we concluded the crystallization pressure of water freezing had fractured the lime particles (although we later concluded the energy required to shear it back to a paste had a greater effect). Looking at the Microtrac curves, we noticed the unfrozen putty had an even curve with a narrow range of particles from 1-15 microns. By comparison the frozen putty after shearing in the Imer had a squater curve with a wider ranger of particle sizes. The additional blip of 100 micron and larger particles is likely to indicate the unsheared lumps of lime that occassionally show back up no matter how hard we mixed.
These small, hard to crush ovals of frozen putty that remain uncrushed look exactly like the blebs of lime seen in most historic mortars (and with greater regularity the older the mortar). Sometimes they initially look like bits of shell in the mortar, but turn to powder when scratched. We had always wondered about the source/cause of these small, soft, ovoid chunks of hydrate.
We had repeatedly tried and failed to repeat them in replica mortars. Traditionally lime putty was stored for long periods before use in mortar. If we accept that these putties would have naturally frozen during that time, then it stands to reason masons in the past were faced with chopping and hoeing and even screening to work out these frozen chunks of "cottage cheese" to make a smooth paste for their mortars. So the only way to get blebs to match what we see in historic mortars is to use frozen lime.
The small particle size and rapid carbonation of the frozen lime result in a putty with wonderful working properties, particularly when stuccoing or plastering. The increased range of particle sizes may be due in part to fracturing of the calcium hydroxide crystals when water in the mix freezes. On the other hand, the considerable amount of energy required to reconstitute the "cottage cheese" into a smooth putty may play an even bigger role. Since the frozen putty is a semi-solid, the energy put into crushing the lumps and shearing the particles against each other leads to an uneven particle size distribution, but also a wonderfully colloidal matrix with excellent working and curing properties... certainly not a material to be discarded.
Later we had some scanning electron micrographs taken of the two putties. It is interesting to note that the frozen lime's increased colloidal nature contributes to a sort of glassy "peanut brittle" appearance.