At the old hut site (the new one being temporarily cut off by flooding) I made lime mortar from the shells of rainforest snails by firing them in a kiln, slaking them in water, mixing them into lime putty. Lime is basically calcium carbonate (CaCO3). The general source of lime is limestone and various other calcareous minerals, though shells, egg shells and coral are other sources of lime. When heated above 840 degrees Celsius, the lime decomposes into calcium oxide (CaO) or Quicklime and releases carbon dioxide (CO2). When water is added to the quicklime it becomes calcium hydroxide Ca (OH)2 or lime putty. From here the calcium hydroxide can then be shaped into a form and allowed to set. Carbon dioxide enters the lime putty as it dries causing it to turn back into calcium carbonate. The new calcium carbonate has then set, remaining solid and water resistant.
In my local geography, calcareous rocks such as limestone are absent leading to a difficulty in acquiring the feed stock for lime making. However, I was still able to make lime by collecting the shells of large terrestrial snails that are native to the rainforest here. The unoccupied shells of these snails were gathered up and stored at the hut. Fire wood was gathered and packed neatly into the kiln. Importantly, the firewood was stacked on top of the grate rather than underneath it in the firebox as is the normal procedure for firing pottery. Using an ordinary updraft pottery kiln in this configuration allows it to reach much higher temperatures than would be possible during normal use. The wood was lit from above and the fire burned down towards the grate. Alternate layers of shells and wood were added on to this burning fuel bed. After adding the last layer of wood to act as a “lid” to prevent heat loss from above I left the kiln to finish on its own, unsupervised. The whole process took about an hour and a half.
When the kiln had cooled down a few hours later, I took out the calcined shells. Not shown in the video was the fact that some shells got so hot, the dirt stuck to them turned into slag and fused to them, possibly with the lime acting a flux lowering its melting point. This extreme heat (+1200 c) should be avoided as the over burnt lime becomes “dead lime”, unable to slake in water. Most shells were still useable though. They were taken out of the kiln and had water added to them. An exothermic reaction then ensued. Heat was produced as the lime quicklime turned into slaked lime. The water heated up creating steam and the shells decomposed into a white paste. The paste was stirred and crushed pottery was added to it as an aggregate (sand is normally used for this, I just had a lot of old pot sherds lying about to dispose of). This lime mortar mixture was then formed into a block shape and left to dry. It took about a week and a half to set as we have had extremely humid, wet weather. The block was observed to have set demonstrating its properties.
What I created is actually lime mortar, typically used for mortaring bricks and tiles together. It’s basically the ‘Glue’ that holds together the building blocks of masonry structures. From my research 20 kg of lime mortar is used on a 1 m square section of brick wall. 5 kg of lime to 15 kg of aggregate (sand, grog etc.) per a 1 m square section of bricks. The shells, though large, are not terribly abundant. A method for finding shells efficiently needs to be made before considering making lime mortar in this fashion. From my experience sand bars in a creek sometimes accumulate snail shells from higher up in the mountains. In these spots, water velocity decreases and shells in the water tend to drop out of the water column. Additionally lime may be partially replaced with ordinary wood ash in mortar without a corresponding decrease in strength. To conclude, making lime in a land without limestone is possible but can be problematic when trying to do so on a large scale.
8 thoughts on “Lime”
Small correction: calcium hydroxide is Ca(OH)2.
LikeLiked by 3 people
Thanks. It’s fixed. Much appreciated.
Amazing video, really interesting. I was wondering why the water bubbled when you put the finished block in? Thanks.
LikeLiked by 4 people
Air was trapped in the brick, it wasn’t a chemical reaction this time. Thanks.
Theoretically would wood ash be a better source of lime than snail shells for a primitive technologist? i.e. Take some wood ash from an open fire (around ~600c), add *a bunch* of water, let it settle and pour it off. What has settled should be nearly pure calcium carbonate since it has such a low solubility compared to everything else (0.001g/100ml vs 22g/100ml for the next least soluble component: sodium carbonate). Leave it to dry in the sun, and then use it in-place of the shells in the same process you’ve demonstrated.
What makes it even more interesting is it will probably contain some silica, potentially kicking off the pozzolanic reaction.
I haven’t seen anyone do it, but the chemistry seems to work out.
LikeLiked by 3 people
I’ve considered it. Wood ash is about 45% calcareous materials. Also from my research it can be used as a 10% replacement for cement. When straight wood ash is added to clay it increases it’s strength for the first 20 days (I think) but then the strength goes away for some reason. I’ve mixed wood ash and clay to see if it can be fired as clinkers and I’ll fire them and see if they slake in water. I haven’t heard of it being done before either. A lot of wood would have to be burnt to make enough for mortar though. Thanks.
LikeLiked by 2 people
isn’t it more or less this: https://en.wikipedia.org/wiki/Lime_kiln#Australia
anyway, nice video again. I have to understand why does it work with bark ash. Try to find marble and burn it. when you water it again, be aware, it becomes quickly hot.
There’s no lime or marble where I live, I’ve checked using the vinegar test on rocks here- no fizz. Thanks.
LikeLiked by 1 person