Lime silicate refractory testing

My lime silicate refractory exploration and ‘thought bubbles’ for DIY refractory tinkerers.

Introduction

I have already tested many minerals (excluding pure lime) that combine with sodium silicate to make highly heat resistant and robust DIY refractories. Some of my best and cheapest refractories are made from simple dried, crushed and sifted clay-free loamy subsoils.

This breakthrough first came when I could not find suitable clay on my farm. So in frustration, I gathered some crab hole mounds that bring deep subsoil to the surface effortlessly. “The effort for me, that is, not the poor crabs. Being smarter than the average bear or crab, I discovered that post-hole soil from my farm fencing was a good substitute!”

For the benefit of others that also may tinker with fire, I have described these and many other cheap refractories in a post: DIY refractories from sodium silicate, Born in fire to survive in fire.

Yes, my soil refractories are literally dirt cheap and they become like indestructible hard refractory rocks that can be used to sharpen knives on. And yes, the process of preparation entails a little searching, experimenting, deep digging, drying, grinding to fine powder, careful sifting through a fine mesh and lastly very thorough mixing with sodium silicate in a stiff dough-like mixture that is somewhat slow to mix well.

Mothy The Elder

My soil refractory is dirt cheap, but it does entail a lot of effort if a large item is to be made with it. Let’s make it clear. My life is not just a long succession of pucking fiddling. So, here is a photo of an experimental stick burner that I made from soil silicate refractory. It was made as a one-piece cast body with a complex labyrinth of connecting tunnels inside. It is as hard as a rock and has survived savage combustion at 1,000C+ and heat cycling within it many times with no harm.

It just laughs at the hottest wood fire in which it was born.” It does slowly grow a strange pale white fuzz on its surface. I think this is excess sodium hydroxide that has migrated to the surface and reacted with atmospheric carbon dioxide to become sodium carbonate crystals. It can be wiped off easily with a damp cloth.

If anyone is interested, there is a detailed post on the burner casting and it’s many combustion modes. Please see Experimental stick burner. Also, my DIY Nebuchadnezzar furnace shown in a photo below is also lined with soil silicate refractory that has also survived many high-temperature burns that have melted bog-ordinary white student clay.

The two photo below demonstrates that the furnace gets very hot and the furnace lining made of soil silicate survives these clay melting temperatures.

Can powdered lime and sodium silicate be used to effortlessly make a refractory?

I am sure you could imagine my delight when I watched a Youtube video by NOBOX7. He appears to be able to make a good refractory glue or casting material with lime powder and DIY sodium silicate. He gave his formulation a great wrap and compared it with other formulations that eventually fell apart.

It sounded as though it was as easy as just mixing the lime into the sodium silicate until the desired texture was achieved. So it would be just magic if at $12, a 20kg bag of beautifully finely powdered lime from the Bunnings store could suffice as the refractory matrix! Those crabs might also be happy that I no longer need to steal their carefully constructed fine soil towers.

NOBOX7 silicate formulation. The NOBOX7 formulation went like this: 40g silica gel beads, 60g Sodium hydroxide and 400ml of water (at about 15:20 on the video). The preparation involved boiling the above into a homogeneous mixture. Then mix in garden lime (calcium hydroxide) to a wet sand consistency for ‘concrete’ (presumably he means refractory molding or casting?). Mix in lime to pudding or gel consistency for glue.

The video shows the garden lime glue mix holding refractory materials together strongly. I think it also shows cast objects made from the thicker garden lime mix that may have been fired, but I am still not sure?

Both the castings and glue survive the heat of an oxygen/hydrogen flame. The same flame was shown melting a refractory brick and leaving beautiful smooth glassy holes in it. Also, the same flame burnt a hole through a steel plate and I think the flame was shown to be hot enough to soften the remnants of a fused quartz glass tube (Not sure?). Fused quartz glass is reported as having a softening point that is in the range of 1,500-1,670C.

My try at replicating the lime and sodium silicate refractory with my own DIY sodium silicate formulation

My silicate formulation. My DIY silicate recipe was; 300g silica gel beads, 210g Sodium hydroxide and 570ml of water. At first, I mistakenly thought that my DIY sodium silicate (which is similar to that of many others) was similar to that of NOBOX7s. Anyway, I made up a refractory lime glue mixture and a castable lime refractory from builders lime (calcium hydroxide) and my thick DIY sodium silicate (that I have used successfully for many years for my other refractories).

Immediately, it was an interesting disaster. A small sample of the freshly mixed glue was spread on a sheet of metal while still liquid. It cracked as it cured and clearly was not going to stand up to the test of fire. The jam jar of glue started to very quickly harden to a hard toffee texture within an hour of initial mixing.

The thicker castable refractory mix was put into a mold and it cured more slowly, but it released easily from the mold with absolutely minimal shrinkage. However, when it was cured in air and then put into the firing process it became crumbly and easily fell apart.

My try at replicating the lime and sodium silicate refractory with the NOBOX7 DIY sodium silicate formulation

While considering the possible reasons for the refractory failures, I realized that my sodium silicate was about 5.3 times stronger and half the sodium hydroxide/silica gel ratio when compared to the above NOBOX7 silicate recipe. Consequently, I made silicate according to the NOBOX7 recipe (shown above) and started the tests once more, with not much confidence that things would turn out better.

This time, I included a refractory test puck made from powdered crab hole soil and NOBOX7 silicate, as a type of control sample that may at least demonstrate that the silicate was an effective refractory binder. The castable lime refractory failed and went soft and crumbly before I could fire it. However, the crab hole refractory set up hard during air curing. and it made a dense and solid test puck that became very hard, stable and strong during firing in glowing charcoal for 1-2 hours.

Conclusion

I could not make a satisfactory lime and silicate refractory glue or castable according to the NOBOX7 video. I will make a comment to this effect on his Youtube video and see if he or anyone else has some suggestions as to what I may be doing wrong or what I need to do to make this exciting idea a reality.

I did learn that the NOBOX7 more dilute sodium silicate solution was very much easier to make than the potentially explosive preparation of my concentrated silicate. The dilute silicate was also much easier to mix into my soil refractory. This may find a place in my refractory tinkerer’s tool kit. More experimentation is needed in that regard.

Tim