Curing times for refractory mortar

[Simonl]

Hi all,

i have finally started my dome construction on my 39" pizza oven. Yay!

I'm using a 1:1:1:3 mix of CAC cement, fireclay, lime and sand.

The question I have for others here who have used the same mix is how long should it take to set hard?

After 24 hours it is still a bit gooey. Its firming up but still not hard. Is this normal?

ambient temps are around 68F or 20 deg C and 70 to 80% humidity.

Any info on how this mix behaves would be much appreciated.

Regards,

Simon

[david s]

I’m surprised that you’ve ended up with that result. CAC is very temperature dependant. Typically the mix achieves full strength in 24 hrs, but I always give it 48 hrs before de-moulding. Apart from CAC having a higher tolerance for heat it is the fast setting and lack of damp curing requirement that give it an advantage for volume production.
CAC does have a relatively short life so it’s possible you have a bad batch. I live in the tropics and have to use chilled water in the hot months otherwise a mix will start to go off before it can be placed, reducing working time. Not sure where you got your recipe, but the addition of lime into the mix only accelerates the reaction. Even a trace of lime can cause this so cleaning barrows, mixers and tools scrupulously is recommended and important. Normally after an hour the heat generated by the reaction can be easily felt when placing a hand to the side of the casting such is the speed of the reaction.
Most home oven builders use the homebrew recipe 3:1:1:1 sand Portland cement, hydrated lime, powdered clay which produces a cheap, user friendly mortar with decent working time and is suitable for the temperature range to which we fire.

[Simonl]

Thanks David.

Prior to building I did a bit of research, most for which from this forum. The recipe im using i got from this forum.

I have since done about 7 test batches with varying amounts of lime and fire clay.

Did that about 4 hours ago and so far it seems that the mix performs better without lime. The test batches where I used no lime or much less lime have already set hard whereas the batches with lime and/or fire clay are still much softer.

My conclusion is maybe I have the wrong type of lime. It's called Builders lime here in australia. Also I'm using bricklayers sand, perhaps the clay content is upsetting the addition of fire clay?

Simon

[david s]

Whoever recommended that recipe obviously had not tried it. I have warned a number of times on this forum that CAC and lime are incompatible. If you have builders lime then you have the correct animal. Both sand and clay are not cementious but should be considered aggregate. Clay has really fine particles but is not cementious. Because of these fine particles it tends to impart more shrinkage. Because of this I halve its addition in the recipe. Any sand should be ok but for a mortar sieving out any coarse grains will help. As all sands are pretty much silica either river sand or silica sand work ok. Silica sand has sharper grains which provide better grip, whilst river sand has rounded grains, but more impurities. For this reason I use silica sand. Silica will not turn to glass at the service temperatures we use but in the 500-650C range it experiences considerable sudden thermal expansion, so avoid over firing. It is a mistake to see how hot you can get the oven.This is likely to cause damage.
The cheapest form of powdered clay is Cement Australia’s Bricklayers Clay. Bunnings don’t stock it (or any Cement Australia products) so you will have to obtain it from a building supplies vendor like Lyndons.
I'd be most interested in your tests as would other builders. so please post your results. Following your post I was thinking of doing the same, so you've saved me the trouble, thanks.

This advice on mixing calcium aluminate castables also applies to calcium aluminate mortars.

Mixing
Mixing is best done in a paddle mixer, but it is essential that all vessels and implements are clean. Contamination by lime or Portland cement will have a detrimental effect on setting time and properties. For small casts, mix with a shovel on a clean surface. For mechanical and hand mixing, water is worked into the material gradually until its texture changes from “harsh” to “workable”. The correct water addition is achieved when a handful of mix can be formed into a ball and be tossed 30 – 40 cm into the air and caught as a cohesive mass. If too little water is present, the “ball” will break up. If too much, the “ball” will slump (See Note 2. below). Where vibration is used to assist placement, slightly lower water contents will give satisfactory results. Excess water reduces strength. Thoroughly mix before placing and never mix more than can be properly handled inside 20 to 30 minutes.
Ideally temperature of mixing water will be 10ºC to 25ºC. If site ambient temperatures exceed 25ºC, chilled water should be used. Where site ambients reach 35ºC or higher, loss of workability may be encountered.
The refractory concrete can be consolidated by rodding, tamping or vibration. Finish off the surface to the correct profile with a trowel or screeding level. Do not overwork and/or slick to a wet smooth finish. For restoration of vertical surfaces in particular a fine mix of the appropriate grade of castable is recommended..

[Simonl]

Thanks David for your in-depth replies.

Test batches using Ciment Fondu as the binder and varying combinations of fire clay and lime has revealed the following;

No lime seems to work better. Creates a stronger bond and faster cure times. Using half the clay has also given similar results.

I suspect the above results are because I'm using brickie sand which has a high clay content.

The above batches set harder in 6 hours than the mortar used on my pizza oven from 2 days earler.
yhopefully I will do some more testing using silica sand as well.

I suspect that a mix of ciment fondu, silica sand and clay in the ratio 1:3:1 will make a better mix.

I see no real benefit using lime in the mix but I will do a test batch with it using silica sand.

Cheers,

Simon

[Simonl]

Further, I suspect I may have to rip down 3 rows of my pizza oven dome. I'll give it few more days and make a decision. Pain in the bumper but I want it to be right.

[david s]

Interestingly the combination of Portland cement and lime seems to work the other way. That is it extends working time. Or so I’ve found, but we live in the tropics and high temperature is a big factor to consider. In the hotter months I always leave concrete casting jobs whether calcium aluminate or calcium silicate based concretes for the late afternoon so it has the cooler night time temperatures, out of the sun to react slower, avoiding shrinkage cracking and too rapid a reaction. Chilled water is also often required. Folk in other parts of the world have the opposite problems of temperatures lower than ideal and freezing issues.

[david s]

We’ve been here before. Discussion on Ciment Fondu here
https://community.fornobravo.com/for...ment#post12800

[Simonl]

Yea I understand what your saying. I'm struggling to make sense of my findings. Interestingly, my first chain of brick which I stood upright where mortared using the same mortar mix but with washed sandpit sand and its set rock solid. It's the same mix and ratios and the mortar that is still soft. Only difference is the type of sand.

Last night I bit the bullet and removed all the brick that did not set. It was the right choice. 3 days on and the mortar was soft and removal with a scrubbing brush.

So big learning point for me and anyone else reading this; don't use brick layers sand with this type of mix.

[david s]

That is a surprising conclusion because there is no cementious quality in sand, it’s just a fine aggregate. However as calcium aluminate cement is far more sensitive than calcium silicate cement (Portland) it is presumably other chemistry contained in the unwashed sand that is interfering with the hydration of the calcium aluminate reaction. Apart from its far greater sensitivity to temperature it is also far more sensitive to the effects of superplasticiser. It is very easy to overdo it and get separation of materials. I found it needs a bit less than half of what I’d use if making a concrete or mortar with Portland.
Perhaps the washed sand cleaned out impurities.
I’ve not tried it, but citric acid is used to delay setting with calcium aluminate cements. Maybe a ph test of your bricked sand might reveal something?

[Simonl]

Hi David, once again thanks for taking the time to reply.

One thing I did notice when I used washed sand pit sand was that the mix lacked workability. The mix was very gritty and did not resemble the smooth paste I have seen online. Hence the reason I tried using brickie sand. It indeed produced a beautiful peanut butter type paste but obviously no good!

Also one other thing I discovered; soaking the bricks before using them made little difference to the ultimate bond strength.

The only difference I found was that pre-soaked bricks were easier to lay and set in as the mortar mix stayed softer after laying the brick.

[david s]

Hi David, once again thanks for taking the time to reply.

One thing I did notice when I used washed sand pit sand was that the mix lacked workability. The mix was very gritty and did not resemble the smooth paste I have seen online. Hence the reason I tried using brickie sand. It indeed produced a beautiful peanut butter type paste but obviously no good!

Also one other thing I discovered; soaking the bricks before using them made little difference to the ultimate bond strength.

The only difference I found was that pre-soaked bricks were easier to lay and set in as the mortar mix stayed softer after laying the brick.

Yes The brickies sand contains some clay usually. Adding powdered clay gives the mix workability and stickiness, but too much creates excessive shrinkage resulting in shrinkage cracking.Brickies usually add a mortar plasticiser or sometimes bricklayers clay.

[Simonl]

Ok. Just laid my hands on some silica sand. $9 per 20Kg bag which seems reasonable. It seems to have a variety of grain size in it. So I tried it out with a mix of ciment fondu, sand and fire clay at a ratio of 1:3:1

I left the lime out at this stage although I may try with lime later. The mix was not as smooth, certainly grittier but still usable. With this mix it definitely helps to pre soak the bricks.

I should know in a few hours. Fingers crossed.

[david s]

By leaving out the lime in the recipe you are therefore increasing the proportion of clay in the mix. This will increase stickiness and shrinkage. You may want to try reducing the clay proportion and sieve out any coarse grains from the sand.

[Simonl]

Yes I understand. I've been a bit gun shy when it comes to both lime and clay so I've been adding half the amount of both lime and clay.

The results using this recipe has been a success. For sure the biggest learning experience has been not to use brickie sand. At least the stuff i have anyway.