Using Concrete Block for bell

[mat73]

Hello, i

have been reading a lot of stuff and i am about to start my project,

Anyway , i have seen this thread donkey32.proboards.com/thread/3857/vortex-dsr3-sksshel-build?fbclid=IwAR00qRIXxpwlgu4tB7E-bZf1XkxBgXbeLGccLXpKxXHwDPwQUcsME122yKY

I would like to know if the use of concrete block klike it is use for this experiment is a good alternative (easy to use) for building the mass .
(core will be in refractory materia)

Thanks for your replies !

[mat73]

I have my answer here
if any of you have experience using this material i will be happy to have advice

peterberg
Global Moderator
*****

peterberg

Posts: 3,055

May 30, 2013 at 8:02pm QuotelikePost OptionsPost by peterberg on May 30, 2013 at 8:02pm
Yes, could be done with normal bricks. And concrete blocks, cob, granite or whatever. The top part, in general everything above the mouth of the riser do need something fireproof. A liner of fire bricks, split fire bricks (the thin variant), superwool, steel, etcetera. Or simply an oil barrel.

[martyn]

Standard, building site, concrete blocks are not the best choice for mass as they contain a lot of air, many even being quite porous.
In order to hold heat the mass needs to be dense with no air voids like brick or vibrated concrete.

[mat73]

Thanks for the answer , the plan is to try to reduce cost and easely built the bell. maybe with clay blocks filled with some concrete ? i don'know the name in english 

[martyn]

Bricks and clay are possibly the most popular choice.

[mat73]

Thanks for the reply

[masonryrocketstove]

Regular concrete made of portland cement has very poor heat resistance properties because the lime (calcium) hydration bonds that hold it together are not very thermally stable.

Here's a study showing that lime-based concrete does not handle thermal cycling well at all, even under moderately low temps (page 33)
inis.iaea.org/collection/NCLCollectionStore/_Public/28/057/28057515.pdf

Thermal cycling causes progressive degradation of concrete. Usually it affects the compressive strength more than the modulus of elasticity with an increasing number of cycles, though most of the damage occurs in the first few cycles. The influence of thermal cycling on the mechanical properties of concrete has been investigated in References 3, 6, 18, 33, and others. Reference 3 indicates that cyclic heating generally causes greater reduction in mechanical properties than a single heating even at a relatively moderate temperature rise of 65°C (149°F) [...]
Progressive deterioration of the mechanical characteristics with an increasing number of cycles was observed. At higher temperatures, 200 to 300°C (392 to 572°F), most of the damage is caused in the first few thermal cycles and is usually associated with a loss of bond between the aggregates and the matrix. The breakdown of bond between aggregates and mortar was least for fireclay brick aggregates and greatest for limestone aggregates.

Same study found that long-term heating up to 315ºC (600ºF] causes lime-based concrete to lose about half of its compressive strength and stiffness/elastic modulus (page 37)

At temperatures of 149°C (300°F) and higher, the increase in long-term exposure results in reduction in the mechanical properties (compressive strength and stiffness), and the rate of reduction increases with increasing temperature as shown in Figures 21 and 22 (Reference 11) and Figure 23 (Reference 50). It appears that most of the loss occurs during the early stages of exposure to elevated temperatures, i.e., within the first month. For example, at 300°C (572°F), a residual compressive strength of about 65% is observed after two days and 50% at the end of four months as shown in Figure 20. Similar behavior is observed in Figure 20 for the modulus of elasticity.

That decrease in elastic modulus can then be a problem that causes cracking between hotter and cooler areas of the concrete as time progresses, and as the number of heat cycles increases.



Another study shows that moisture / water vapor can very quickly destroy all load-bearing capacity of portland-cement / lime / calcium-bonded concrete, even below the (400ºC) temperatures that destroy the calcium hydration bonds themselves:
www.ncbi.nlm.nih.gov/pmc/articles/PMC8780578/#:~:text=The%20reasons%20for%20concrete,resulting%20in%20a%20decline%20of

The reasons for concrete degradation at high temperature are as follows: high temperature vapor pressure, temperature stress, difference in thermal expansion coefficient between aggregate and paste and thermal decomposition of hydration products [2,3,4,5,6,7], among which the main reasons for concrete degradation are high temperature vapor pressure and thermal decomposition of hydration products. Under 400 °C, the hydration products of concrete do not obviously decompose, the deterioration of concrete is mainly due to the high temperature vapor pressure; when the temperature exceeds 400 °C, the calcium hydroxide (CH), C-S-H gel and calcium carbonate (CaCO3) of concrete decompose, resulting in a decline of concrete strength [8,9,10]. Compared with the degradation of concrete caused by thermal decomposition of hydration products, the degradation caused by high temperature vapor and thermal stress is often more serious, because the accumulation of high temperature vapor, to a certain extent, will instantly destroy concrete and cause it to lose all load capacity, ultimately leading to the failure of concrete structure.

The last point is a big one to consider with a mass heater that might experience cycles of moisture condensation and re-vaporisation along with the thermal stress of its heating cycles.



..That's not to say that you can't use lime-based portland cement concrete at all in a heater bell.. but it's probably not a great idea to use it on the inner skin where temperatures are highest and where condensation is most likely. ..And using it for an outer skin would probably require careful planning of thermal expansion gaps to avoid cracking from differential expansion.

[martyn]

There are many hundreds or thousands of pizza ovens made from Home Brew, it is a 3:1:1:1 mix of sand, Portland cement, hydrated lime and powdered clay, measured by volume.
This mix will last for years on end in the form of a pizza oven dome which ‘is self supporting’ but can be run at 500c for as long as you like.
Interestingly this recipe does not seem to last very well without the portland cement, or vey well if the standard cement is substituted with high temprature refractory cement.

[Dan (Upstate NY, USA)]

My 30 feet of 8 inch pipes are encased in a concrete bench. It's fed with a J-tube.

The steel heat ducts rusted out long ago and were pulled out.

It is holding up fine (going on 12 years now), BUT it is reinforced with steel rebar and is only 3 feet tall.

Now the fire bricks, they didn't hold up so well...

[mat73]

Sorry for the late response but thank you for your explanations ! for now i am looking for bricks from clay wich seems more appropriate.