Concrete and mortar made with Portland cement, cures to about 60% of full strength in 24 hours and nearly all of the water used to mix the mortar has been used up in the "hydrolic", or chemical, reaction. Portland based concrete and mortar, however, dosen't reach full strength until cured for 28 days, so many masons tell you to wait 28 days until you use the fireplace.

While there may be some moisture left in the masonry it is more likely to be excess moisture or moisture from rain or washing and even in 28 days the masonry may still contain moisture. You don't want to heat this moisture up so fast or hot that it boils, which could damage the masonry, but a good way to dry out the chimney is to build a fire in the fireplace. The refractory materials (firebrick, throat, smoke chamber, & flues) which are the only materials in the fireplace likely to get very hot, have been made and fired in a kiln, don't need to be curred and are fully ready to use.

The refractory mortar used to lay the firebrick and lining components has not been curred but it is a modern refractory product that can be dried or cured and fired within 24 hours. Modern refractory materials have been developed mostly for industrial furnaces and kilns. They are not going to shut down a steel mill or glass factory for 28 days after repairs to the kiln and in fact the instructions for refractory products recommend curing after 24 hours by bringing the temperature in the kiln or furnace up to operating temperature at a rate of 50 degrees F per hour. The old guys say "we just used to build a wood fire in the kiln to cure it". So, after 24 hours, build a modest wood fire in your new fireplace to cure it and break it in.

Discusion
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9/24/97 Mr. Buckley, I have just had a 42" rumford installed in new construction, and am very anxious to start a fire in it (will warm the framing!). In your instruction manual, it says that a fire can be built 24 hours after the fireplace is finished. My mason says that is way too soon, and I need to wait at least 28 days or it will crack. If my mason is right, some clarification is needed in the instructions because it would be a shame to see someone ruin a brand new fireplace. Answer Concrete and mortar made with Portland cement, cure to about 60% of full strength in 24 hours and nearly all of the water used to mix the mortar has been used up in the "hydrolic", or chemical, reaction. Portland based concrete and mortar, however, dosen't reach full strength until cured for 28 days, so your mason has a point. My view is that while there may be some moisture left in the masonry it is more likely to be excess moisture or moisture from rain or washing and even in 28 days the masonry may still contain moisture. You don't want to heat this moisture up so fast or hot that it boils, which could damage the masonry, but a good way to dry out the chimney is to build a fire in the fireplace. The refractory materials (firebrick, throat, smoke chamber, & flues) which are the only materials in the fireplace likely to get very hot, have been made and fired in a kiln, don't need to be curred and are fully ready to use. The refractory mortar used to lay the firebrick and lining components has not been curred but it is a modern refractory product that can be dried or cured and fired within 24 hours. Modern refractory materials have been developed mostly for industrial furnaces and kilns. They are not going to shut down a steel mill or glass factory for 28 days after repairs to the kiln and in fact the instructions for all of the refractory products I've seen recommend curing after 24 hours by bringing the temperature in the kiln or furnace up to operating temperature at a rate of 50 degrees F per hour. The old guys say "we just used to build a wood fire in the kiln to cure it". So, after 24 hours, build a modest wood fire in your new fireplace to cure it and break it in. Best, Jim Buckley ___________ From: "Tim" To: Subject: Concrete curing - how soon can I build a fire? Date: Mon, 29 May 2006 Just wanted to say that I strongly disagree with your recommendation suggesting that after 24 hours a fire would be fine and in fact "a good way to dry out the chimney". That is not only a poor recommendation, it is reckless and misguided. It is even more incredible that such a recommendation would come from someone so intimately related to the concrete/masonry industry. Controlling the rate of moisture loss and in fact creating high moisture environments during curing is the single most important factor to creating maximum strength concrete structures. Concrete is not "drying" like paint...it is a chemical reaction which is taking place. It is why concrete can be successfully poured even in underwater environments. The curing is not about removing moisture, in fact just the opposite. The highest strength concrete/mortars are created by supplying extra water or humidity to the poured structure. And doing this over the full 28 days gives the best result. We are not firing clay plates here...we are bulding an important structure out of concrete. Accelerating the drying time of concrete through heat or air movement, both of which would happen by building a fire, are both very bad recommendations. There is plenty of information available on the internet and from industry sources on the science behind curing concrete in moisture enhanced environments. A paper that touches on much of the science (while evaluating curing aid compounds) is here: http://www.mnconcretecouncil.org/2003meetings/03-Nancy%20Paper_MCC.pdf While it could be argued, how much compressive strength and reduced permeability does one really need in a chimney...when it comes to major part of my new home...why would I do something to weaken the structure and compromise its potential? I selected Rumford fireplaces because I believe them to be the best...it is that same spirit of excellence that went into most of the other construction decisions...and I would bet most of your customers share this desire for the best. Building a fire after 24 hours after construction is not in line with that theme. Thanks for your time and consideration, Tim Dykema Douglas, MI _______________ Tim, Thank you for your comments and for the reference to the concrete curing compounds study. I assume you refer to the statement in our Instructions at http://www.rumford.com/inste.html where we say: When You Are Finished: We hope you have enjoyed building a Certified Buckley Rumford fireplace. And that it has been the easiest and best quality fireplace you have ever built and that our customer is well pleased with it. The fireplace is ready for the first fire 24 hours after it has been built. Leave the Manual for the homeowner and encourage him to read the section on "Building a Fire in Your Rumford Fireplace". Perhaps you also read my comments at http://www.rumford.com/tech4.html which I wrote in response to a customer who's mason had the same concern you have. There is nothing wrong with your recommendation to wait 28 days. In fact it would often result in stronger masonry. But it is not necessary. The refractory mortars can be fired after 24 hours according to their manufacturers, the clay lining materials have long ago been fired and the surrounding masonry and concrete won't get hot enough to interfere (much) with the 28 day curing process and will be strong enough. Use of the fireplace may dry out the masonry a little more than would be optimal in a hot dry climate but the chimney will still stay moist longer than exterior masonry veneers exposed to the sun and air. And in cool moist climates we often try to limit the moisture content in the masonry because it can lead to efflorescence. I do understand that Portland based concretes and mortars cure rather than dry and that they will be stronger if they can be kept moist by a coating or plastic cover for the 28 day curing period. However, I also noted that the study you cite shows that compressive strengths of around 4000 psi are attained in three days. In homes where the builder covers the veneer walls with plastic for 28 days I think we should recommend not building a fire for 28 days. In most houses, however, the fireplaces and chimneys - even if the fireplaces are fired after 24 hours - will stay moist as long as and be at least as strong as the exterior masonry veneers. Since you do make a good point, even though I think it is in the margins, I will add it to our comments linked to that recommendation so people can decide for themselves. Thanks for commenting. Best, Jim Buckley

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