Masonry Magazine January 1968 Page. 49

Masonry Magazine January 1968 Page. 49

Masonry Magazine January 1968 Page. 49


Cold Weather Mortar Admixtures

(Continued from page 36)

The chemical reaction of cement and water generates heat, and the larger the mass, the more heat generated - and since volume increases much faster than surface area through which heat can escape, the thicker the concrete, the more heat that is generated and retained. This is basically the reason why large animals such as polar bears roam the arctic regions, but mice do not. Compared to concrete, the typical mortar mass is tiny, like the mouse.

It is obvious that the small amount of cement-lime heat generated in the mortar mix will quickly disperse into surrounding air and masonry if these are chilled. This is a key point. It is essential to provide sufficient heat to generate and sustain the chemical reaction through the set if we are to achieve satisfactory cold weather mortar construction results.

The word "Sustain" is vital. Take, for example, a typical mortar with a mix temperature of 70 degrees F. If that temperature is maintained, the mortar will reach its initial set in about 2½ hours. Let the temperature drop to 50 degrees F., however, and the initial set will stretch out to about 6 hours and at 40 degrees F., we'll record an eight hour delay.

For this reason, many authorities recommend mortar mix temperatures of between 70 and 90 degrees F. Heating mortar ingredients, however, can be very costly, and at those times when temperatures fluctuate widely - in the early spring or late fall - heating may be out of the question. You, the contractor simply can't afford to haul around heating equipment with its associated scheduling and labor complications just on the off-chance that you might need them.

This is where quality chemical accelerating admixtures can demonstrate their value, for these mixtures basically are chemical heat stimulants for the mortar.


ACCELERATORS

Many brand name admixtures are based upon a liquid solution of calcium chloride. Sometimes, varying amounts of other chemicals are used in combination with calcium chloride to achieve other benefits.

Calcium chloride and proprietary accelerators both will achieve roughly similar accelerating effects.

Proprietary accelerators, however, have several advantages:

* They are ready for immediate use, in carefully controlled mixture concentrations that can quickly be measured out for the job at hand.
* They provide highly uniform and predictable performance because of this uniformity.
* The manufacturer often builds in other performance characteristics as well.
* Reputable manufacturers will provide competent technical service and advice on its products upon request. This is a valuable service and should be utilized by the mason.

While quality accelerating admixtures are invaluable aids for achieving good cold weather mortar construction at reasonable cost, they are not and cannot be considered the complete and final answer to the entire cold weather problem, just as light bulbs alone are not the full answer to illuminating problems.

As their name clearly states, they are accelerators. They are not anti-freeze compounds in the sense that, say, automobile winterizing mixtures are. They will not prevent the freezing of water or mortar but what they will do, properly used, is accelerate the initial set of the mortar so that hydration occurs before freezing does.

This means that the mortar must have the proper mix temperature, and that when the temperature of the air is low enough for freezing to occur before the initial set develops, even at the accelerated pace, then the surface must be shielded or covered, particularly the exposed edges. Obviously, masonry units should not be cold enough to rapidly chill the mortar mix, either.

One word of caution; accelerating admixtures may tend to darken mortar, especially white mortar. This results from the increased density of the mortar fostered by the admixture.

Similar color differences often arise in mortar that does not contain admixtures; generally this stems from rapid temperature changes, which affect the chemical reaction of the mortar and in turn, the density. This is what we or the architect-see as discoloration.

Most accelerating admixtures reduce efflorescence by virtue of the chemical reaction of the admixture and the cement and lime, as well as by the increased density it promotes in the hardened mortar joint.


MORTAR AND METAL

No discussion of accelerating admixtures would be complete without a look at metal corrosion. While full treatment of this subject would be an article in itself, we can perhaps look at a few key elements of the problem and see how they relate to admixture usage.

Some metals, notably aluminum, must never be placed in direct contact with mortar or concrete, and all other metals except for steel or galvanized metals in contact with masonry should be suitably protected. This is true whether admixtures are used or not.

While iron and steel that is fully imbedded in masonry mortar does not need protection, hardware that will be partially exposed will corrode if not protected. Steel door bucks and casement window frames should be properly painted on the surface to be in contact with mortar before they are placed into the masonry wall.

Unpainted or poorly painted areas will tend to corrode if not in full contact with the mortar, because the hollow areas formed by the plastic mortar will function as centers for condensation during cool weather; the continued presence of water will induce corrosion.
(Continued on page 50)


Masonry Magazine December 2012 Page. 45
December 2012

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Masonry Magazine December 2012 Page. 46
December 2012

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Masonry Magazine December 2012 Page. 47
December 2012

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Masonry Magazine December 2012 Page. 48
December 2012

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