Tricks of the (Cement) Trade

At some point, I will probably have to organize this section.  But for now, just cruise through the random titles at your leisure. Over the years, I have consulted with a number of Ph.D.’s who have written books on the subject, and a lot of 80-year old masons who’ve mixed enough mud to cover the Moon.  And, while each certainly qualifies as an “expert”, I learned that they disagree on just about as many things as they agree on.  But between them, I’ve picked up a lot of useful information.  So, take it all with a grain of salt…(or sand), and use it as you see fit.  I’ve found a few things along the way that have come in quite handy.  I hope you do too.  And do be sure to also take a look at the “Mixing Basics” technical paper as well.  It focuses on getting everything together properly.

Sand.  How to tell what’s really going into your mix.
A couple of the really savvy old Masons I’ve spoken with both laughingly pointed out an interesting observation. They both noted that they had observed a lot of “smart” people in the concrete business who would spend months and tons of money testing and evaluating all the new “wonder drugs” for concrete…then dump whatever sand was handy into the mix and blame the chemicals if it didn’t work out. The best of the old-timers consistently said that because sand was the dominant ingredient in their mix, they never used ANY that they hadn’t examined under a pocket magnifier AND…tested in solution.  Both simple processes we all can employ.
The magnifier is to check for “sharpness”.  Good masonry sand is sharp and angular, never smooth, rounded  and polished looking. It forms a more secure matrix by virtue of its’ shape locking into the paste.  Smooth sand simply does not lock into the cement paste as well as the sharp variety.  So, if what you see under the glass looks like little billiard balls…keep looking.

The “Solution Test” is an easy way to tell what you are buying besides sand.

Take a large Mason jar and fill it half-way with a sample of the sand (or any aggregate, for that matter) that is in question.  Fill it up to about the 3/4 mark with warm to hot water, secure the lid and shake the fool out of it for at least a couple of minutes, then set it aside to settle out undisturbed for 24 hours. The results can really be surprising. If it consists of clean sand, that’s all you’ll see when you come back…sand and clear water.  If there’s a lot of dust, dirt, or any other material present, it will form separate, visible “strata”. Individual layers of different materials formed primarily on the basis of their relative weight.
Sticks, twigs & most organic matter will float to the top, as will any oily residue or lighter than water residual material. Shine a flashlight through it and at an angle to the surface and see what’s there to see. It’s either clean or it’s not. Not exactly rocket science or laser spectrography, but it’s worked pretty darned well for a few hundred years (and yes…they had to use candles instead of flashlights way back when).

The other “big mistake” they consistently warned me of making was assuming that the sand suppliers had any idea what it was that they were actually selling at any given time. Just because they point you to a pile that’s supposed to be a particular type of sand, doesn’t mean that’s what somebody else’s truck dumped there. The “old-timers” advised that you assume nothing and always check to see what it is that you are actually paying for.   Hey…not bad advice for a lot of things in life.

Curing.  Some of the basics & some handy hints.
The cure is by definition…
Cure: The method by which sufficient internal humidity and proper temperature are maintained to insure that the hydration process of the cement within the freshly placed concrete or mortar is uninterrupted allowing full and proper hardening to take place.”
If you are working with any cementitious mix, it is imperative that you appreciate just how important proper curing is to the success of your project.  It is equally important that you understand that nearly all of the information available through most sources is either intended for, or based on, commercially placed concrete.
Artwork & sculpture are very different animals & require tailoring much of the available information to your individual needs  and circumstances.  For example…nearly all commercial formwork, such as foundations, driveways, bridges and the like, are commonly closed on five of six sides.  A piece of sculpture or any work that is not confined in a mold, is much more exposed to the environment, and consequently subject to much greater and more rapid moisture loss.  “Maintaining sufficient internal humidity & proper temperature” is a great deal more difficult for artists than it is for most contractors. But it’s just as important.

Hydration vs. Evaporation
In order for any cement-based mix to achieve full strength, it must fully hydrate. That’s the chemical process by which cement paste hardens.  It begins as soon as water is introduced and it must not be interrupted until hydration is about 95% complete (that is about what is considered a “full” cure.  The other 5% can take centuries.). If the mix loses too much water, for any reason, at any time before it is fully cured, hydration stops cold at that point and it does not start up again. Even getting close to an insufficient moisture level can be  problematic.
“Plastic Shrinkage Cracks” are the direct result of inadequate moisture near the surface.  They occur in the exposed surface of fresh concrete during or within the first few hours after the concrete has been placed and while the concrete is still plastic, but before attaining any significant strength. Such cracks are caused by the evaporation of surface water, and the consequent drying and shrinking of the exposed surface of the still plastic concrete.  The surface must therefore never appear dry during the first few critical hours that it is in this plastic state.

The duration of this “critical” stage of the cure is dependent on two separate considerations. First; what’s in the mix (Rapid Setting concrete vs regular cement; accelerator, retarder and any other admixtures) and secondly, the environment. You will have to refer to each manufacturer’s specs for any curing considerations regarding cement & admixes, but there are some general guidelines for dealing with the environment. Ambient temperature, humidity and wind velocity all play a role in removing moisture from your mix and must each be compensated for. Here’s how they work against you and a few things you can do about it:

The higher the ambient temperature, the more rapidly moisture evaporates. But, ironically, cement paste also cures (hydrates) faster as the temperature rises, so as long as you can control moisture loss, higher temperatures are not necessarily a terribly bad thing. The commercial/industrial concerns expressed in the definition above are primarily directed at the internal chemical reaction temperatures that are generated by very large masses of concrete that are placed at once.  The good news is that, unless you are placing twenty yards of concrete at a time, you won’t have to be greatly concerned about that type of heat.  Just try and keep the ambient temperature within reason. Hydrating cement paste really likes it best around 70 degrees F.  If you have a nice air-conditioned workspace, just adjust the thermostat. If you are like me and have to do most of your work outdoors, then you’ll just have to keep the following in mind and apply shade, heat or cooling as best you can.

  • Do not place or apply a mix when the working environments temperature is, or will be below 50 degrees F, anytime during the working and curing periods.
  • Avoid using water that is extremely cold, as it can dramatically slow or interfere with the curing process. However, the use of slightly “cool” water on a hot day can help offset the heat’s tendency to accelerate the initial set somewhat.
  • You should also avoid using “hot” water in your mix as it will greatly accelerate the initial set and can even contribute to cracking. This is actually a more common problem than you might imagine because a hose lying in the sun can build up enough heat to literally burst.  Consider what it will do to your mix. Check the water’s temperature by spraying it onto your hand before you add it.  If it scalds your hand, don’t use it.  Run it until it is cooler. If it is neutral to comfortably cool to you, your mix will be comfortable with it as well.
  • When working on hot, sunny days, try to keep your work shaded from strong direct sun and or misted. Cement & concrete absorb and store heat with remarkable efficiency.  Applying fresh material onto a broiling hot surface almost guarantees problems and should always be avoided. If the surface is hot, hose it down until the excess heat is thoroughly dissipated before proceeding.
  • Check your mixer or mix container’s temperature. Regardless of whether it is metal or plastic…if, it has absorbed a lot of heat…that heat will be going right into any mixture you put in it. More than once I have had to cool my metal mixer with a hose before I could continue.
  • Here is an old masons’ rule of thumb regarding how temperature impacts setting & curing:
“When planning your work, you would do well to bear in mind that concrete placed at 90°F will set in one half the time as concrete placed at 70°F, and concrete placed at 50°F will take twice as long to set as concrete placed at 70°F.”

Humidity definitely plays a role, but to a much lesser degree than temperature or wind velocity. Low humidity levels can be compensated for by occasional light mistings with a hose; high humidity generally works in your favor by slowing the evaporation process. Heavily saturated air simply can’t hold much more water, so it tends to stay where it is longer.

Wind Velocity
This just may be the greatest single cause of dehydration and failure. Even small amounts of air moving across the surface will have a profound effect on the amount of moisture that is removed from the mix. And the amount of moisture lost goes up almost exponentially as wind speed increases. This is true both while you are working and while your work is curing. And if you are working on a three-dimensional object that has most of its surface exposed, the effect is even greater and the loss more rapid.
Take a lesson from the old-timers…shield your work from the wind. If you can’t work indoors, erect some type of “screen” to block the breezes while you are working. And, whatever you do…don’t even consider working in front of a fan, no matter how hot it gets. Indoors or out.

Tricks for Maintaining Sufficient Internal Humidity
As you have no doubt guessed by now, successfully accomplishing this feat means managing all of the above and will require a slightly different approach to overcome each new days’ weather and every different mix. Once you take into account how hot or cold it will be, the humidity and the wind, there are a few tricks that can help you deal with them. They include…

While Working…

  • Like I just said…shield your work from the wind & sun. A movable “V”-shaped screen made from 2×2’s and opaque painters’ plastic will help redirect breezes and provide a measure of shade. Scale depends on your work, but the taller & wider the better. And, it obviously needs to be made so that it won’t blow over in a stiff breeze.  Small camping tents are inexpensive these days at discount sporting goods stores and can serve quite well to control environmental influences such as direct sun & wind.
  • Keep a hose handy. Preferably one with an adjustable nozzle that includes a very fine mist setting. On hot, windy days, you may have to mist your project regularly, even while you are working on it. Just don’t overdo it before the initial set or you risk adding excess water content into the mixture which will weaken it. Just a very light misting from a few feet away to keep the surface damp.
  • If you are up to the task, you can create a high-humidity environment for you & your work by installing an area misting system.  These consist of a hose-end fitting and some length of tubing with fine misting nozzles at regular intervals.  They can usually be found at the Big Box stores or Patio Specialty stores. The only “brand” I know of is called the “Arizona Mister”. The mist they produce is so fine that even after several hours of continuous use, the ground in the area never actually gets “wet”, it just remains highly humid. Just what your work needs. And it makes for mighty comfy working conditions on a scorching hot day, I might add. Some are advertised as being capable of lowering the temperature as much as 20 degrees.

After Working, but before fully set…

  • What you do next depends on the state that the mix is in.  If the mix has not yet achieved its’ final set, you will need to continue lightly misting it until it does. But remember, too much too soon and you are watering down & weakening the mix as well as washing away details. And keeping the airflow off is very important at this stage, so be prepared to cover your work.  At this stage the surface will still be quite soft, so you may need to avoid laying any covering material directly in contact with your work as it will alter the finish. I keep a couple of sets of prefabbed support posts for this purpose. They are simply 4 x 4 blocks that have a 1/2″ dowel rod pressed into a hole. I position them around my work to hold the plastic sheeting up and off the surface.

After Working and fully set…

  • Once the mix has achieved its’ final set, you can apply more generous doses of water. In fact, you can submerge it in water if the size is convenient.  One of the miracles of Portland cement is that it is fully hydraulic and will set under water. This technique not only ensures adequate moisture for full hydration but also solves the problem of air flow over the work as well. However, if submerging your work is not feasible, you will have have to handle those two issues in some other fashion.
  • One option is to try and maintain adequate moisture by covering or wrapping it in moisture retaining materials such as wet fabric (old towels, burlap) or wet sand or even sawdust. Covering that with an air-blocking material, like plastic sheeting, will also help keep the moisture from escaping. Using this approach, you will still have to constantly monitor and maintain its’ water supply by removing the plastic and hosing it down regularly.
  • Today, many concrete professionals rely on a chemical solution. Literally. Remember, you aren’t trying to add any water…just prevent what’s already there and adequate for complete hydration from escaping. That’s how the industry came up with a line of products that are called “Membrane Curing Compounds”. Simply put, they are liquid compounds that can be sprayed or rolled onto freshly placed concrete that form a thin film or membrane over the work that prevents moisture loss through evaporation. Great for flat surfaces, but less than perfect for anything 3-dimensional. Plus many of them seriously interfere with any post cure processes such as acid-etch staining or applying paints. So, while they may be a boon for contractors, their use by artisans is somewhat limited.
  • Another approach is to seal your work in an airtight containment so that any moisture the mix does give up, stays available for continuous hydration.  Depending on the size of your work, either large plastic trash bags or the roll type plastic sheeting can be used.  The large bags have the advantage of being easy to seal. Plastic sheeting for larger work must be taped closed along all seams to achieve airtight status.  Using either method, wet the surface thoroughly or apply some cloth and soak it down, then slightly inflate the container by just blowing into it and then seal it tight.
    And once it is sealed, I leave it that way for at least 72 hours, then open it and check its’ water supply. As long as the surface is damp to wet, you are in good shape. I usually mist it again before resealing, just to compensate for anything lost by breaking the seal. Just how long you leave it in this environment depends on the mix, but 5 to 7 days should be adequate for the full hydration of just about any mix. Note that 5-7 days is NOT full strength. That takes 28 days.  But for most artwork, I have found about a week to 10 days to provide ample strength. If a piece of work is more architectural in nature or has to carry significant loads…then curing longer is a good idea,
  • One last trick involves what I call a “Poor Mans’ Steam Cure”. Many super-critical concrete placements (such as nuclear reactors) undergo a high-tech process called steam curing.  The curing concrete is kept in a high-temperature, steam laden environment.  “Oh, but what about all that heat?”, you ask.  Heat is not the enemy for conventional cement product mixes.  Up to about 290 degrees F. anyway.  In fact, AAC (Autoclaved Aerated Concrete; see the “Glossary” on this site for a definition) is cured at 3500 degrees F.!  Moisture loss is the issue when curing concrete or any cementitious mix.  So as long as you either PREVENT moisture from escaping…or REPLACE it with an equal amount as it does…hydration will be continuous & complete and the final product will be as strong as it can possibly be.  And as witnessed above, heat can actually be used to accomplish near perfect curing.
    The poor mans’ version simply involves sealing the work in a large black plastic bag, lightly inflating it, sealing it, then,  just as above…putting it in the sun. The black plastic absorbs a great deal of heat and creates a very high humidity, steamy environment within the closed bag or wrapping that recycles just about 100% of the moisture.  Just be sure that you seal it as air-tight as possible to keep the much-needed moisture from escaping.

And…It Can Also Serve as a Cure For Your Winter Blues…
Of course, I do live in the coastal Texas region, where we have an ample supply of excess heat that can be put to work for much of the year. But you can accomplish the above in soggy Seattle or even sultry Saskatchewan…at any time of the year. Just invest in a heat lamp or two. A “sun-lamp” will transfer radiant heat into the black enclosure the same way the real sun does. In fact, using this approach to curing will allow you to accomplish work during times of the year when it would be otherwise impossible.  Granted, you absolutely must conduct enough testing to ensure that the bag heats properly while NOT catching on fire or melting (unlikely, unless you position the heat source within inches), but this technique will allow many of our Northern friends to continue their crafting activities all throughout the year.