Hypertufa and Cement: An in depth look into how they work

Hypertufa was created to mimic the properties and look of the Tufa stone, http://en.wikipedia.org/wiki/Tufa, which was used for feed and water troughs in Europe and going as far back as the Greeks was used to construct the aqueducts.  The antique troughs became sought after for planters in alpine gardens, the porous quality of the stone as well as its esthetic lends its self  easily to hearty plants that prefer well-drained soils.  Due to cost Hypertufa quickly became the substitute and now is a material in its own right.   The following  is an in-depth look at not just how its made but also how concrete works.  

There are a few things that when understood, can demystify what is actually going on and show us how to predict the quality of thefinish product.  Concrete is a mixture of three basic ingredients, Portland Cement, aggregate, and water.  Everything else is an additive to achieve a specific result.  70%-80% of the volume of concrete should be aggregate, leaving 20-30% Portland.   These percentages are based on physical strength, too much aggregate and there is not enough Portland to glue it all together.  Chemically there are three calcium structures in the Portland Cement that react with water during hydration.  These calcium structures react at different times during the curing process taking electrons from the water molecules creating crystalline structures that bind everything together, as the first reaction is winding down a week after casting, another begins.  This can continue for up to 48 days, but only if the material is allowed to stay fully hydrated during this time.  Once the material dries out all the reactions that were left have now stopped forever.  In most cases, two-week cure time is fine but if your pots will be outside in the winter and some need extra protection from repeated freezing then maximizing the cure time for added strength is advised.  I do this by wrapping the wet pots in plastic shopping bags to seal in the moisture.  The tricky part when mixing is that a high water to cement ratio results in a weak material and a low water to cement ratio is stronger but difficult to mold, cast, and shape.  Your mix should contain the least amount of water to reach the consistency chosen for your molding techniques.  This may sound confusing because we just discussed how keeping the material wet for as long as possible will make it stronger, more water in the mix sounds like it would help with this but there are enough available electrons for these reactions in a dry stiffly mixed batch of cement.  When too much water is used small pockets and veins are left behind where the excess water drains out of the material.  This can be seen to some extent with any batch of concrete, water will work its way out of the mix in every direction possible.

You will notice that nowhere do I give a ratio or percentage for water.  This may seem strange because its chemistry and should be predictable, but it’s actually very unpredictable  This is because we don’t work in a lab and can’t control all of the variables.  The peat and perlite both have an unknown moisture content as well as does the air which can create condensation due to temperature.  It might seem like a stretch to say temperature could affect this, but I have seen concrete take 30% less water from 1 day to the next because it was hot, humid and the sand we were using was cold resulting in water from the air condensing on it.  This was not in any way visible other than there was a very obvious difference in the volume of water needed to make the same volume of mortar.  After a few mixes, if you are keeping tabs on how much water you are using, you will learn how much you need, use some sort of graduated container to monitor the volume used.

In Hypertufa, there are four ingredients, Portland cement, perlite, water and peat moss.  We now know why we need Portland cement and how it works but why perlite and why peat moss?  In the case of Hypertufa perlite is the aggregate, it is specifically chosen because it is light weight and drains moisture quickly.  The peat is not quite as utilitarian, it is light weight and holds some moisture balancing out the drainage of the perlite but also adds an ascetic value as well.  Large chunks of peat in the surface of the finished pot will rot or fall out when they dry, leaving  puck marks that look more like the natural volcanic aspects of the tufa stone.  As you now know, too much water will make concrete weak and not enough is going to make it hard to work with.  The trouble here with Hypertufa is both the Perlite and Peat absorb water and very differently.  The Perlite is of a mostly even density and graded for size, it holds moisture but will release it quickly acting in a consistent manner from batch to batch.  The peat is like a sponge and being a plant-based material is unpredictable in how much water it absorbs.  Like a sponge it will hold on to its moisture for an extended period of time, this is benefit during curing as keeping moisture in the concrete for as long as possible will improve the overall strength.  I choose a very stiff mix for hand packing methods and cure for 20 days before letting them dry which can take up to 5 additional days.  In New England, I want to be sure the pots can stand up to the abuse of freezing and thawing repeatedly all winter without cracking into pieces. 

Concrete Admixtures:

The two most common ones we could use are plasticizer and fibreglass.  When plasticizer is added to a high viscosity or dry mix, it lowers the viscosity to that of a very wet mix without the added water.  This allows you to use the properties of the wet mix to your advantage, like being able to pour it while keeping the strength of the dry mix.  It will literally cause a pile of material to turn into soup.  The fiberglass adds small strong fibers throughout the mix that help hold it altogether preventing small cracks from becoming worse.  I personally don’t use either in making hypertufa because generally with a proper mix I can achieve the strength I need from the final product, without adding cost.  Although if I had a complicated mold that had to be poured I might opt for the plasticizer or add more water plus fiberglass.

Tints and Dyes:

The home centers do carry a few basic colors that are designed for concrete slabs and patios, but color selection is limited.  Through online retailers, there are hundreds of colors in liquid and powder forms but remember a super bold color choice can be too much for Hypertufa.  The Hypertufa esthetic to me is about heavy looking basic shapes highlighting small plants or when used to make larger planters the result can seem like a miniature landscape.  Very light shades of colors work best but we all see the world differently and thinking outside the box is the key to design, so go nuts!

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