Well, one of my previous video and post was about the Hypertufa Making Marathon that we did a while back. But I have a whole big bunch of hypertufa planters to cure now. So I thought I would take this opportunity to talk about curing hypertufa. It seems to create a lot of confusion about how to do this and for how long. I have explained this on some of my posts and pages, but the information gets confusing, I guess. So I will try and give you my take on this question.
Curing Hypertufa – The Green Stage
First of all, there are two steps to curing hypertufa. Once you have made your project or its pieces if it has to be assembled, there is the initial curing which is when the actual chemical process of the Portland cement working its “magic” to turn that pile of oatmeal or fudge into a solid rock. This is really what I call its “green stage.”
After I have formed my hypertufa mixture into a mold, I typically place it inside a bag or wrap it inside some plastic material. If you have small pieces or a big enough plastic bin with a lid, you can set all your pieces unwrapped inside this bin. But the purpose is to create a tent to enable the process of curing to occur. This curing or hydration of the cement generates a small amount of heat and I sometimes open and touch my newly-forming hypertufa and feel the medium in its mold. Warmth is good, hardness is good. It is during this first 24 hour period, your project seems to already be rock-hard.
Either inside your plastic container or inside the plastic bag, you should see condensation forming or a watery sheen over your project. This watery atmosphere is important in the curing process for the chemicals to form the hardness that you are looking for. Remember that the moisture present during this initial curing process is essential for the project to reach its maximum strength. That’s why I put mine in a “tent” of plastic bags or garbage bags. I want it to cure in a high humidity environment.
Since this chemical reaction occurring generates a small amount of heat, the addition of extra heat is not good for the long-time durability of the piece. I believe that your curing should be in the shade if possible. Setting a piece in the sun to “dry” is just not good for your hypertufa piece and could cause it to crack or fall apart. Some people have had success setting pieces in the sun to dry, but I would not recommend that at all.
Unwrapping the Hypertufa after First Cure
My video for this post shows my unwrapping stage, or the end of the Green Cure, for my creations. Typically I do this after 24-48 hours. Even the larger hypertufas will be rock hard after this initial period of time. They are very solid and hard and you can tell this just by touching them, and it would be at this time that I do any carving, sanding, filing to get the perfect edge that I want on the piece. Sometimes your project will seem a bit moist and if you carve or sand it, a small amount of “mud” comes off in small sawdust-like amounts. This feels like clay and shouldn’t worry you.
During this period, the piece will be heavier than the end product and will be darker in color. As it continues to cure, the true color will emerge. Sometimes I wish I could halt those changes in color because I like some of the transitions that it goes through.
The Second Cure – An Important Step
After your piece or pieces are all unwrapped and “manicured” to your liking, then re-wrap them in more plastic and put them into the shade once again. In my case it is either the basement or in the yard under bushes etc. I try and forget them for this period because I get so impatient to have them ready. Many arguments now occur for the timing of this Second Cure.
In my opinion, ten days to two weeks in the suitable amount of time for this cure. We have discussions all the time in The Hypertufa Gardener Group on Facebook regarding the length of time that each hypertufa maker feels is the correct time to leave them cure. All of us want them to be durable and last for many years. I want mine to last a long time also. Maybe you feel that if you speed up the process you can have a lot to sell at an upcoming sales festival of some kind. But remember. A buyer will always remember that yours fell apart its first winter season. So I would not want to disappoint a buyer who has had faith in me and had given me money for my special hypertufa container. ( I have to mention here that I don’t sell mine, but many do and are very successful at it.)
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But that being said, I feel that two weeks gives a strong enough cure to work for a planter. Many of the sources say that cement/concrete does most of its curing in the first 7 days and reaches a full cure by about 28 days.
Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit.
Based on all this information about how long a BRIDGE DECK needs to cure, I think my planter is ready to be leaching by it’s third week. I am not planning to use it as a bridge or canal lining or retaining wall. Just to hold some soil and plants. Have you ever had a concrete driveway poured? Did you not drive on it in a few days? If it is strong enough to hold my car and pickup, my small bit of “concrete” hypertufa style will be great for my uses after a two week second cure.
The Curing Hypertufa Controversy
There are many opinions about how to cure and how long to cure, but I feel there are a few basic rules so I wanted to write those all out here so that you can make the decision on what works for you. Your climate, humidity, materials you use…all these factor into your finished project.
But for both Curing procedures, here are some general guides:
- Wrap it in plastic
- Keep it out of the sun if possible
- Moisture during curing is great & some will be generated by the heat given off the piece as it cures
- Patience (the hardest one of all!)
Next we will discuss Leaching as this is a whole different process. Stay tuned!