Most of our users will use concrete to cast our molds. You can find a list of ready made mixes we have tried here.
What is concrete?
Concrete is the mixture of Cement + Aggregate + Water. This mixture is what is needed to get the chemical bonds necessary to cast items, and not have them crack. I would not recommend just using with cement + water. The balance of the ingredients is even more important as it will create a strong casting. What might work for a smaller casting, won't always work for a larger casting. As the surface area increases, the mixture becomes even more important.
What are the correct proportions?
A quick search on concrete recipes will say 3-2-1 of Aggregate - Cement - Water. There is exhaustive research done on concrete strength for buildings, but those same theories don't necessarily work in trying to get an artisan concrete pot or planter. Using a large aggregate just doesn't give the smooth finish many of us are looking for. Also, a thicker mix, is not always the easiest to work with.
Water ratio is one of the most important aspect of your mix. Too much and you can run into two problems.
- You get a thinned out bond which results in a loss of strength. One solution to trying to get a more workable mix is to add water reducer.
- Greater amount of shrinkage during the drying process and the mixture will get cracks.
- Water Ratio will also depend on temperature and humidity, which is why adding small amounts of mix or water after your combine might be needed.
- 20% Portland type I cement replacement with CSA cement, this should speed cure time
- a water reducer, usually .5% of cement weight. This will strengthen the mix and reduce cure time