When casting concrete into ABS or rubber molds, should you use Portland cement and sand? Premixed bags of concrete? Premixed Bags of Sand Mix? Moulding Plaster? There are two primary rules, and a couple of secondary considerations that will determine what to use. First and foremost, follow the instructions on the product you are using. And be sure to wear protective clothing and equipment as specified by the cement manufacturer.
Mold Thickness and Size Matters - You can use bags of premix 'Concrete' or bags of premix 'Sand Mix' depending on what thickness you are casting. The main differences are the size of the aggregate in each type. Concrete has larger aggregate or gravel in with the sand and Portland cement, and is designed for castings of two-inches and thicker. These premixed bags of concrete should meet or exceed ASTM C-387 and will yield a compressive strength of 4,000 psi at 28 days, if mixed per instructions. Sand Mix is Portland cement and sand, as the name implies, and is designed for use at half-inch thick and up to two-inch deep mold thickness. Sand Mix used as directed and in accordance with ASTM C-387 yields a compressive strength of 5,000 psi after 28 days.
Where to Use Bags of Sand Mix - Cement tiles and thin brick veneer, which are usually about half-inch thick or thinner are best made with a Sand Mix. Stone veneer under two-inches thick, and most wall plaques would be cast with Sand Mix as well. If the items being made are for an interior application on a vertical surface, such as a brick or tile wall, you may use a plaster instead of Sand Mix. Using plaster would produce a lighter weight tile, brick or stone as well as allowing the use of ready-made wall tile mastic to install them. This comes in buckets, and is readily available in most home supply stores.
It should be noted that a half-inch thick cement tile, paver or brick can handle the weight of vehicular traffic, as in a driveway, if they are installed over a solid concrete base. The same holds true for covering a concrete patio. Stones of a half-inch thick, properly installed with thin-set and or mortar, and made with Sand Mix have a compressive strength of over 5,000 psi. The savings in raw material costs between pouring concrete into a half-inch thick mold, versus a two to three inch thick mold is substantial. So if you are covering a concrete sidewalk, patio or driveway, you may want to consider using a thinner mold.
Where to Use Bags of Concrete Mix - Driveway pavers, stepping stones, thick cast manufactured stone, garden and landscape edging, and anything that is going to end up being thicker than two-inches would require using pre-mixed Concrete-- the stuff with the gravel. The gravel makes it stronger and able to withstand the extra weight these concrete products are expected to handle when installed in a packed sand and gravel installation. A properly prepared base will give you years of service with very little maintenance.
Alternative to Using Bags of Premixed Concrete - For large projects, to save money, and to have more control over your mix design, I recommend using Portland cement, which comes in 94 lb. bags. You will also need bulk sand and/or gravel, additive, concrete color, etc., to make your stones. Or in the case of needing to make batches of light or medium weight stone for walls, you would want to start with Portland cement and a lightweight aggregate like lava rock, Vermiculite, Perlite, expanded shale, or other lightweight aggregate.
When making stone or tile for large projects, or commercially for resale, the savings in raw materials cost will make a big difference in your total cost of the project versus using bags of pre-mixed concrete or Sand Mix. Be aware that there are some negatives though. When designing and mixing your own batches you need to consider that you'll have to purchase bulk sand, and have a place to store it. The same holds true for gravel if the items you are making are thicker than two inches. If you need additional help or advice, there are a variety of mix designs, and other tips and techniques on our Training and Instructions website, as well as hundreds of other free articles relating to the concrete casting industry.