Builders often ask for “the” cement ratio for soil blocks. The honest answer is that it depends on your soil. A sandy soil usually needs more cement to bind it than a well-balanced one, while a very clayey soil may need sand added before cement can do its job.

A common starting range

Many practitioners start trials at roughly 5% to 10% cement by weight of dry soil. Treat that as a place to begin, not a final answer. Use the results of your soil tests to decide where in that range to start.

Make trial batches

1. Prepare three small batches at different cement levels, for example low, medium and high within your starting range.
2. Measure materials by weight or with the same container every time, and write the proportions down.
3. Press a few blocks from each batch and label them.
4. Cure them properly (see our guide to curing soil blocks).
5. After about four weeks, test them: check for crumbling at the edges, try breaking one by hand, and soak a block for 24 hours to see if it softens or loses its shape.

Choose the lowest cement content that gives blocks which stay hard, keep sharp edges and survive soaking. Extra cement beyond that point only adds cost.

Dry mix first, then add water

Mix soil and cement dry until the colour is uniform, turning the heap several times. Only then add water, a little at a time. Uneven mixing is one of the most common causes of weak, patchy blocks.

The moisture check

A reliable field check is the hand-squeeze and drop test. Squeeze a handful of the mix firmly in your fist. It should hold together in a lump without leaving water on your palm. Drop the lump from about chest height onto firm ground. If it breaks into a few pieces, the moisture is about right. If it splatters, it is too wet. If it simply crumbles into dust, it is too dry.

Use the mix quickly

Once water meets cement, the mix starts to set. Mix only what you can press within a short time, and do not re-wet mix that has stiffened.

Working with a manual press

Machines such as the Standard Straight Interlocking Block Press compact a mixture of soil, a small amount of cement and water into blocks. Because the machine delivers consistent pressure, the biggest variables you control are the soil, the cement content and the moisture, so it pays to get those right.

• cement ratio
• mix design
• stabilised soil blocks