Soil-cement is a mixture of Portland cement, natural soil, and water used to form a hard, semi-rigid paving surface. It is most often used in highways or as a sub-base for asphalt or other forms of paving, but it can also be used as a cheap stand-alone paving surface for driveways, sidewalks, patios, or garage floors. According to the Portland Cement Association, "Its advantages of great strength and durability combine with a low first cost to make it the outstanding value in its field." The finished surface will be nearly as solid as concrete or asphalt but with much less effort and expense. Because it continues to hydrate and strengthen over time, soil cement can actually be stronger than traditional Portland cement concrete.
The components of soil-cement could not be more basic—just the on-site soil, a little Portland cement, and some water. The soil already exists in your yard, and the water is waiting for you at the tap. The only thing you have to buy is the dry cement powder. You will also need to borrow or rent a tiller and a tamper or roller. With these ingredients and a little labor, you can create a smooth, durable paving surface. Soil-cement may not work well in soils that have a high clay content or a lot of organic content, but it is suitable for most soil types. The look of soil-cement is not as uniform and aesthetically pleasing as traditional cement concrete, so it may be best suited for rural or informal settings.
Off-Site vs. On-Site Mixing
In commercial or road-work applications, soil-cement can be mixed at processing plants or in a portable mixer, then poured in much the way that standard cement concrete is poured. But for residential DIY applications, it is more often created in place, by thoroughly tilling the existing soil on the paving, blending in dry Portland cement powder and water, then leveling and compacting the surface.
Tools and Supplies You Will Need
- Treated dimension lumber for forms
- Carpentry tools (saws, hammers)
- Portland cement (3 to 4 pounds for each square foot of surface)
- Garden rake
- Garden hose with a spray nozzle
- Rubber boots
- Tamper or roller
- Sheets of plastic
Prepare the Site
Begin by removing all grass and plant life from the location, using a shovel. Also remove any of the dark topsoil that contains organic material, because this will compromise the soil-cement's strength and durability.
As with standard Portland cement concrete, it is best to build forms to contain the soil-cement surface. If building permanent forms, use pressure-treated dimension lumber (2 x 4s are generally best) nailed together and staked in place. Use a level to make sure the forms have a slight pitch to allow for water to drain off. A pitch of 1/4 inch per foot is the standard recommendation; this is especially important where the paved surface abuts a building, to ensure that rainwater flows away from the foundation.
Mix the Soil-Cement
Next, use a power tiller to churn up the existing soil inside the forms to the proper depth—about 4 inches for a walkway or 6 inches for a driveway or other large slab. This work can be done with a shovel, but it is laborious work. A power tiller is the preferred tool.
After tilling is complete, spread the required amount of dry Portland cement powder over the surface of the paving site. Generally, a layer about 1/2 inch thick (or 2 to 4 pounds per square foot) will be sufficient. A 40-pound bag of dry Portland cement will cover about 20 to 40 square feet. Spread the cement in an even layer using a garden rake.
- Note: Soils that are high in clay or organic content may require more Portland cement, while sandy soils can get by with less cement powder.
Now churn the Portland cement into the soil with the tiller, while at the same time lightly spraying the surface with water. The soil-cement should have a moist, but not sloshy, texture. This can be a messy process, so wearing tall rubber boots will help protect your clothes.
Level and Compact the Surface
When the entire slab is mixed, use a long straight board to level off the wet soil-cement between the edges of the form. The action is much like that used to screed traditional cement concrete—using a back and forth sawing motion as you draw the board along the forms. Having a helper makes this job much easier.
When the soil-cement surface is level and just dry enough to not be sticky, pack the surface down with a hand tamper or garden roller. This is a critical part of the process, so take your time to get the surface as flat as possible.
Unlike cement concrete, soil-cement does not require floating to create a smooth surface. A soil-cement slab will have a rougher texture and color, best suited for more informal paving applications.
Allow the Surface to Cure
For at least a week, keep the surface covered with sheets of plastic, removing the plastic every few days to lightly mist the surface with water. This will limit evaporation and allow for slow absorption of the water, which helps strengthen the paving. Do not allow traffic on the slab until the curing is completed.