Making concrete bricks requires three basic ingredients (cement, sand, and aggregate), a mold, and a few days of patience for curing. The process is straightforward enough for a DIY project, but the quality of your bricks depends heavily on getting the mix ratio right and compacting the mixture thoroughly before it sets.
Choosing the Right Mix Ratio
The ratio of cement to sand to aggregate determines how strong your finished bricks will be. For most home projects like garden walls, raised beds, or non-structural partitions, a 1:2:4 ratio (one part cement, two parts sand, four parts aggregate) produces bricks rated around 15 MPa, which is plenty for residential use. If you need something stronger for load-bearing walls or pavement, bump up to 1:1.5:3 for roughly 20 MPa strength.
For simple footpaths or decorative uses where strength isn’t critical, you can stretch your cement further with a 1:3:6 ratio. On the other end, a rich 1:1:2 mix delivers 25 MPa bricks suitable for heavy structural loads, though it uses significantly more cement and costs more per brick.
Use fine gravel or crushed stone (no larger than about 10mm) as your aggregate. Sharp sand, sometimes called concrete sand, works better than soft builder’s sand because its angular grains lock together more tightly.
Getting the Water Ratio Right
Water is the ingredient most people misjudge. The water-to-cement ratio controls both workability and final strength, and the ideal range for most brick applications falls between 0.35 and 0.45 by weight. That means for every 10 pounds of cement, you add 3.5 to 4.5 pounds of water (roughly 1.7 to 2.2 quarts).
Going below 0.35 makes the mix very stiff and hard to pack into molds, but it produces denser, more durable bricks. Concrete permeability drops sharply once you get below 0.40, which means your bricks absorb less water and resist weathering better. Ratios above 0.55, on the other hand, leave excess water that evaporates during curing, creating tiny voids that weaken the brick and can double or triple the rate of surface deterioration over time.
The mix should be damp enough to hold its shape when squeezed in your fist but not so wet that water drips out. If you see a sheen of water on the surface after compacting, you’ve added too much.
Building or Buying a Mold
Standard concrete bricks measure roughly 7⅝ by 3⅝ by 2¼ inches (the actual dimensions are 3/8 inch smaller than their “nominal” size to account for mortar joints). You can build a simple mold from plywood or dimensional lumber, or use steel molds for more consistent results over many bricks. A multi-cavity mold lets you produce several bricks at once.
Line wooden molds with a thin coat of motor oil or a commercial release agent before each pour. This prevents the concrete from bonding to the wood and makes demolding far easier. Steel and plastic molds need the same treatment but are easier to clean between batches.
Mixing and Filling the Molds
Combine the dry ingredients first, mixing thoroughly until the color is uniform with no streaks of pure cement visible. Then add water gradually while mixing. A wheelbarrow and shovel work fine for small batches. For larger runs, a portable cement mixer saves enormous effort and produces a more consistent mix.
Fill the mold in two layers. After adding the first half, tamp it down firmly with a flat piece of wood or a tamping tool. Then add the remaining mix and tamp again. This compaction step is critical. Vibration and pressure force aggregate particles to rearrange and push trapped air bubbles to the surface. Without proper compaction, air pockets remain inside the brick, reducing strength and creating weak spots that crack under load.
If you have access to a vibrating table or even a palm sander clamped to the mold, 30 to 60 seconds of vibration dramatically improves density. You’ll see small air bubbles rising to the surface during vibration, which is exactly what you want. Be careful not to overdo it, though. Excessive vibration causes the heavier aggregate to sink and the cement paste to rise, a problem called segregation that creates a weak, uneven brick.
Curing for Maximum Strength
Curing is where most DIY bricks either succeed or fail. After filling and compacting, leave the bricks in their molds for at least 24 hours in a shaded area. Then carefully remove them from the molds and begin wet curing.
Wet curing means keeping the bricks consistently moist for a minimum of 7 days. You can do this by misting them with water two or three times daily, covering them with damp burlap, or wrapping them loosely in plastic sheeting. The goal is to prevent the surface from drying out while the cement continues its chemical reaction with water. Bricks reach roughly 70% of their final strength at 7 days and continue gaining strength for weeks afterward. For best results, cure for a full 28 days before putting them under any structural load.
Temperature matters during curing. If the air temperature drops below 40°F for three consecutive days, the hydration reaction slows dramatically and can stop altogether. In cold weather, store curing bricks in an insulated area or use curing blankets. Hot, dry, windy conditions are equally problematic because they pull moisture from the surface faster than you can replace it, leading to surface cracks.
How Many Bricks You Can Expect
An 80-pound bag of premixed concrete yields about 65 standard-sized bricks. A 60-pound bag produces roughly 48. If you’re mixing from raw materials, a 94-pound bag of Portland cement combined with sand and aggregate at a 1:2:4 ratio will go considerably further, since the cement is only one part of the total volume. Plan your materials based on the number of bricks you need, and always mix 10% extra to account for waste and any bricks that don’t turn out right.
Adding Color and Water Resistance
Plain concrete bricks cure to a utilitarian gray, but pigments can produce nearly any color. Iron oxide pigments create reds, yellows, and browns. Chromium oxide produces green, cobalt blue makes blue, and carbon black creates dark gray or black bricks. Add pigment at no more than 6% of the cement weight to avoid weakening the mix. Blend the pigment into the dry ingredients before adding water for even color distribution.
For bricks exposed to rain or ground moisture, you can mix in a water-repelling additive. These are typically stearate-based compounds (sold as concrete waterproofers at hardware stores) that make the brick’s pores resist water absorption. For more serious waterproofing, crystalline admixtures react inside the concrete to physically block pores and capillaries, though these are more common in commercial production.
Safety While Working
Dry cement and sand contain crystalline silica, a fine dust that causes serious lung damage with repeated exposure. OSHA sets the exposure limit at just 50 micrograms per cubic meter of air, an amount so small you can’t see it. Wear an N95 respirator or better whenever you’re pouring, mixing, or sweeping dry materials. Never use a leaf blower or compressed air to clean up cement dust.
Wet concrete is strongly alkaline and will burn exposed skin with prolonged contact. Wear waterproof gloves and long sleeves when handling the mix. Safety glasses protect against splashes, which are especially harmful to eyes. If wet concrete gets on your skin, wash it off promptly with clean water rather than waiting for it to dry.