How concrete blocks are made

Introduction

Concrete blocks, also called concrete masonry units (CMUs), are essential building materials for wall construction. These precast products are formed and hardened before reaching the job site, typically featuring one or more hollow cavities with either a smooth or a designed surface. They are stacked using concrete mortar to form walls.

Historical background

The Romans pioneered concrete mortar around 200 BC for binding shaped stone. During Emperor Caligula's reign (37-41 AD), small concrete blocks became a construction material in Naples, Italy. This technology was lost after the fall of the Roman Empire in the fifth century. English stonemason Joseph Aspdin developed Portland cement in 1824.

Harmon S. Palmer designed the first hollow concrete block in 1890 in the United States, patenting it in 1900. His original design measured 8 x 10 x 30 inches and required cranes to move. One person could produce approximately 10 blocks a day through hand casting. Modern machinery now produces up to 2,000 blocks per hour.

Raw materials

Standard concrete blocks combine powdered Portland cement, water, sand and gravel to create light grey blocks with a fine surface texture and high compressive strength. A typical concrete block weighs 38-43 lb (17.2-19.5 kg).

Cinder blocks use granulated coal or volcanic cinders, producing dark grey blocks weighing 26-33 lb with good sound deadening properties and thermal insulation.

Lightweight blocks incorporate expanded clay, shale or slate, heated to 2000°F, resulting in blocks weighing 22-28 lb. These suit non load bearing walls and partitions.

Additives include admixtures for altering curing time and increasing compressive strength, pigments for uniform colouring, and glazes made from thermosetting resin, silica sand and colour pigments.

Block design

Standard block dimensions measure 8 x 8 x 16 inches, including the mortar bead. Manufacturers offer variations for aesthetic appeal and specific applications, including split faced blocks with a stone like texture and water resistant designs.

Manufacturing process

The production process involves four main steps: mixing, moulding, curing and cubing.

Mixing

Sand and gravel are stored in outdoor silos and transferred via conveyor belt, while cement remains in silos for moisture protection. A weighing batcher measures each material precisely, then a mixer blends the dry components for several minutes. Two mixer types are used: planetary (shallow pan) and horizontal drum mixers. After dry blending, small amounts of water are added. In warm climates, water passes through heaters or chillers to maintain temperature. Chemical admixtures and pigments are added, and mixing continues for 6-8 minutes.

Moulding

Mixed concrete transfers to a bucket conveyor, then to an elevated hopper. It flows into moulds containing outer boxes and inner liners that shape the blocks and their cavities. Five to 15 blocks are moulded simultaneously, depending on machine capacity. Hydraulic or air pressure compresses the concrete, with some machines using vibration. A rotating brush removes loose material before blocks are pushed onto steel pallets.

Curing

Pallets transfer to automated stackers that arrange them in curing racks holding several hundred blocks. When full, racks move via rails to curing kilns.

In low pressure kilns, blocks remain 1-3 hours at room temperature, then steam raises the temperature gradually, by around 60°F per hour. Standard blocks cure at 150-165°F; lightweight blocks at 170-185°F. After reaching target temperature, blocks soak in hot, moist air for 12-18 hours. Total curing takes approximately 24 hours.

In high pressure steam autoclaves, temperature reaches 300-375°F at 80-185 psi pressure, and blocks are held for 5-10 hours. Though more energy intensive and expensive, autoclaves produce blocks faster.

Cubing

Cured racks exit the kiln, pallets are unstacked onto chain conveyors, and blocks are pushed off the steel pallets. Empty pallets return to the block machine. Split face blocks, moulded as pairs, pass through a splitter with a heavy blade after curing. Blocks then move through a cuber for alignment and stacking into units three blocks wide by six blocks deep by three or four blocks high. Forklifts transport the cubes outside for storage and dispatch.

Quality control

Electronic scales weigh raw materials before mixing. Ultrasonic sensors measure water content in sand and gravel, automatically calculating the required water amounts. Water temperature is controlled by chillers or heaters for different conditions. Laser beam sensors check block height after blocks exit the machine. Temperature, pressure and cycle time are automatically monitored and recorded in curing chambers to ensure proper hardening and maximum strength.