Choosing Quality Sandcrete and Hydraform Blocks

Walls make up a large part of most buildings, and in Nigeria they are usually built from blocks. The quality of those blocks has a direct effect on how strong the wall is, how well it keeps out heat and rain, and how much plaster and repair work you will need later. Yet blocks are often bought on price alone. This guide explains two common types, sandcrete blocks and interlocking blocks made with Hydraform-type machines, and what to look for when buying.

Sandcrete blocks

Sandcrete blocks are made from sand, cement and water, moulded and then cured until they harden. They are the familiar hollow or solid blocks used throughout Nigeria. Hollow blocks are lighter and cheaper to lay, while solid blocks are heavier and usually used where extra strength is wanted, for example in foundations or boundary walls. The strength of a sandcrete block depends on the proportion of cement to sand, the quality of the sand, the amount of water, how well the mix is compacted and, very importantly, how the block is cured afterwards.

Interlocking blocks

Interlocking blocks are made on machines that press a damp mixture of soil or sand and cement into a mould under high pressure. The Hydraform system is a well-known example. The blocks have shaped faces that fit together, so walls can be built with little or no mortar between the courses, and the pressed blocks have a regular size and a smooth finish. Because the shapes lock together, walls can go up quickly and neatly, and the channels in the blocks can take reinforcement and services. Block makers who use this technology usually need training to operate the machine and prepare the mix correctly, and for that reason certification and practical experience matter.

What to check before you buy

• Size and shape. Blocks should be uniform. Variations in size make walls uneven and use more mortar and plaster.
• Edges and corners. Good blocks have sharp edges and square corners. Crumbling corners are a warning.
• Surface and texture. A well-made block has a close, even texture. A surface that is very sandy, with loose grains rubbing off on your hand, suggests too little cement or poor compaction.
• Sound. Knock two blocks together or strike one with a hammer. A good block gives a clear ring; a dull thud and easy breakage point to weakness.
• Breakage test. Drop a block from about waist height onto firm ground. A sound block should not shatter into pieces.
• Age and curing. Blocks that are delivered fresh from the machine are still weak. They should have been kept damp and left to cure and harden for some days before use.
• The producer. Visit the yard if you can. Look at how the sand is stored, how the mix is measured, whether the blocks are cured under cover and kept damp, and how they are stacked.

If you are building something important or large, ask your engineer whether the blocks should be tested for compressive strength in a laboratory, and agree the required strength in advance.

Why curing matters

Cement gains strength through a chemical reaction with water, and it needs moisture to do so. In a hot dry climate such as Northern Nigeria, blocks left in the sun dry out quickly and stop gaining strength, ending up much weaker than they should be. Good producers keep their blocks in the shade, sprinkle them with water regularly and allow time for hardening before they are stacked high or delivered. Ask your supplier how long and how they cure.

Storing blocks on site

• Stack blocks neatly on level ground, not in a heap, and keep stacks to a safe height.
• Keep them off wet ground and away from places where water will pond.
• Handle them carefully when unloading; throwing blocks from a truck leads to cracked and chipped units.
• Order a small allowance for breakage, but not so much that the surplus is wasted.

Laying blocks properly

Even good blocks give poor walls if they are laid badly. Mortar joints should be of consistent thickness and fully filled. Courses should be level and plumb, checked with a spirit level and a line. Joints should be staggered so that vertical joints do not line up in successive courses. In hot weather, wet the blocks lightly before laying so that they do not draw the water from the mortar too fast, and cover freshly built walls to protect them from the sun. Reinforcement and lintels should follow the engineer’s details. With interlocking blocks, the first course must be laid very carefully, because any error will be repeated up the wall.

Which type should you choose?

There is no single best block for every project. Sandcrete blocks are widely available and well understood by masons. Interlocking blocks can give faster construction and a neat finish and can reduce mortar use, provided they are made and laid correctly. What matters most is quality, consistency and proper curing, whichever type you pick. Ask for the opinion of your engineer or builder, and check the supplier’s work before ordering large quantities.

Fani Nigeria Limited produces interlocking and solid blocks at its own yard and has been involved in a bricks project and Hydraform-related training; see our blocks page. To ask about supply, you can contact us, or read our guide to planning a building project in Borno State.