The strength of the finished block is determined by the mixture. The recommended mix is 2 level wheelbarrows of sand, 1 x 50kg bag of 42 MPa cement and 2 level wheelbarrows of chipped stone (Not larger than 13mm stone), gives a block of roughly 5,2 MPa strength. The required strength in most countries for a concrete block (Reference B-Block) is 3,5 MPa. The block can be strengthened to any level by simply adding more cement. The above mixture makes 28 large blocks.