Cement and concrete

Burn a limestone that contains roughly 10–20% clay, at an ordinary lime-kiln temperature — see lime. The clay combines with the lime and the result sets underwater instead of waiting months for air.

Find it by testing: burn small samples of different limestones, slake them, make a pat of each and put it under water after a day. The ones that set hard in a week are hydraulic. It is entirely adequate for foundations, tanks, cisterns and most masonry, at a fraction of the fuel of Portland cement.

Lime plus volcanic ash — or crushed fired clay, brick dust or certain ashes — gives a hydraulic set with no special kiln at all. The silica and alumina in the ash react with the lime in the presence of water.

Mix roughly 1 part lime putty to 2 parts pozzolana, then add aggregate. Roman harbour concrete made this way is still standing in seawater two thousand years later. Test any candidate material the same way as hydraulic lime: pats under water.

Portland cement is limestone and clay, ground together to a fine powder in roughly 3:1 proportion and burnt to about 1,450 °C — hot enough to partly fuse the mix into clinker. That is a serious step up: a white heat held steadily, which needs a well-designed kiln, good refractories — see brick and tile — and good fuel.

Reach for it for high-strength or fast-setting work — machine bases, large dams, reinforced structures — not for walls that hydraulic lime would carry.

Fineness controls strength and setting rate: the reaction happens at the surface of each grain, so a coarse grain leaves an unreacted core — most of what you burnt, wasted.

Grind in a ball mill or edge-runner mill until the powder feels like flour between the fingers and very little is left on the finest sieve you can make. Add a few percent of gypsum while grinding to control the setting time; without it, cement can flash-set in minutes. Store in sealed, dry containers — cement that has absorbed moisture forms lumps and has lost strength.

About 1 part cement, 2 parts sand, 4 parts coarse aggregate by volume is a general-purpose mix. Clean sand and aggregate — clay and silt coating the stone ruin the bond.

Water is the variable that matters most. Aim for roughly half the weight of the cement in water — a water-cement ratio of about 0.5. Excess water is the commonest cause of weak concrete: the surplus leaves voids as it evaporates. The mix should be stiff enough that a handful squeezed holds its shape and leaves the hand only damp.

Place it in layers and compact each — rod, tamp or vibrate — until air bubbles stop rising. Voids left in are weak spots.

Concrete does not dry, it reacts, and the reaction needs water. Keep it damp for at least 7 days — covered with wet sacking, sand or sheeting, sprinkled regularly — and out of frost. Strength keeps developing for months. Leave formwork on walls a few days, and supports under slabs and beams longer — around two to three weeks.

Concrete is roughly ten times stronger in compression than in tension. Anything that bends — a beam, a slab, a lintel, a tank wall — cracks on its tension side unless steel bar carries that load.

The steel goes where the tension is — near the bottom of a beam spanning between supports — and must be well covered: around 25 mm inside, and 40–50 mm or more outdoors or in the ground. Thin cover lets water in; the steel rusts, expands, and splits the concrete off from within. Tie bars so they do not move during the pour.

Cast sample cubes — 150 mm is standard — from every pour, cure them beside the structure, and crush them at 7 and 28 days. Without an hydraulic press, load a cube with a lever and measured weights until it breaks, and record the load.

Without testing you have no idea what you built. Label each cube with the date, the mix and the water used, and keep the results. Anything that fails its test does not carry its design load.