Curing means keeping fresh cement damp so it can keep reacting and harden dense and watertight: keep a ferrocement tank wet for at least 14 days by covering it with wet burlap or plastic and spraying it twice a day, and cure each coat of cistern plaster for at least 7 days. Curing is not waiting for the mortar to dry; drying early is what cracks a tank, and rushed curing is the commonest reason one leaks.
Part of Cisterns and Rainwater Storage
Curing is the controlled keeping of newly placed cement mortar or concrete in a moist, protected state while it gains strength. For a water tank the aim is a dense wall with the fewest cracks, since cracks are leaks.
Cement Hardens By Reacting With Water, So Drying Out Stops It
Cement sets by a chemical reaction with water, not by drying. If the surface dries in sun or wind, the reaction stops at the surface and the mortar shrinks. A shrinking surface on a rigid wall cracks. The mortar then looks hard but is weak, porous and leaky.
The cisterns page gives the rule for ferrocement: keep the tank wet for at least 14 days, covered with wet burlap or plastic and sprayed with water twice daily, and states that rushed curing means a cracked tank. For the rendered lining of an underground cistern, each of the two coats is cured for at least 7 days and kept damp (Cement Plaster).
What You Need
- Burlap, cloth sacking or old blankets — to hold moisture on the surface
- Plastic sheeting — to hold the moisture in
- A sprayer or a bucket and a soft brush — water must reach the surface without washing it away
- Shade, from poles and cloth if there is none
- A calendar, with the day each coat was placed
Method: Cover, Wet, Repeat
Step 1: Start early, gently. As soon as the surface is firm enough to take a wet cloth without marking, cover it. Before that, shade it from the sun.
Step 2: Cover the whole surface with wet burlap or plastic, inside and out where reachable.
Step 3: Spray twice daily, more often in hot or windy weather. The covering should never be seen dry and pale.
Step 4: Keep going for the full time. Count from the day of the last plastering.
Step 5: Do not fill the tank early. A tank filled before the cure is complete can crack under the load.
Worked Example
Calendar for a cistern lined in two coats, from day 0 when the first coat is placed:
- Day 0: first coat placed
- Days 0–7: kept damp (7 days)
- Day 7: second coat placed
- Days 7–14: kept damp (7 days)
- Day 14: earliest the lining is ready to take water
Ferrocement runs on the same total of 14 days from the last plastering. If a heat wave or wind dries the cover during the cure, count the dry day as lost and extend the cure.
First-Fill Water
New cement leaches lime, so the first water in a new tank is alkaline and not good to drink. Fill it, let it stand, empty it, and repeat before putting it into use. Disinfect before drinking: chlorine at 50 mg/L for 24 hours is the dose used for a new well in Well Development; that is the dose added, not a residual.
Common Mistakes
- Cure counted from the first coat, not the last. The final layer is the one at risk.
- A cover that blows off. Weight the edges.
- Spraying hard. It erodes a surface that has not hardened.
- Letting the burlap dry between sprays. Dry burlap pulls moisture out.
- Stripping props or forms early. A slab or shell is still weak.
Safety
Fresh cement burns and a damp cover can hide a hazard
Wet cement is alkaline and burns skin and eyes; wear gloves when handling it and when lifting wet cover. Do not climb into a covered, roofed or lidded tank to check the wall without ventilation and a rescue line: the air in a closed tank can be short of oxygen.
What’s Next
- Learn next: Ferrocement Tanks — how curing fits the whole build
- Combine with: Cistern Roof — slabs need the same patient curing