Cement needs water to gain strength. If a slab or column dries out too soon, the surface stops gaining strength, dusts up and cracks. Hot sun and drying wind, common across much of Kenya, strip moisture from fresh concrete quickly.
Start curing as soon as the surface can take it without damage, and keep it going for at least seven days. For slabs: pond the surface, lay wet hessian or sand and keep it wet, or use plastic sheeting weighted at the edges. For columns and beams: wrap in wet hessian or plastic.
Pour early in the morning or late in the afternoon where you can, and shade fresh work from direct midday sun. Never let a slab dry out and then soak it again; interrupted curing is almost as bad as none.
Rapid (R) classes gain early strength faster but still need the same curing attention. Early strength is not finished strength.