Moisture Control and Additives
Water is part of the design
Moisture control covers mixing water, drying, and service water. Earth gains strength as it dries, but stabilised earth also needs enough water for the cement or lime reaction. Aim for optimum compaction moisture at placement, then protect the wall from rapid drying. Capillary rise from the ground and wind-driven rain are the main service threats.
Additives modify behaviour. Cement, typically 4–10% by dry mass, suits sandy, low-clay soils. Lime, often 6–10%, works better with clay soils because it reacts with clay minerals, though it needs time and moisture. Gypsum, bitumen emulsion, natural fibres, animal dung, and casein have traditional uses, but each has limits: bitumen can impair breathability, fibres can rot, and dung varies widely. Water should be clean and low in salts.
Traps: adding cement to a very wet mix and expecting strength; using lime in clean sand with no clay to react; assuming an additive replaces good compaction; and allowing stabilised walls to dry in direct sun before curing.
Worked example: for cement-stabilised rammed earth, dry-mix soil and 6% cement thoroughly, add water to the Proctor optimum, and compact within about two hours of wetting. Cover the finished lift to prevent premature drying. Rule of thumb: cement for sandy soils, lime for clayey soils - and always confirm with trial specimens.