When "stabilising raw earth" comes up, the mind almost automatically jumps to lime and cement — the two most common routes, already covered in this material's main guide. It's an understandable but incomplete shortcut: stabilising earth is a much older and far more varied practice than those two industrial binders suggest, and contemporary research is returning to explore natural and biological additives that further reduce the material's already-low environmental footprint, without necessarily introducing a cementitious component.
The first stabilisation is mechanical
Before adding any binder, earth can be improved in two ways that cost almost nothing environmentally. The first is correcting its grading. A good building soil contains a balanced mix of clay, silt, sand and gravel: clay binds, sand and gravel form the skeleton. If the site soil is too clayey, sand is added; if it is too lean, a soil richer in clay is added; this is often enough to reduce shrinkage cracking and considerably increase strength. The second way is compaction. The closer the particles are packed, the less room there is for water and the denser, stronger and more durable the material becomes.
The compressed earth block, often abbreviated CEB, is based on this principle. In 1952 the Colombian engineer Raúl Ramírez developed the CINVA-Ram, a small hand-operated lever press that made it possible to produce regular, compact blocks directly on site, without firing. The press spread quickly through social housing programmes around the world. Since then much more powerful hydraulic and mechanical presses have appeared, and research centres such as the Auroville Earth Institute in India have built thousands of buildings with compressed, lightly stabilised blocks, developing technical know-how that is now an international reference.
The historical precedent: prickly pear mucilage
In Andean and Mesoamerican building traditions, well before the arrival of Portland cement, earth mixes were often supplemented with nopal mucilage — from the prickly pear (Opuntia) — extracted by steeping the plant's pads in water. The resulting mucilaginous gel acts as a natural organic binder: its long polysaccharide chains interact with clay particles, reducing the porosity of the hardened mix and significantly increasing its resistance to water erosion compared with untreated earth. This technique, documented both archaeologically and in vernacular building traditions still alive in Mexico and Peru, is today the subject of renewed scientific interest precisely as a prototype of a bio-based stabiliser with negligible environmental impact — a direct conceptual ancestor of the biopolymers studied today in the laboratory.
The prickly pear is not the only natural additive in the tradition. In India and many regions of Africa cow dung is mixed into earth plasters and floors: the partly digested plant fibres reinforce the mix and some of its components improve water resistance. European tradition used milk casein, especially combined with lime, and linseed oil, which is still used today to treat rammed-earth floors to make them hard-wearing and washable. In the south-western United States, from the 20th century onwards, adobe producers have often added a bitumen emulsion to the mix, which makes the bricks much less sensitive to water but compromises their recyclability. Every additive, even a natural one, changes the material's properties in different ways and must be assessed case by case.
Natural fibres: not just straw
Fibre reinforcement of raw earth — traditionally straw in adobe, but also sisal, jute, hemp or animal hair in other traditions — works through a different mechanism from chemical stabilisation: fibres distributed through the matrix limit the propagation of shrinkage cracks during drying and improve the composite's tensile strength, conceptually much as steel or polymer fibres reinforce certain special concretes today. Contemporary research is systematically characterising less traditional fibres — industrial hemp, coconut, even fibres recycled from textile waste — to optimise dosage according to soil type and climate, a characterisation effort that empirical tradition had already sensed but never quantified with rigorous statistical methods.
In site practice, quantities matter more than the type of fibre. In mud bricks straw is usually added in modest proportions, of the order of one or two per cent by weight, cut into pieces a few centimetres long: enough to counter shrinkage cracks, not so much as to create voids that weaken the block. Plasters use shorter, finer fibres so as not to spoil the finish. Fibres must be dry and clean, free of mould, and well distributed in the mix, because clumps of straw become weak points. In compacted techniques such as rammed earth, by contrast, fibres are used little or not at all: there it is compaction that ensures strength, and straw would make the wall less dense.
Enzymes and biopolymers: the laboratory frontier
The most recent frontier of raw-earth stabilisation research explores enzymatic and biopolymer stabilisers that promise mechanical strength comparable to cement stabilisation, but with a much smaller carbon footprint. Enzymatic stabilisers work by catalysing calcium carbonate precipitation reactions within the soil matrix (a process conceptually similar to the bacterial biocementation also studied for self-healing concrete), while biopolymers — derived from starch, xanthan or other microbially derived polysaccharides — form a network that binds earth particles in a way analogous, at a different molecular scale, to what nopal mucilage did empirically for centuries. As of the mid-2020s, these systems remain mostly at the research and laboratory or prototype-testing stage, not yet in commercial production as widespread as lime- or cement-stabilised CEB — but they represent the most promising direction for further reducing the one non-negligible environmental-impact item of conventionally stabilised raw earth.
Among these systems, the most studied is enzyme- or bacteria-induced calcium carbonate precipitation. In the enzymatic version, the urease enzyme breaks down urea in the solution and creates the chemical conditions for calcium to precipitate as carbonate between the soil grains, cementing them together. The result can be remarkable, but there is a by-product to manage: the reaction releases ammonia, which must be controlled and disposed of. This line of research, documented in scientific publications, should also be distinguished from the commercial products sold generically as "enzyme stabilisers" for unpaved roads, whose effectiveness on building soils is highly variable and rarely documented independently. Before adopting them, it is worth asking for laboratory tests on the specific soil of the site.
The practical choice: it depends on climate and use
None of these systems is universally superior to the others — the choice depends on climatic context, budget and the level of performance required. In a climate with moderate rainfall and for non-structural elements (plasters, finishes), natural fibres and bio-based stabilisers can be enough to guarantee acceptable durability with almost no environmental impact. In areas with intense rainfall or for load-bearing structural elements, lime stabilisation today remains the most balanced choice among performance, cost, availability and environmental compatibility — Portland cement, while offering the highest strengths, should remain the last option, reserved for cases where the others fail to meet the required performance, not the default first choice.
A practical rule helps in choosing between lime and cement when one of them really is needed. Lime reacts with the clay minerals, in a slow reaction called pozzolanic, and therefore works best in clayey soils, which it makes less plastic and less sensitive to water. Cement instead forms its own hardened skeleton around the grains and works best in sandy soils poor in clay. Using cement in a very clayey soil often gives disappointing results; using lime in a sandy soil achieves little. The choice of binder therefore depends first of all on the soil available.
Testing the soil before building
Many decisions can be made with simple tests carried out on site. The jar test consists of putting a soil sample in a clear jar with water, shaking it and leaving it to settle: within a few hours the materials settle in layers, gravel and sand at the bottom, then silt and clay, giving a first idea of the composition. The shrinkage box test measures how much a strip of damp soil shortens when left to dry in a wooden box about sixty centimetres long: high shrinkage indicates too much clay. The cigar test, finally, involves rolling a cylinder of damp soil by hand and pushing it over the edge of a table: the length at which it breaks gives an indication of cohesion. None of these tests replaces laboratory analysis for an important building, but they help in choosing the soil and understanding which way to correct it.
For formal verification, some national standards now exist. New Zealand published a series of standards on earth building at the end of the 1990s; Germany has had specific standards for earth blocks, mortars and plasters since 2013 (DIN 18945, 18946 and 18947); Spain has a standard for compressed earth blocks (UNE 41410). Italy still lacks a comprehensive national standard, and projects rely on these foreign standards, on the technical literature and on laboratory tests. It is a gap that weighs above all on public works, where the lack of normative references makes it difficult to specify earth in a tender.
Stabilise only where needed
There is a decisive argument often missing from the discussion: unstabilised earth can be reused indefinitely. A demolished earth wall can be wetted, remixed and laid again, or returned to the ground with no impact at all. Adding cement, or bitumen, breaks this cycle: the material becomes construction and demolition waste like any other. That is why many experienced designers prefer to stabilise only the parts that really need it, such as the plinth, the lowest courses most exposed to splashing water, or the external render on façades exposed to driving rain, leaving everything else natural. It is a way of thinking that combines durability and circularity: the wall is protected where it is vulnerable, without giving up the material's most valuable property.