Estonian clay could replace part of cement

Clay from Estonia could partially replace cement in some construction materials, according to tests by researchers from Tallinn University of Technology and the University of Tartu, ERR reported. Cement is the world’s second-most widely used material by mass after water, but its production releases large quantities of greenhouse gases.

The researchers, led by Andres Trikkel of Tallinn University of Technology, examined clay from the Arumetsa and Aseri quarries. They heated the material in a special furnace at 750, 825 and 900 degrees Celsius before mixing it with slaked lime and sand to measure the strength of the resulting test blocks.

All the blocks made with treated clay exceeded the strength limit set for such materials in the concrete industry. A material is considered successful if, after hardening for a week, it can withstand pressure of at least four megapascals. That is equivalent to carrying about 40 kilograms for every square centimetre.

Measurements showed that clay heated to 825 degrees Celsius produced the best results. Arumetsa and Aseri clay treated at that temperature withstood pressure of about seven megapascals. Arumetsa clay performed slightly better because it contains more kaolinite, a clay mineral that makes the material more chemically active.

Although clay-based mortar does not collapse under minimal stress, it remains considerably weaker than conventional cement. Even the foundation of an ordinary private house must withstand about four times greater pressure. The researchers therefore see clay as a partial replacement for cement rather than a complete substitute. Further tests are needed before engineers can rely on the material.

Heating disrupts clay’s regular crystal structure, releasing silicon and aluminium that react with lime to form a strong binding gel. Temperature is crucial: insufficient heat does not activate the clay enough, while heating it to 900 degrees or more causes new strong crystals to form, reduces the material’s surface area and eliminates its initial reactivity.

The findings suggest that Estonian clay, which has so far been used mainly to produce bricks, could eventually help the construction sector make more environmentally friendly concrete. The results were published in the Journal of Thermal Analysis and Calorimetry.