Cold sintering of sand-evaporite mixtures for sustainable brick applications in hot, arid climates: A laboratory upscaling study

In the Sahara Desert, various historical sites were built using vernacular bricks made from sand-gypsum (sand roses) and sand-halite (karshif) mixtures, and they have survived for centuries in the hot, arid climate. Cold sintering has been effective in densifying gypsum and halite at room temperature, producing small-scale compacts with high unconfined compressive strength (UCS). This study explores the potential for laboratory upscaling of the cold sintering process for gypsum-sand and sand-halite mixtures, aiming to optimize parameters that result in adequate UCS values. The effects of evaporite type, pressure duration, drying duration, and drying relative humidity to on the UCS of cold-sintered samples were studied. The results showed that, at the scale of a 2-in. cube, applying a uniaxial pressure of 50 MPa to equal amounts of evaporite minerals (dihydrate gypsum or halite) and sand powders for 5 min, followed by drying for one day at 50 °C and 40 % relative humidity, is adequate to produce cold-sintered compacts with UCS values surpassing American Society for Testing and Materials (ASTM) standard for load-bearing masonry bricks (13.8 MPa). Low drying relative humidity levels (20–40 %) played a key role in improving UCS values. However, as the humidity approaches the deliquescence point of evaporites, the UCS values drop significantly due to weakening caused by recrystallization pressure. These findings confirm that, under controlled laboratory conditions, cold sintering of sand–evaporite mixtures can yield sustainable brick-quality compacts, thereby providing a foundation for future engineering efforts toward full industrial-scale implementation.