Insights into natural and carbonation curing of ancient Chinese rammed earth mixed with brown sugar

Abstract

Rammed earth materials are extensively used in the past to construct shelter, rampart or fort. The behaviour, in terms of strength and durability, of rammed earth varies from region to region. In this work, the response of rammed earth is studied with the mixtures of hydrated lime, clay, sand and water to understand the wall behaviour of Fujian Tulou. Unconfined compression tests and immersion tests are conducted on rammed earth specimens under the natural and carbonation curing conditions. To improve the workability of rammed earth, water-reducing admixture was often added anciently. The effect of brown sugar, as a traditional water-reducing admixture, on strength and water resistance of rammed earth is also tested. Finally, mineralogical and microstructural analyses are conducted to understand the mechanism. It is found that the strength and water resistance both increase when the carbonation curing is implemented with a higher content of CO2. The strength is positively correlated with the lime content, whereas the water resistance is negatively related to the clay content. Portlandite explains the strength gain of rammed earth cured in the air, which is further consumed and replaced by the formation of calcium carbonate. The use of extra brown sugar can be detrimental, given that calcium saccharate is formed to retard the carbonation reaction. It is suggested that a dosage of 0.5% can be used for brown sugar, without having any adverse effect.