Earth stabilisation by plant-derived urease enzyme for building applications

Abstract

The present work investigates the hygro-mechanical performance of compacted earth as an alternative to conventional energy-intensive building materials. Cement and lime have been widely employed as stabilisers to improve the strength and durability of compacted earth for building applications. Nevertheless, the use of these chemical binders partly compromises the energy efficiency of earthen materials while increasing their carbon footprint. This has recently led to the study of alternative stabilisation methods that are equally effective in improving the properties of earthen materials without however compromising their green credentials.
The present work adopts a recently proposed method for the manufacturing of earth bricks. The method is based on the application of high compaction pressures up to 100 MPa (hypercompaction) to increase the density of the earth and hence to obtain mechanical properties that are similar to those of traditional construction materials such as fired bricks, concrete blocks and stabilised earth. A wide campaign of laboratory tests was performed on samples made of different earth mixes that were hyper-compacted at their respective optimum water contents. Stiffness and strength were measured by unconfined and triaxial compression tests while vapour adsorption/desorption was assessed by measuring moisture buffering value (MBV). Durability to water erosion was also evaluated by performing suction, immersion and drip tests according to the norms DIN 18945 (2013) and NZS 4298 (1998), respectively.
Results showed that hyper-compaction largely improves the mechanical performance of compacted earth but that a marked increase in ambient humidity can produce a considerable reduction of strength. Compacted earth is also characterised by an excellent capacity of adsorbing/releasing ambient moisture, which increases the hygro-thermal inertia of the material. Nevertheless, durability tests highlighted that the unstabilised compacted earth cannot be employed for the construction of structures exposed to natural weathering. The experiments also demonstrated the dependency of strength, stiffness, moisture buffering capacity and water durability on particle grading. In particular, it was shown that a fine and well-graded earth mix exhibits higher levels of strength, stiffness, moisture buffering capacity and durability than a coarse and poorly-graded one. This suggests that careful selection of the soil is necessary to optimise the manufacture of earth bricks.
One important challenge lies in the improvement of the earth durability against water erosion by adopting novel stabilisation techniques which exhibit small environmental impacts while preserving the advantageous properties of compacted earth in terms of mechanical and moisture buffering behaviour. In this work, the exploitation of knowledge at the interface between physics, biology and chemistry has led to the development of an original stabilisation method based on the utilisation of plant extracts. The method is consistent with the principles of Enzymatic Induced Calcite Precipitation (EICP), which utilises the action of the urease enzyme to catalyse the hydrolysis of urea. This reaction produces carbonate ions, which then react with the calcium ions dissolved in the pore water to produce the precipitation of calcium carbonate (i.e. calcite), thus binding the soil together.
The urease enzyme is a widely occurring hexameric protein that is the product of the metabolism of microbes and is also found in the tissues of many plants. The novelty of the present work resides in the utilisation of crude plant-derived urease enzyme instead of pure reagent-grade products available from chemical suppliers, which reduces environmental and financial costs. In particular, the urease enzyme was obtained from a liquid soybeans extract, inside which the urea and calcium chloride were subsequently dissolved to induce the precipitation of calcite. A fundamental study of the relevant microbiological and biochemical processes pointed out that the concentrations of urea and calcium chloride play an important role in the activity of the urease enzyme and on the amount of precipitated calcite. Measurements of pH, electrical conductivity and precipitation ratio indicated that the optimum equimolar concentration of urea and calcium chloride (leading to the largest precipitation of calcite) is 2.5 mol/L.
An experimental campaign was finally undertaken to implement the proposed biostabilisation method into the manufacture of compressed earth bricks. The efficiency of the treatment was initially assessed by means of water immersion tests to quantify the improvement of the material water durability. The most promising versions of the proposed bio-stabilisation method were also the object of further investigation to assess the hygromechanical behaviour of the stabilised earth by means of unconfined compression and moisture buffering value tests. The findings, although preliminary, suggest that a noticeable improvement of strength and water durability can be achieved by the proposed stabilisation protocol, in spite of the difficulty in replicating exactly quantitative results. Further tests are still necessary to make the proposed treatment competitive with conventional stabilisation techniques based on the use of cement and lime.