Waste plant fibers could reduce construction costs and improve sustainability

Earth mortar could be more resilient, flexible and stronger by adding plant fibers to the mix. The finding comes from research to better understand the strength and performance of raw earth mortar, which has been carried out by the University of Portsmouth’s School of Civil Engineering and Surveying. The work is published in the Journal of Building Engineering.

Dr. Muhammad Ali, Associate Professor in Materials and Environmental Innovation at the University, said, “The focus of research in construction using earth has increased in response to demand for low-cost and sustainable building materials. However, while the properties of compressed earth blocks have been widely investigated, there has been less attention given to the make-up of earth-based mortar—a rudimentary mix of water with natural earth consisting of sand, silt and clay.”

Building using materials where raw earth is the fundamental component is typically seen in middle and low income countries around the world. Earth-based mortars are widely used to bind together compressed earth blocks mainly because the materials are structurally compatible, breathable and, when combined, help interiors stay warm or keep cool as needed. The two materials are also sustainable and have the appeal of giving the finished structures a pleasing, natural appearance.

Raw earth mortar has a low environmental impact due to the availability and accessibility of earth—an abundant natural resource that can be locally sourced—and the minimal, simpler processing methods needed to produce it. But, when cement is used in the raw earth mortar mix, energy intensive production methods are required, and this has an adverse environmental impact.

The fresh and hardened properties of three raw earth mortar mixes were examined to determine the extent to which plant fibers from agricultural waste could replace the cement content of raw earth mortar to create an effective, more sustainable binding material.

The researchers used a variety of testing techniques, including compression testing, numerical modeling, finite element analysis and statistical analysis, working as far as possible to British Standards (BS), Eurocodes (EN) and criteria set by the American Society for Testing Materials (ASTM).

According to the German Institute for Standardisation (DIN), load bearing structures bound by raw earth mortar must be able to cope with the pressure of weight equivalent to 2.00MPa (megapascal—a unit of pressure) or more and a shear strength (the amount of force it can bear before sliding apart) of 0.04MPa.

It was found that incorporating plant fibers in the raw earth mortar mix can prevent shrinkage, reduce cracking, and improve strength. When compared to stabilized earth mortar alone, adding 20 mm jute fibers at a dosage of 0.25% of the dry soil’s weight improved the raw earth mortar’s mechanical properties.

The result was a 12% increase in compressive strength—meaning the amount of pressure the mortar can withstand without fracturing or deforming went from 5.64 megapascals (MPa) to 6.31 MPa, and there was a 20% increase in flexural (shear) strength—i.e. the ability to bend or fold—from 1.20 MPa to 1.44 MPa.

Dr. Ali added, “Through this research we have been able to identify potential improvements for raw earth mortar to help further develop low-cost and environmentally friendly building materials around the world and support the creation of sustainable communities.

“It also aligns with the UN Sustainable Development Goals, which includes a target to ensure everyone has access to adequate, safe and affordable housing and basic services by 2030.”