Plant fibres can increase compressed earth blocks’ strength by 12%, a study by the University of Portsmouth has found.
Researchers at the School of Civil Engineering and Surveying examined the fresh and hardened properties of three raw earth mortar mixes to determine the extent to which plant fibres from agricultural waste could replace the cement content of raw earth mortar to create an effective, more sustainable binding material.
Although raw earth mortar has a low environmental impact due to the availability and accessibility of earth, it requires carbon-intensive production methods when cement is used in the raw earth mortar mix.
The researchers used a variety of testing techniques, including compression testing, numerical modelling, finite element analysis and statistical analysis. They followed, as far as possible, British Standards (BS), Eurocodes (EN) and criteria set by the American Society for Testing Materials (ASTM).
They found that incorporating plant fibres in the raw earth mortar mix can prevent shrinkage, reduce cracking and improve strength. Compared with stabilised earth mortar alone, adding 20mm jute fibres at a dosage of 0.25% of the dry soil’s weight improved the 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. There was also a 20% increase in the ability to bend or fold from 1.20 MPa to 1.44 MPa.
Demand for sustainable building materials
Dr Muhammad Ali, associate professor in materials and environmental innovation at the University of Portsmouth, 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 cool as needed.
The two materials are also sustainable and give the finished structures a pleasing, natural appearance.
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.”
