Microbially Indurated Rammed Earth: A Long Awaited Next Phase of Earthen Architecture

Rammed earth possesses low embodied energy, high recyclability, and low toxicity while having little impact on biodiversity and virtually no depletion of biological nutrients. Although rammed earth is an inherently sustainable building material, it fails to meet the high compressive strength requirements of contemporary building standards. Attempts to rectify this shortcoming by importing advances from concrete construction have resulted in a degradation of its sustainable properties. Inspired by biomimicry, we propose to stabilize rammed earth using biomineralization through a process we are calling microbially indurated rammed earth (MIRE). This process offers the opportunity for earthen architecture to harmoniously reconnect to the natural world while simultaneously meeting contemporary performance demands. The microorganism, Sporosarcina pasteurii, known to effectively induce calcite precipitation, was suspended in a solution containing calcium chloride (CaCl2) and urea. The CaCl2 is the calcium source for calcite precipitation and the urea is used as a nitrogen fertilizer to accelerate microbial growth. This microbial solution is mixed into a base soil at sufficient quantities to approximate the optimum moisture content, thereby achieving maximum bulk density of the soil material through a standard compaction process, which is common in rammed earth construction. The hyperactive ~1 m long microorganisms are dispersed throughout the densified soil matrix and rapidly begin to modify solution chemistry to induce calcite precipitation at grain-to-grain contacts, cementing the material. In this experiment, MIRE cylinders achieved compressive strengths exceeding 2.5 times the strength of non-stabilized rammed earth. While this result is promising, related literature suggests that resistance to moisture degradation may also be substantially improved. This is significant because compressive strength and moisture resistance are the two greatest challenges in rammed earth construction. While much work is needed in order for MIRE to be a feasible alternative to cement-stabilized rammed earth, these preliminary results suggest much promise.