Fluvial barrage tufa as a natural lab for the improving of geotechnical engineering properties of sand by microbially induced calcium carbonate precipitation (MICP)

In the last two decades one innovative approach to decrease the harmful impact on the natural environment, is to use microorganisms to produce sustainable biomaterials, such as microbially induced Ca-carbonate precipitate (MICP). MICP has been studied for ground improvement to enhance shear strength and stiffness after introducing Ca-carbonate cementing agents into the pores, and for other applications including environmental remediation, production of construction material, improved durability and remediation of building materials, cations removal in wastewater, and carbon sequestration. However, even if MICP could represents a good alternative to the utilization of natural resources and to reduce CO2 and NO2 emission of industrial activities, the process produces an inevitable environmental impact as the waste and chemicals involved in lab activities. With the aim to develop a bio-cement for increasing the geotechnical properties of a common multimineral sand, a more sustainable process of MICP was tested introducing the sand in the water flow of a tufa-forming river with active microbial Ca-carbonate deposition. The tufa deposits are covered by a lithifying biofilm composed of a microbial community including autotrophic and heterotrophic bacteria, algae, viruses, and extracellular polymeric substances. Biominerals forming the tufa deposit replace the organic substrates starting with an amorphous phase rich in Ca, Si, and other cations, followed by massive precipitation of fibrous to polyhedral Ca-carbonate crystals and subordinately lamellar/fibrous Mg-clay crystals. During the 16 weeks of the experiment, the biofilm colonized rapidly all the surface of the sand grains, which were gradually encrusted by neoformed Ca-carbonate biominerals showing the same crystal structures and composition of those forming the tufa deposit. Moreover, the sand showed a progressive increase of the internal friction angle from 28.6° to 35.4°, with a trend very similar to MICP lab experiments that used a single bacterial species. This suggests that is possible to use natural Ca-carbonate biominerals, induced by natural microbial communities, for engineering applications with very sustainable procedures.