High strength biomineralization using synthetic and human urine

Abstract Biomineralization enables the production of carbon-neutral construction materials that are chemically similar to natural calcareous sandstone but are shaped using formworks. The aim of this study was to investigate whether specimens with high compressive strength could be produced using human urine instead of synthetic urea, the production of which is energy-intensive. Biomineralization experiments demonstrated that the relatively low urea concentrations present in human urine are not a limiting factor. However, the composition of synthetic urine—and even more so that of human urine—negatively affected the biomineralization process. Omitting sulfate compounds from the synthetic urine resulted in a slight improvement. To prevent urea decomposition, human urine was stabilized during collection and storage by adjusting the pH to either strongly acidic (pH 3 using aqueous HCl) or strongly alkaline conditions (pH >11.2 using solid Ca(OH) 2 or pH = 11.2 using aqueous NaOH). When the urine was stabilized with HCl or NaOH, phosphates were precipitated by adding MgCl 2 for nutrient recovery. Subsequent biomineralization experiments showed that the initial urine treatment influenced the compressive strength of the resulting specimens. The highest compressive strength, 63.3 MPa, was achieved using urine stabilized with Ca(OH) 2 and concentrated by heating at 50 °C.