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The weak bond between the shives and lime binder results in low mechanical strength of hemp concrete used as material filling for frame walls. The composite’s strength decreases due to its saturation with water. To prevent these effects, there is a need to modify the composite with an admixture showing binding and waterproofing properties. Because there is an identified gap in the knowledge of the influence of acid casein on properties of hemp concrete, the objective of this study was to verify the effect of this admixture in different amounts (1%, 3%, and 5% by weight) on hemp concrete properties. Density, porosity, thermal conductivity, sorption moisture, water vapor permeability, compressive strength (also on samples soaked in water), and Young’s modulus were examined. The effect of casein on vapor permeability turned out to be insignificant due to the fact that porosity also changes minimally. For 3% and 5% of casein, there is no visible effect on the thermal conductivity, which only slightly increased, as an effect of increasing the density and decreasing the porosity of the composites. Only a 1% admixture reduced thermal conductivity by 1%–8% at all relative humidity (RH) levels, which may be related to its lower density and highest porosity. Each amount of casein increased the ability of the composite to store sorption moisture at all RH levels, but the greatest difference in relation to the reference composite (by 42%–60% for sorption and 60%–100% for desorption), due to the increase in porosity, was shown by the composite with 1% of casein. Dissolved casein in the amount of 5%, due to its binding properties, improved the bonding of the binder with the shives. As a result, composites with this amount of casein showed higher compressive strength than the reference one in each condition (initial, saturated, dried after saturation). The use of casein (3% and 5%) allowed for the reduction of the amount of mixing water due to the liquefying properties of biopolymer.
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