Hygric Behavior of Cement Composites Elaborated with Flax Shives, a Byproduct of the Linen Industry

The reuse of flax shives, an agricultural waste, in the elaboration of cement composites offers an interesting alternative to meet the challenge of their elimination and solve an environmental problem. The composites studied are developed by adding flax shives to Portland cement and water. Due to their honeycomb structure the flax shives are used as lightweight aggregates and provide insulating properties to the composites. However they are hygroscopic and can release water-soluble molecules responsible for setting retardation, large dimensional variations and low mechanical strengths. In previous works these drawbacks have been reduced by the development of coating processes using lime, linseed oil and paraffin as coating substances. When coated shives are used the composites exhibited compressive strength 8 to 27-fold higher, drying shrinkage two to threefold lower compared to the raw shives composite, and belong to the insulating class of the lightweight concrete. The thermal conductivity of the composites may be negatively affected by environment humidity, so their hygric behavior must be determined. The establishment of the sorption isotherms and the evaluation of the water vapor transport allow the assessment of the hygric behavior. The treatment of the flax shives involves a decrease in the moisture adsorption capacity of the composites up to 48% for a paraffin coating. The composites can be ranked in descending order of hygroscopicity: raw, lime-coated, linseed oil-coated and paraffin-coated shive composites. Only the water vapor permeability and resistance factor of the paraffin-coated shive composite are significantly lower (46%) and higher (73%) compared to those of the raw shive composite. The analytical models (GAB, Janssen and Roels) adjust the experimental results and the influence of flax shives treatment on the composites hygric behavior is demonstrated. Graphic