Thesis title: Effect of Fibrillated Cellulose in Mortars and Concrete
Cement is the most widely used building material and its production is responsible for a large share of greenhouse gas emissions. For this reason, several efforts have been directed towards the development of a «green concrete», e.g. blending or replacing the ordinary Portland cement with alternative binders or using eco-friendly additives to reduce the environmental impact of concrete, thus benefiting the sustainability. Natural fibres, commonly used during ancient times to reinforce and to reduce the shrinkage in mortars and concrete, are nowadays receiving a growing interest since they represent a renewable, economical and abundant resource. In particular, nanocellulose materials can enhance the mechanical properties of modern concrete due to their high aspect ratio and Young’s modulus. Moreover, due to their high hygroscopicity they can act as an internal curing agent of cement, preventing self-desiccation and promoting hydration. Their use as viscosity modifiers in self-compacting concrete (SCC) allows to stabilise the fresh concrete, inhibiting bleeding and segregation phenomena. The use of such natural fibres in concrete can also influence the composite porosity and its hygrothermal behaviour. Indeed, cellulose fibres enhance the moisture transfer and the storage capacities of mortars and have been classified as excellent hygric regulators.
The use of nanocelluloses in traditional lime-based mortars is a promising solution for green buildings in the frame of limiting the CO2 emissions resulting from Portland Cement production. This new material may also be evaluated for the restoration of historical mortars and plasters. Nevertheless, to our knowledge, no extensive study of the influence of nano- and micro-fibrillated cellulose materials on the hydration and on the mechanical properties of lime-based mortars has been performed so far. For this reason, the influence of the fibrillated cellulose (FC) on pastes and mortars based on cement was first studied in order to compare the performances of our composite with similar ones described in literature. In a second stage the FC was incorporated in lime-based pastes and mortars to investigate their compatibility with the lime matrix. The FC was added at dosages of 0%, 0.1%, 0.2% and 0.3%wt by weight of Portland cement or natural hydraulic lime. The pastes were subjected to thermal and nitrogen gas sorption analyses to understand if FC affects the formation of hydraulic compounds and the mesoporosities volume distribution. The setting and early hydration of the mortars were studied with isothermal calorimetry. The mechanical performances were investigated with compressive and three-point-bending tests. Furthermore, fragments resulting from the mechanical tests were microscopically studied to understand the reinforcement mechanism of the fibres. Finally, the hygrothermal performances of two selected mixes (one for the cement and one for the lime) were studied by investigating how the FC affect the water adsorption by capillary rise, the water vapour permeability, the sorption isotherms, the heat conductivity and the heat capacity. For this purpose, cement-based mortars with 0.1%wt FC and lime-based mortars with 0.3%wt FC were prepared.
The mechanical performances of cement-based mortars were minorly affected by our type of FC. As for the lime-based mortars, it was found that 0.3wt% of FC enhances the flexural and compressive strengths respectively by 57% and 44%, while the crack propagation after the material failure is not affected.
On the other hand, the use of FC influenced the composite hygrothermal behaviour for both types of binders. In particular, the capillary water absorption coefficient AC for cement-based and lime-based mortars was respectively reduced by 35% and 27% with FC. These findings lay the groundwork for additional studies about the employment of our material for retrofit solutions.