Microstructure and moisture transport in carbonated cement-based materials incorporating cellulose nanofibrils

Carbonation of cement-based materials may lead to durability problems, so that it is necessary to find a way to reduce the effect of carbonation on concrete durability. This study investigated the effects of cellulose nanofibrils (CNFs) on the durability of carbonated cement-based materials. Two dosages of CNFs were used to prepare cement pastes and mortars. Before and after accelerated carbonation (4 % CO2 concentration and 57 % relative humidity), their microstructure, mineralogical composition, moisture retention capacity, drying kinetics, and water absorption were measured. Results show that the contents of hydration products (CH, C-S-H, and ettringite) slightly decrease with the increase of CNFs dosage. After carbonation, the carbonation ratios of hydration products decrease with CNFs dosage so that more hydration products remain in the carbonated materials. The calcite content was found to increase with the dosage of CNFs which may help the transformation of other metastable calcium carbonates to calcite. The coarsening effect of carbonation on pore structure was clearly observed in measured pore size distribution for all materials, while our results show that this effect is weakened by CNFs. Moisture transport is clearly accelerated by carbonation, but the acceleration rate is diminished with the increasing dosage of CNFs, suggesting that CNFs are able to reduce the microstructural damage by carbonation.