Impact of Miscanthus Lignin and Arabinoxylan on Portland Cement

Miscanthus biomass can be used to produce lightweight concrete. However, cell wall polymers leached in the alkaline cementitious medium can disturb cement setting. This is the case for grass lignin and grass arabinoxylan due to their specific alkali solubility. The main objective of this paper was to study the impact of lignin and of arabinoxylan from miscanthus biomass on the hydration of Portland cement and by electrical conductivity. To this end, dioxan lignin (DL) and arabinoxylan (AX) were extracted from miscanthus by methods preserving the main structural specificities of the native polymers. These DL and AX fractions were added to Portland cement (1-5% w/w in cement) and their impact on the electrical conductivity of cement/water mixtures was time-monitored. The novelty of this study lies in using polymers structurally similar to those of miscanthus fibers rather than commercially available ones, such as kraft lignin (KL). The addition of DL or of KL to cement/water mixture differently affected the electrical conductivity, which is most likely assignable to the severe structural degradation of KL during kraft process. The conductivity curves suggested that cement hydration was substantially delayed when DL % in cement was 3% or more while lower values had no impact. The results support the hypothesis that the access of water to cement grains was impeded by the adsorption of ionized lignin entities at their surface. When co-added to the cement (1.6 wt% each), the DL and AX fraction delayed cement hydration more substantially than when the same amounts were separately added. This unexpected synergy suggests that the miscanthus lignin and arabinoxylan polymers form lignin-carbohydrate complexes efficiently adsorbed on cement grains.