Rheology of natural hydraulic lime pastes modified by non-traditional biopolymeric admixtures

Abstract

Viscosity enhancing admixtures, widely used to improve characteristics of concrete and ready-mix mortars, are mainly different derivatives of cellulose. Due to the nature of cellulose processing, the environmental-friendlier alternatives should be studied in order to reduce the impact of the building industry on the environment. The rheological study of natural hydraulic lime (NHL) grouts modified by four different biopolymers is carried out to investigate their behaviour in the NHL-based mortars. The biopolymers studied are of seaweed (sodium salt of alginic acid (ALGNA) and carrageenan (CG)) and microbial (diutan gum (DG) and xanthan gum (XG)) origin. The effect of addition of these admixtures in the doses of 0.1%, 0.5%, and 1% was studied using hybrid rheometer with DIN concentric cylinders geometry. The flow properties as well as viscoelastic properties were studied. The addition of any of the admixtures led to the increase in yield stress, with DG being the most effective admixture. Desirable increase in consistency coefficient was observed within the pastes with CG and DG addition having growing dosage dependency, the ALGNA addition also increased the coefficient noticeably, but it was furtherly decreased with growing dose of admixture. The fluidity index lower than 1 expressed shear-thinning behaviour of studied pastes, except the pastes with highest dose of admixtures, and all of the XG pastes. The addition of CG and DG supported the stability of the grout expressed as the increase in critical strain, thus prolongation of linear viscoelastic region. The flow strain was increased by all of the studied admixtures promoting the gel-like behaviour of the pastes. Complex modulus and viscosity measured at 1Hz frequency were unaffected by the DG addition while they were increased notably by addition of other admixtures with ALGNA and XG supporting the resistance to deformation of the grouts studied. Correspondingly to complex modulus increase, the loss tangent is diminished, reporting more elastic behaviour of the material. All of the admixtures studied increased the yield stress, and the influence of most of them had similar trends within other properties. Noticeable differences in efficiency and dosage-dependency were observed. The xanthan gum was overall the worst performing admixture. This was mainly due to higher sensitivity of xanthan to the concentration of bivalent ions in the solution.