Potential of starch as bio-admixture for cementitious composites

Abstract

This study was inspired by the need to find an environmentally friendly bio-admixture. In recent times, chemical admixtures are being used indiscriminately to obtain high performance concrete. This leads to release of toxic chemicals such as formaldehyde to environment especially after demolition. Modern construction materials are opting towards green technology. As a part of that, search for alternative renewable bio-admixture is on going. In this study, the effects of starch admixture on mortar and concrete were investigated. Two water soluble starches, laboratory starch and arrowroot, were used as bio-admixture. Starches were added to the proportion of 0.5%, 1%, 1.5% and 2.5% to cement weight. The experimental programs were set up to investigate properties of cement, starch, fresh mix of mortar and concrete and hardened mortar and concrete. Special attention was directed towards durability aspects of concrete. Durability parameters were assessed in terms of permeability of mature concrete. A closer look to the microstructure of mortar via SEM imaging of specimens without and with starch admixture aided the explanation of behavior of starch in cement and mortar matrix. The initial and final setting time was increased by 62.50% and 64.71% respectively with respect to that of control sample for the addition of laboratory starch between 1% and 1.5%. Presence of starch admixture reduced the surface tension on cement particles and increased the workability of fresh mix for both mortar and concrete. A better workability led to a better mixing of the constituents that resulted into reduced permeability. Permeability parameters of hardened concrete were reduced by 36.46%, 25.28% and 42.02% with respect to control specimen as found from sorptivity, RCPT and RMT results respectively. This observation was based on the data obtained at 120 days of curing of concrete with 1% to 1.5% arrowroot addition. Addition of starch had no adverse effects on compressive strength of mortar and concrete. Overall, laboratory starch had greater retardation effect whereas arrowroot preformed better in durability aspects. This study recommends arrowroot as the alternative set retarding and viscosity modifying admixture. The optimum dose is suggested to be within 1% to 1.5% of cement content.