Retardation and compressive strength enhancement effect of upcycling waste carrot as bio-admixture for cement mortars

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The use of bio-admixtures instead of traditional chemical admixtures to prepare green concrete has attracted immense attention. In this study, a new and clean method to upcycle waste carrot into a novel natural admixture for cement mortars was proposed, and the effect of carrot extract (CS) on the properties of cement-based materials was systematically studied at the macro and micro levels through multiple means of detection. The results revealed that CS effectively inhibited the early hydration of cement and prolonged its setting time. The setting time of the pastes containing CS was 1.1–2.7 times that recorded for the control group (without CS) (CON), and the retarding effect was positively correlated with CS concentration. The compressive strength of the mortar of the M-CS1 (the value of CS to water was 1:100) group was increased by 12.5%, 11.6%, and 4.5% at 3-, 7-, and 28 d, respectively,which was due to the fact that CS accelerated the generation of calcium hydroxide (CH) and calcium silicate hydrate(C–S–H). In summary, CS can be used as a retarder and compressive strength enhancer in concrete simultaneously, and the experimental results in this study can provide experimental data and a basis for the preparation of green concrete. • The setting time of the pastes containing CS was 1.1–2.7 times that recorded for CON, and the addition of CS with appropriate concentration can also improve the compressive strength of the mortar. • CS contains a large number of hydrophilic group - hydroxyl, which has good water retention performance in cement. • The CS solution did not lead to the formation of new hydration products but promoted the formation of CH and CSH in the system, which has a positive effect on the compressive strength of mortars. • CS can improve the micro pore structure of mortar. • The use of low dose CS exhibited better sustainability and higher cost-effectiveness as confirmed by the economic and environmental analysis.