Waste baby diapers constitute a significant part of hygiene waste and currently are primarily landfilled or incinerated. The present study explores the applicability of shredded waste diapers (SWDs) as an innovative viscosity modifying admixture for cement grouts and concrete. A model was proposed which formulates the chemicals that SWDs add to concrete. The model was combined with the building and environmental standards to present the legal framework about using SWDs in different types of concrete. Cement grouts and self-consolidating concrete were designed to validate the viscosity modifying properties of SWDs. A Bingham viscosity model and flow tests were used to evaluate the viscosity modifying performance of SWDs. The present results show that the SWDs are a sustainable route to manufacture highly-effective viscosity-modifying admixtures for concrete.