Dispersing agents are widely used in paste processing to preserve good rheological properties while increasing the solid content in blends. These chemicals are better known as superplasticizers in the cement industry. They are used to maintain the workability of fresh cement pastes having high particle volume fractions. This improvement enables to formulate concretes with lower carbon impacts by incorporating mineral additions such as industrial by-products (fly ashes, slag, silica fumes…), but also with higher mechanical strength and durability. Despite dispersing agents are commonly used, the way they modify colloidal interactions and their impact on rheological properties are not yet fully understood. Such understanding requires to work with model systems which have properties that can be finely controlled [1]. In this work, we experimentally study the influence of a non-ionic surfactant, which mimic superplasticizers, on both the yield stress and the shear elastic modulus of concentrated suspensions of silica beads and crushed quartz grains dispersed in an ionic solution of calcium salt. By combining rheometry, Total Organic Carbon, zeta potential, confocal microscopy and laser tweezers experiments, we show that variations of rheological properties scale very well with the surfactant adsorption on particles. Two regimes are then observed: one where the surfactant coverage is low and both the yield stress and the shear elastic modulus increase due to hydrophobic interactions; and another one where the surfactant coverage is high enough to induce steric repulsion which leads to a drop in rheological properties. Both regimes are currently assessed with laser tweezers to investigate how the surfactant modifies contact properties between particles [2],[3].