Influence of viscosity modifier addition methods on the rheological behaviour of alkali-activated slag systems

Alkali-activated materials (AAMs) are emerging as sustainable alternatives to traditional binders in the construction industry. While the compatibility of various admixtures, such as superplasticizers, water reducers, and retarders, has been studied in one-part AAMs, a significant gap exists in understanding the effects of viscosity-modifying admixtures (VMAs). This study investigates the compatibility and impact of incorporating VMAs into one-part alkali-activated slag (AAS) paste, as well as the associated environmental impacts of using VMAs.Two factors were considered: the effect of different addition of VMAs methods (adding VMA separately, undissolved in water, or dissolved in water) and timings (early addition, 1 min after dry mixing the powder ingredients, and delayed addition, 25 min after mixing and resting). Initial flow diameter, flow retention, setting time, rheological properties (yield stress and plastic viscosity), heat of hydration, compressive strength (at 3, 7, and 28 days), and drying shrinkage were evaluated. Findings showed that delayed and separate addition of VMA significantly prolonged setting times due to slower distribution within the paste, regardless of the water-to-binder (w/b) ratio. Increasing the w/b ratio diluted the paste and reduced the significance of the method and timing of VMA addition on all measured properties. This research provides critical insights into the strategic use of VMAs to enhance the control of the rheological performance of AAS systems, contributing to the advancement of sustainable construction materials. • Adding VMA separately or combined with mixing water will impact AAS rheological properties differently. • Separate addition of VMA than mixing water significantly prolonged setting times • The timing of VMA addition greatly influences the rheological properties of one-part AAS. • Adding VMA reduces the drying shrinkage of one-part AAS. • Minor influence of timing and adding method of VMA at high w/c mixtures.