Alireza Kashani

Effect of molecular architecture of polycarboxylate ethers on plasticizing performance in alkali-activated slag paste

A range of anionic and cationic polycarboxylate ether (PCE) plasticizers with different molecular architectures (molecular weights, side chain lengths, and ratios of side chain density to backbone charge) are synthesized and tested to determine their effects on the rheological properties of fresh alkali-activated slag (AAS) pastes. A higher density of long side chains in the lower molecular weight polymers can provide a noticeable yield stress reduction, indicating a mild increase in workability compared to that of an unmodified AAS paste. It is hypothesized that side chains may have two important roles, i.e., providing steric hindrance to disperse particles after PCE adsorption on a particle surface, and also providing partial protection of the backbone charges against attachment of one PCE molecule to two or more slag particles, which is called bridging. This enhances the likelihood of adsorption on single particles, and thus increases the plasticizing action. A very similar plasticizing mechanism is observed for PCEs with similar structures but differing charge signs (cationic/anionic), which indicates that both anionic and cationic adsorption sites are available on AAS particle surfaces. The measured flow curves of all pastes are well described by the Herschel–Bulkley model with shear thinning behavior.

Effect of ground granulated blast furnace slag particle size distribution on paste rheology: A preliminary model

Ground granulated blast furnace slag is widely combined with Portland cement as a supplementary material, and is also used in alkali-activated binders (geopolymers) and in supersulfated cements, which are potential replacements for Portland cement with significantly reduced carbon dioxide emissions. The rheology of a cementitious material is important in terms of its influence on workability, especially in self leveling concretes. The current research investigates the effects of different particle size distributions of slag particles on paste rheology. Rheological measurements results show a direct relationship between the modal particle size and the yield stress of the paste. An empirical model is introduced to calculate the yield stress value of each paste based on the particle size distribution, and applied to a range of systems at single water to solids ratio. The model gives a very good match with the experimental data.

Time-resolved yield stress measurement of evolving materials using a creeping sphere

Physicochemical phenomena influenced by aging or reaction can result in rheological changes across several orders of magnitude, but the classical rheometry methods available for analysis of concentrated suspensions can face challenges in correctly measuring the yield stress of aging/reacting (evolving) materials and need some precautions to enable precise measurement of the evolution of the yield stress with time. Here, a creeping sphere method has been applied to measure time-resolved yield stress; the force required to pull a solid sphere at very low velocity is used to calculate yield stress using previous analytical solutions for local flow of a creeping sphere in yield stress materials. The measured yield stress values agree well with the data recorded using vane-in-cup geometry for time-independent measurements using Carbopol gel. The creeping sphere is less affected by shear history because of the constantly changing shear region and therefore measures yield stress changes in evolving materials such as cement for a long time period in a single run, without altering ongoing structural network bond formation.

Effects of Different Polycarboxylate Ether Structures on the Rheology of Alkali-Activated Slag Binders

Polycarboxylate ether (PCE) admixtures are generally used in concrete at relatively low concentrations, enabling the reduction of mixture cost and enhanced flow properties due to reduction of cement content and significant enhancement of rheology, respectively. However, typical PCE polymer structures that are used in Portland cement have little or no effect on alkali activated slag (AAS) binder rheology due to ineffective consumption of polymer by a number of mechanisms, including degradation of the polymer chains within the high alkaline environments present in AAS systems. In this study, a range of PCEs with long and moderate PolyEthyleneGlycol (PEG) side chain lengths, and with high and low molecular weights (M), are examined. Co-polymers containing a higher density of backbone charges, as is typical for a Portland cement superplasticiser, increase the yield stress of alkali-activated slag. A co-polymer with longer side chains and lower M show a yield stress reduction, indicating a mild increase in workability compared to an unmodified AAS paste. It is suggested that in the high ionic strength environment of an AAS binder, a more charged polymer is consumed through interactions with other ions and charged particles, which can bring an increase in yield stress and plastic viscosity of AAS.