Rheological behavior of alkali-activated slag

© 2014 Dr. Alireza KashaniPortland cement based concretes are the most used construction materials worldwide. However, in recent years some new systems have been developed to reduce the cement content of concrete by replacement of cement with other materials, or even introduction of cement-free concretes. This has been motivated by the high energy consumption of Portland cement production, landscape alteration, and high greenhouse gas emissions. Any cement-replacement material must provide satisfactory mechanical properties, durability, and rheological properties. One such option is alkali-activated slag (AAS), which has shown comparable performance while offering major reductions in CO2 emissions compared to Portland cement. However, AAS provides some processing challenges regarding its rheological properties, and the workability is considerably lower at the same water to solids mass ratio compared to Portland cement. Also, water reducing admixtures which are normally used in Portland cement appear to have little or no plasticizing performance in AAS. This investigation focuses primarily on the interrelationship between rheological behavior and AAS chemistry. High pH and electrolyte concentration due to addition of alkali activators affect reaction rate and particle surface charges, which increase interparticle forces, causing yield stress and apparent viscosity changes. Understanding of this chemical environment has resulted in the design of effective polycarboxylate ethers (PCE) as plasticizers for AAS. This was achieved by manipulation of molecular structure of different PCEs and analysis of the plasticizing performance of each synthesized polymer in AAS paste yield stress reduction. In addition to chemistry, the physical properties of precursors are also able to affect rheological behavior. For example, particle size distribution (PSD) has been shown to have a considerable impact on yield stress. A broader particle size distribution provides a higher packing density of particles which reduces the water volume required to fill the voids. The excess water is then used to disperse particles and reduces inter-particle forces, hence yield stress. A very low yield stress can be approached if the PSD is broad enough, in the absence of any rheology-modifying admixtures. Here, a model is proposed to predict the yield stress of blended cement systems with slag and fly ash from particle size distribution data. Although the time-resolved yield stress measurement of a reacting AAS system is of great practical importance, current rheometry methods have limitations in continuous yield stress measurement of an evolving binder because of the conflict between structural network bond formation at rest and disruption of structure formation by shear. Here, an effective method of measuring time-resolved yield stress is developed based on a creeping sphere geometry by minimizing any effects due to residual shear forces. The force required to pull a solid sphere at a very low velocity is used to calculate yield stress using analytical solutions for local flow of a creeping sphere in yield stress materials. The method presented here is capable of measuring time-dependent yield stress changes in reacting or evolving materials for a long time period in a single run, without interfering with ongoing structural network bond formation