Prospective retarders for alkali-activated slag: stability at high alkalinity, effects on setting behavior and strength of paste

Alternate binders are becoming essential to counteract the carbon emissions caused by manufacture of ordinary Portland cements. Geopolymers or alkali-activated systems with precursors as fly ash, slag or metakaolin, or a mixture of these, are possible substitutes. There are several hindrances to their use in construction due to the lack of understanding and control of properties of the system, starting from initial product formation rate all the way to long-term durability. Ordinary Portland cement systems have been around for a much longer time in history and so have been admixtures that can bring about desired changes in the properties of the material. Similar admixtures for the alternative binders are much less well-established. Through this study, the author hopes to contribute to this field of admixtures for the alternate binders. The scope was narrowed down to one of the major challenges of control of fresh properties. The current investigation looks at an alkali-activated slag system, which has a short, or fast, hardening time. This is not conducive to its use in mainstream construction and a control of the setting or hardening time is essential. Several retarders have been conventionally used for this purpose in the ordinary Portland cement systems. This investigation hopes to study the effectiveness of these retarders in the significantly different alkali-activated system, while also exploring novel retarders. Conventional admixtures used here were organic in nature such as lignosulfonate-based, polycarboxylate-based and naphthalene formaldehyde-based. Novel retarders were a mixture of inorganic and organic substances selected to achieve specific effects (adsorption, complexation, seeding, etc.) that might affect reaction rate. These novel retarders were, namely, dodecyltrimethylammonium chloride (DTAC), poly(diallyldimethylammonium chloride) (PDADMAC), ethylenediaminetetraacetic acid (EDTA) and zinc oxide nanoparticles. Isothermal calorimetry was the primary test conducted to observe effects on the reaction rate. Stability in the high pH environment was assessed visually and also chemically through infrared spectroscopy. Compressive strength tests were conducted for the novel admixtures and the hardened product was analyzed through infrared spectroscopy. Additional tests such as zeta-potential measurements, adsorption measurements through UV-Vis-NIR spectroscopy and ultrasonic wave reflection (for hardening) were performed, but quality data was hard to obtain. The results from these tests are not part of the main investigation and have been presented separately in the Appendix. Most conventional organic admixtures were found to be unstable in the high pH (>14) alkali solution. Those that were stable did not retard the reaction when added to paste. The novel admixtures were stable in the high pH environment and retarded the reaction. Some admixtures also had unexpected consequences, unrelated to retardation. The results of this investigation may serve towards the foundation of admixtures for alkali-activated systems and facilitate the exploration or artificial synthesis of such admixtures