Investigation on the replacement of lithium carbonate as accelerator for calcium aluminate cements and its underlying working mechanism

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For the acceleration of alumina cements and ternary binder systems, more than a thousand tons of lithium salts are taken each year from the supply chain and are irretrievably cemented e.g. into floorings. However, this precious element is needed in large quantities for the production of electric cars, resulting in a higher price and poor availability for the construction industry. Against this background, it is highly desirable to find an alternative for lithium in construction applications. Surprisingly, it was found in earlier research at the chair of construction chemistry relating to the intercalation behavior of microbially produced and natural biopolymers into Zn-Al-LDHs that alginates accelerate the hydration of alumina cement. This was completely unexpected as it is known that such biopolymers generally retard Portland cement. The accelerating effect is also very interesting because alginates represent a renewable resource with practically unlimited availability. Furthermore they are completely non-toxic as alginates are commonly used in the food industry (e.g. as thickener). As for the algae, different kinds can be differentiated, e.g. green algae, red algae and brown algae, which in turn are divided into different subclasses and species. Commercial alginates are obtained exclusively by extraction from brown algae (Phaeophyceae). Preliminary investigations conducted so far showed that addition of 0.1 M.% of a randomly selected commercial sodium alginate to an alumina cement containing 50 m.% Al2O3 reduced the setting time by half. It therefore seems to be promising to examine systematically selected alginate samples for their accelerating effect, to determine an optimal structural composition and to investigate the underlying mechanism of this effect. For this purpose, the accelerating effect is to be verified by various methods including heat flow calorimetry (detects the amount of heat released by the cement during hydration), ultrasound measurements (shows the development and densification of the microstructure based on the passage time of the sound wave through the medium) and mechanical strength measurements on mortars. To fully understand the mechanism of action of the alginate, also specific alginate derivations, an investigation of the effect on individual clinker phases such as CA and C12A7, an analysis of the influence of the alginate on the ion content in the cement pore solution and the nucleation and crystallization of the C-A-H phases will be studied. Finally, the effect of alginates on the ternary binder system OPC/CAC/AH will be investigated. This system produces fundamentally different hydrates compared to pure aluminate cement. In contrast to pure CAC, there no C-A-H phases are formed, but instead ettringite presents the main hydration product.