Metakaolin-based geopolymer: Formation of new phases influencing the setting time with the use of additives.

Graphical abstract Highlights • Setting time of geopolymer samples has been controlled with the use of additives. • The addition of boron-based compound can decrease the pH value of geopolymer sample. • Boron coordination seems to be the key parameter to control the geopolymer pH value. • The phosphate species perturb the polycondensation reactions. Abstract The aim of this investigation was the development of a geopolymer grout. To achieve this goal, it is necessary to control the characteristics of the reactive mixture (i.e., between the dissolution of the mineral source and the consolidation of the binder) and its setting time. The targeted application (geological radioactive waste disposal) requires a grout with a setting time greater than 24 h, a viscosity in the range from 1 to 5 Pa·s and a moderate pH value between 10 and 11. To reach these values, a metakaolin-based geopolymer was used with the addition of mineral additives. The viscosity measurements reveal that a focused viscosity is reached and that the setting time can be easily controlled and extended above 24 h with the addition of 2.8% weight of acids (boric and phosphoric) or 10% weight of borax. The acids reduce the reactivity of the mixtures by decreasing the alkalinity and inducing slow silica/alumina dissolution. The borax extends the setting time by increasing the water content and by modifying the polycondensation reactions to form huge structural entities. The decrease in the pH value to the target (10–11) can only be reached using 10% weight of borax or 7.5% weight of lithium tetraborate due to the insertion of B[IV] in the geopolymer network. [ABSTRACT FROM AUTHOR]