Effect of sodium gluconate addition on setting, hardening, and microstructure behaviour of hybrid alkaline mortar

In this investigation, the influence of adding 2 %, 4 %, 7 %, and 10 % sodium gluconate (SG) on the setting time of hybrid alkaline paste, and hardened and microstructure properties of hybrid alkaline mortar mixes were examined, and the results were compared with a control mix that made without addition of sodium gluconate. The hybrid alkaline paste and mortar mixes were prepared using a fixed proportion of OPC/fly ash (25 %/75 %), NaOH solution molarity of 9 M, and sodium silicate to hydroxide ratio of 1.5. The results of this investigation found that the quick setting and abrupt loss of flowability of the control mix were improved by adding sodium gluconate, however, more promising results were observed at 7 % and 10 % sodium gluconate. The delay in polymerization due to lower availability of calcium ions, and adsorption of carboxyl and hydroxyl groups associated with sodium gluconate on the unreacted particles of binding materials and earlier formed products assisted in prolonging the time required for hardening. Further, the compressive strength declined at a higher dosage of sodium gluconate, however, the mortar mixes made with 2 % and 4 % sodium gluconate exhibited higher compressive strength than the control mix. The peak intensity of albite, nepheline, and C-S-H gel decreased at higher dosages of sodium gluconate, which was supported by a weaker microstructure characterized by more pores, micro-cracks, and partially reacted fly ash particles. Furthermore, the lower bridging oxygen content and greater Si-O-Na/ Si-O-T ratio at higher dosage of sodium gluconate confirms the delayed polymerization process.