Effect of gluconate molecules on the reaction kinetics of C-(N)-A-S-H and N-A-S-H gel formation

Abstract

Organic ligands are widely used to regulate setting in cementitious binders, yet their chemistry-selective action in geopolymers (GPs) remains unclear. Here we isolate the role of sodium gluconate (SG) in early reaction kinetics across a Ca-rich GP that forms a calcium-(sodium)-aluminosilicate hydrate (C-(N)-A-S-H) gel and a Ca-poor GP that forms a sodium-aluminosilicate hydrate (N-A-S-H) gel. Four GPs spanning Ca/Si ≈ 1.01 to 0.01 were prepared with identical activator chemistry and dosages of SG up to 5 wt% of precursors. Setting time, isothermal calorimetry, pore-solution speciation (TOC, ICP-OES, and pH), selective dissolution (reaction extent), low-field ¹H NMR, XRD, FTIR, SEM-EDS, and Avrami analysis were combined. In the Ca-rich GP, SG delayed the main peak of heat flow and setting but increased the cumulated heat in the first 72 h, so increased reaction extent and compressive strength. Mass-balance fits showed strong early Ca²⁺ mobilization per mol of gluconate, consistent with Ca-gluconate complexation and ligand-assisted dissolution. Avrami analysis indicated lower exponents but higher apparent growth rates, and FTIR revealed greater silicate cross-linking. In the Ca-poor GP, SG had little to no retardation and sometimes slightly accelerated setting, with negligible changes in reaction extent and compressive strength due to electrolyte-driven aggregation rather than Ca-controlled nucleation. These results indicated that SG acted as a Ca-complexing retarder in the C-(N)-A-S-H-dominated GP but mainly as an inert electrolyte in the N-A-S-H-dominated GP. Therefore, moderate SG can tune induction and improve early gel yield in the Ca-rich GP, whereas benefits in the Ca-poor GP are limited.