To balance cost and performance of geopolymers, alkalinity of activating solution is critical. Alkalinity affects condensation that determines the final gel structures, but this effect is confounded by dissolution and is not understood from direct experimental evidence. In this study, we investigated effects of alkalinity on condensation for gels synthesized via a sol–gel method that eliminates dissolution process. As alkalinity increased, particle sizes of the gels increased as indicated by SEM, Si/Al ratios of the gels decreased but polymerization extent increased as supported by FTIR, ²⁷Al and ²³Na NMR, and composition analysis. The mechanism for the effects of alkalinity was proposed accordingly: (1) increasing alkalinity lowers the Si/Al ratio (i.e., more incorporation of Al) of the resulting products probably by affecting charging conditions of the Si and Al units; (2) the presence of Al(OH)₄⁻ units promotes their condensation with nearby species to increase the extent of polymerization; (3) enhanced condensation increases particle sizes of the gels even at microstructural level. This understanding on condensation independent of dissolution provides ways to control gel structures and Si/Al ratios and thus tailor properties accordingly, as well as to suggest a strategy (by altering Si/Al ratios during condensation) to develop kinetics-controlling admixtures.

为了平衡地质聚合物的成本和性能，活化溶液的碱度至关重要。碱度影响决定最终凝胶结构的缩合反应，但这种作用会因溶解而混淆，从直接的实验证据中无法理解。在这项研究中，我们研究了碱度对通过溶胶-凝胶法合成的凝胶的缩合作用的影响，该方法消除了溶解过程。随着碱度的增加，凝胶的粒径（如SEM所示）增加，凝胶的Si / Al比降低，而聚合度则得到FTIR，²⁷ Al和^23的支持而增加Na NMR，以及组成分析。据此提出了影响碱度的机理：（1）增加碱度可能通过影响Si和Al单元的充电条件而降低所得产物的Si / Al比（即，更多地引入Al）；（2）Al的存在（OH）₄ ^-单位促进其与附近的物种，以增加聚合程度缩合; （3）即使在微结构水平上，增强的缩合也会增加凝胶的粒径。这种对不依赖于溶解的缩合的理解提供了控制凝胶结构和Si / Al比例，从而相应地调整性能的方法，并提出了开发动力学控制混合物的策略（通过在缩合过程中改变Si / Al比例）。