Mix design formulation range for metakaolin-based geopolymer synthesis

Abstract This article presents a range of mix design formulations to obtain metakaolin-based geopolymer synthesis. Geopolymer samples were synthesized from metakaolin, a 10 M sodium hydroxide solution, and alkaline sodium silicate. These three components were mixed in different proportions to form a paste which was molded in a cylindrical shape, set at room temperature for 24 hours, and demolded. The workability of the paste, the integrity of the samples, development of efflorescence, and the presence of undissolved metakaolin particles in the microstructure of the geopolymer matrix were observed. Since sodium and water play a vital role in the geopolymerization and were added at fixed proportions, a mix design was employed based on the molar proportions of aluminum, silicon, and sodium plus water. The simplex design plot was able to separate the regions of a mix of the components that showed different behaviors according to the observed responses. The dissolution of metakaolin, condensation of ortho-sialate and silicate species, which are expected to interact with each other according to the model of geopolymerization of Davidovits, can explain the observed results. “Good results” (no release of white powder, no efflorescence, no shrinkage, and no cracks) after 28 days of curing at room conditions were observed for the molar proportions 0.097 < Al < 0.112, 0.120 < Si < 0.178, and 0.711 < Na+H2O < 0.775 (Na/H2O = 0.185).

摘要 本文提出了一系列用于制备偏高岭土基地质聚合物（metakaolin-based geopolymer）的配合比设计方案。研究以偏高岭土（metakaolin）、10 mol/L氢氧化钠（sodium hydroxide）溶液与碱性硅酸钠（sodium silicate）为原料，通过调整三者的配比制备浆体，将浆体浇筑为圆柱形试件，于室温下静置养护24小时后脱模。随后对浆体的工作性、试件的完整性、泛碱现象的产生情况，以及地质聚合物基体微观结构中未溶解偏高岭土颗粒的存在状态进行了观测。由于钠与水在地质聚合反应中起到至关重要的作用，且二者的添加比例固定，本研究基于铝、硅以及钠与水的摩尔比开展配合比设计。单纯形设计图（simplex design plot）可依据观测得到的响应特征，区分不同原料配比区间对应的反应行为。偏高岭土的溶解、邻位硅铝酸盐（ortho-sialate）与硅酸盐物种的缩聚反应——根据达维多维茨（Davidovits）提出的地质聚合模型，此类物种本应相互作用——可对本研究的观测结果作出合理解释。在室温条件下养护28天后，当摩尔比满足0.097 < Al < 0.112、0.120 < Si < 0.178以及0.711 < Na+H₂O < 0.775（Na/H₂O = 0.185）时，试件可达到“良好性能”：无白色粉末析出、无泛碱现象、无收缩及开裂问题。