Accentuating the ambient curing behavior of geopolymers: metamodel-guided optimization for fast-curing geopolymers with high flexural strength†

Abstract

A geopolymer, consisting of –Si–O–Al– covalent bonds in a polymeric network, has a simple manufacturing process with low CO₂ emissions and excellent high-temperature performance, making it a promising modern refractory material. In particular, owing to its low-temperature and fast-curing conditions, geopolymers can be used for practical on-site applications. However, the properties of geopolymers are significantly dependent on the composition and content of various additives, and this complexity limits our understanding of the composition to a narrow scope. In this study, we investigated the optimal composition designed for fast and low-temperature curing geopolymers with additives, including Ca(OH)₂, fumed silica, and chopped carbon fiber. A multivariate compositional optimization was systematically conducted using design of experiments and metamodeling. By utilizing the metamodel, we successfully developed an optimized geopolymer composition with only 45 sets of experiments. The flexural strength obtained was 27.83 MPa, the highest recorded value for a bulk fast-curing geopolymer to date. Furthermore, the curing speed was modulated to be swift at ambient conditions, achieving 98% of the full strength in 6 days at 20 °C (whereas it typically takes 1 to 4 weeks at 40 °C). We also investigated how superior strength could be achieved while curing at low temperatures for a short duration. It turned out that fumed silica slowed down the growth of the Ca compound, balancing two different effects stemming from Ca ions: strength degradation and rapid curing. The developed geopolymer is expected to be widely used in applications that require rapid curing at room temperature, such as external cement replacements for fire spread prevention structures, acid-exposed environments, or repair and finishing materials.