UBC Theses and Dissertations
Process engineering of functional metakaolin based geopolymers Pernechele, Matteo
Geopolymers (GPs) are a class of inorganic materials which can be used as construction and refractory cements and as functional materials for environmental applications. GPs are low CO₂ emissions binders with high durability that can replace traditional cementitious materials. However the effects and interactions of processing parameters on the different stages of GP setting (“geopolymerization”) are still under scrutiny and the molecular mechanisms and rate limiting steps controlling the setting kinetics are unknown. The crystallization in GPs, which ultimately controls their performance in advanced applications such as water purification and toxic waste encapsulation, is a poorly investigated topic. This dissertation provides new experimental evidences on the role of chemical composition and curing process on metakaolin-based GPs. Steady state and dynamic rheological studies, contact angle tests, microstructural (SEM), structural (XRD and FTIR) and mechanical analyses lead to better understanding of the fundamental transformations occurring during geopolymerization. GPs were seeded with different oxides and zeolites to determine the rate limiting step, increase the reaction rate and control the crystallization. This work contributes to clarification the complex effects of soluble silica on the geopolymerization process. It is shown that soluble and colloidal silicates (such as Na₄SiO₄ and Na₂SiO₃) can act as seeding agents, changing the geopolymerization rate limiting step at temperatures T≥35°C. However, they also slow down the reaction rate, possibly by forming passivation layers on the metakaolin particles, thus producing a more chemically stable and mechanically stronger amorphous gel. Silicates also decrease the water requirement in GPs and thus the porosity. Under certain conditions silicates can increase the percentage of crystalline Faujasite in GPs, but the crystallization process requires higher curing temperatures and times (T>40°C and t>4 days, depending on the amount of silicates). The alkali metals have also a structure-directing role in crystallization of GPs in the form of zeolite, favoring faujasite structure. Water has a templating effect in GPs, favoring the structure of zeolite LTA-type over hydrosodalite. This work also illustrates the compromises that need to be made when selecting appropriate processing parameters to tailor the rheology, structure and properties of geopolymers for specific applications.
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