Understanding the role of calcium on the reaction mechanism of geopolymer cements through addition of nucleation seeds

Geopolymer cement is an innovative binder proposed by Davidovits as an alternative to conventional Portland cements for construction use. It is made from minimally processed industrial byproducts (fly ash, slag) activated by a low concentration alkaline solution (Na, K) and cured at room temperature. The use of low concentration alkaline activators (2 M), unlike the high concentration used for conventional geopolymer binders (8-12 M), makes these cements environmentally friendly (using industrial waste products) and user-friendly. Geopolymer cements gain strength quite rapidly and have been formulated to achieve nearly 100 MPa in 28 days, however, at the cost of low workability. The loss of workability is usually attributed to the presence of free calcium. It is predicted that calcium silicate hydrate/calcium aluminosilicate hydrate gel (C-S-H/C-A-S-H, a main binding phase of fly ash modified Portland cements binder) and aluminosilicate gel ((Na,K)-A-S-H, a main binding phase in geopolymer binders) co-exist in these systems. The precipitation of C-S-H/C-A-S-H is known to initiate the rapid hardening which then is hypothesized to act as a nucleation site for the precipitation of aluminosilicate gel. This study verified the hypothesis through the addition of synthesized C-S-H/C-A-S-H as an external seed during the processing of geopolymer cement. Isothermal conduction calorimetry, scanning electron microscopy, shear wave ultrasonic wave reflection method, and Fourier Transform Infrared spectroscopy are employed to study the effects of seed on reaction kinetics, extent of product formation, nature of reaction products, and dissolution of raw materials. Through the addition of seeds, it has been concluded that the reaction mechanism in fly ash-slag cements depends on the activator solution. In hydroxide activated fly ash-slag geopolymers, the rate controlling step is the nucleation-growth controlled reaction, early age hardening behavior in these systems can be controlled via mechanisms that increases or decreases the rate of nucleation and growth of C-S-H. The reaction can be accelerated by adding synthesized C-S-H seed or a small percentage of nanoparticles which will promote nucleation of the product in the pore space. In silicate activated systems, the rate determining step is gelation which depends exclusively on the extent and the rate of aluminosilicate oligomers formation in the solution. Any factor that will affect the availability of silicate species or the rate of aluminosilicate oligomer formation will effect the reaction mechanism. Retarders that will selectively polymerize with silicate species in the pore solution can be used to develop retardation in these cements. The addition of nucleation seeds to study reaction kinetics is shown to effectively capture the shift in the reaction mechanism from nucleation-growth controlled to the gelation with varying silica concentration in the solution. The protocol developed in the study to separate the responses of two amorphous gels (C-A-S-H and (Na,K)-A-S-H) from the amorphous precursors can be extended to understand the structural evolution of phases in other alkali activated blends of complex chemistry. The fundamental understanding gained through this research can pave a way for large-scale adaptation of alkaline cement technology. The research output will enable engineers to understand the early age reaction mechanism, the knowledge of which will provide greater control over the length of induction period, setting time, and workability of geopolymer cements. Research will provide a unique tool for tailoring the nanostructure of the reaction products through the addition of synthetic seeds for optimizing engineering performance