Experimental Study and Numerical Simulation of the Reaction Process and Microstructure Formation of Alkali-Activated Materials

Alkali-activated materials (AAMs) show promising potentials for use as building materials. Before the utilization, AAMs must satisfy the properties required from the construction sector, such as good durability and long-term service life. These properties mainly depend on the chemical and physical properties of the microstructure of AAMs. In the literature many studies have been presented about the reaction process and microstructure formation of AAMs, but still some aspects are not given due attention, such as the pore solution composition of AAMs and the origin of the induction period during the reaction of AAMs. Furthermore no computer-based simulation models have been developed so far for simulating the reaction process and microstructure formation of AAMs. It is still a big issue and challenge today to numerically obtain the microstructure of AAMs.This research adopted two routes to study the reaction process and microstructure formation of AAMs, i.e. the experimental study route and numerical simulation route. Ground granulated blast furnace slag and fly ash were used as the aluminosilicate precursors and sodium hydroxide and sodium silicate were used as the alkaline activators. The experimental study route provided new results for better understanding the reaction process and microstructure formation of AAMs. Then the new insights from the experimental study route helped to develop the GeoMicro3D model for simulating the reaction process and microstructure formation of AAMs.(1)Experimental study routeFirstly, the pore solutions of alkali-activated slag, alkali-activated fly ash and alkali-activated slag/fly ash pastes with different activators and reaction conditions were studied by means of ICP OES. It was found the pore solution composition of AAMs depended on the activation conditions, such as the type and concentration of alkaline activator and curing temperature. Then, the reaction kinetics of alkali activated slag, alkali activated fly ash and alkali activated slag/fly ash pastes were investigated by using isothermal calorimetry. The sodium content, silica content and curing temperature affected the reaction kinetics of AAMs. The origin of the induction period was different for alkali-activated slag systems and alkali-activated fly ash systems. For alkali activated slag systems, the presence of soluble Si in the activator slowed down the dissolution of slag and caused an induction period. In contrast, the fact that an induction period occurred in alkali-activated fly ash systems was mainly attributed to the passivation of the leached surface layer caused by the absorbed Al.Finally, the microstructure development of alkali-activated slag, alkali-activated fly ash and alkali-activated slag/fly ash pastes was studied using SEM and MIP. It was found that the type and concentration of alkaline activator affected the microstructure formation of AAMs. An increase of Na2O content led to a reduction of the total porosity and a refinement of the microstructure for alkali-activated slag systems. In contrast, an increase of Na2O content did not affect the total porosity of alkali activated fly ash systems very much. Instead, it altered the microstructure by increasing the amount of large pores and decreasing the amount of small pores. The SiO2 content seriously affected the microstructure formation of AAMs. In sodium silicate activated systems, the soluble silicate in the activator led to a relatively dense microstructure with separated small capillary pores. This was different from the relatively coarse microstructure with connected capillary pores in sodium hydroxide activated systems.(2)Numerical simulation routeFirstly, the initial particle parking structure of AAMs, as the starting point for simulating the reactions and microstructure formation, was simulated using real-shape particles of slag and fly ash. In comparison with the spherical particles, using real-shape particles increased the total surface area (up to 23 %) and bulk specific surface area (at least 12 %) of the simulated initial particle parking structures. At low liquid/binder ratios (≤ 0.47), using real shape particles led to a significant shift of the pore size distribution to small pores as compared to using the spherical particles in the simulated initial particle parking structures.Secondly, a dissolution model was developed for simulating the dissolution of aluminosilicate precursors in alkaline solution. The influences of temperature, reactivity of precursors, alkalinity of solution and inhibiting effect of aqueous Al etc. on the dissolution of precursors were taken into account in this model. The simulation results of the dissolution of slag and fly ash in alkaline solution were in agreement with the experimental data.Then, the reactions in AAMs were thermodynamically simulated. A thermodynamic model, i.e. N(C)ASH_ss, was developed to describe the N A S H gel. With this model and the C(N)ASH_ss model in the literature it is possible to perform thermodynamic modelling of the reactions in alkali-activated fly ash systems and alkali-activated slag/fly ash systems. The simulated pore solution compositions of AAMs were in line with the experimental results.Finally, the GeoMicro3D model was built up based on the numerical modules of initial particle parking structure, dissolution of aluminosilicate precursor, thermodynamic modelling and nucleation & growth of reaction products. As a case study GeoMicro3D was implemented to simulate the reaction process and microstructure formation of alkali-activated slag with three different alkaline activators. The simulated reaction kinetics (degree of reaction of slag) and pore solution chemistry (element concentration) were in agreement with the experimental results. The simulated volume evolution of solid phases by GeoMicro3D was consistent with the results calculated by GEMS with regard to the primary reaction products (C (N )A S H) and some crystalline reaction products, such as the hydrotalcite-like phase and katoite. Besides the volume evolution of solid phases, GeoMicro3D also provided the volumes of adsorbed water and gel pore water that were retained by the C (N )A S H gel.To sum up, the reaction process and microstructure formation of AAMs were studied experimentally and numerically. Based on the insights obtained from the experimental study numerical models were developed and validated. With these models it is possible to simulate the initial particle parking structure of slag/fly ash in alkaline activator, dissolution of slag/fly ash, chemical reactions and microstructure formation of AAMs with reasonable accuracy.