Incorporating Geopolymer Binders to Fully Utilize Recycled Concrete Aggregates

This study aims to develop a geopolymer concrete binder that will yield desirable strength and durability characteristics when applied to recycled aggregate concrete. In order to achieve this, a geopolymer binder consisting of a metakaolin-silica fume aluminosilicate source material blend activated by potassium-based alkaline solutions is developed and used in place of ordinary Portland cement (OPC) with four different levels of recycled aggregate replacement ratios. The properties of these concretes were analyzed and compared with the properties of OPC concretes produced with the same replacement ratio levels. The results showed that the geopolymer recycled aggregate concrete (GRAC) achieved greater than 17.5 MPa compressive strength for combined recycled aggregate replacement ratios up to 70% of the total weight of the aggregates. Furthermore, stiffness loss associated with increasing replacement ratios was noticeably less prevalent in the GRAC mixes than in the OPC recycled aggregate concrete (ORAC) mixes. Simultaneously, this study also aims to assess the role of calcium compounds in the strength and microstructural development of geopolymer binders cured in ambient conditions experimentally as well as reactive molecular dynamics (MD) studies. Experimentally, the results of the study showed that calcium hydroxide (Ca(OH)2) performed better than calcium oxide (CaO) as a calcium additive in terms of improving the properties of ambient-cured geopolymers. Moreover, it was found that a Ca(OH)2 dosage equal to 8% of the total mass of the aluminosilicate materials resulted in the precipitation of the potassium aluminosilicate hydrate, the typical geopolymerization reaction product, but also calcium silicate and aluminosilicate hydrates. Additionally, it is shown that the presence of Ca2+ ions in the geopolymer solution aids in the dissolution of the aluminosilicate materials. These results are corroborated by the results of the MD study, which showed that the three previously mentioned reaction products were formed when the K:Ca molar ratio is sufficiently low. In summary, the importance of utilizing recycled concrete aggregates in new construction is established, while the current state of research regarding recycled aggregate concrete is surveyed, including methods to improve its performance. A geopolymer binder is successfully developed for use in recycled aggregate concrete, and the resulting concrete successfully meets minimum standards for concrete performance. Finally, the role of calcium compounds in the development of ambient-cured geopolymers is analyzed and affirmed