Mechanical, Microstructural, and Durability Characterization of Fly Ash/Slag-Based Geopolymer with Limited OPC and Different Alkaline Activator Ratios

Different building materials have been used in construction for centuries to meet the accommodation needs. Building materials were primarily designed to tolerate the environmental impacts and to meet the demand. However, some of these materials did not survive in the past due to their engineering limitations and durability concerns. Concrete is among the survived ones and is considered one of the primary materials used on earth. Concrete is a composite material primarily composed of ordinary Portland cement, water, coarse aggregate, and fine aggregate. Portland cement is the main binder component of regular concrete. Producing Portland cement requires a high amount of energy and heat, which corresponds to releasing carbon dioxide (CO2) in the atmosphere and negatively impacts the environment. Geopolymer concrete is a promising material that can be produced using by-product materials such as fly ash and slag as prime binders of the concrete and sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) as alkaline activators. Gel phases formed from fly ash/slag-based geopolymer concrete are mainly sodium aluminosilicate hydrate (N-A-S-H) and calcium aluminosilicate hydrate (C-A-S-H), and the limited replacement of OPC may improve the coexistence of these gels. In geopolymer concrete, aluminosilicate sources (fly ash and slag) do not react entirely to form sodium-aluminosilicate-hydrate and calcium-aluminosilicate-hydrate gel phases, and that produce deficient microstructural formation. Besides the gel formations deficiency, a part of the binders remains unreacted, which sequentially induces lower mechanical performance and a part of the aluminosilicate sources to be wasted. Thus, in this study, fly ash and slag combinations were used with limited OPC content as a maximum of 12% of the total binder to improve properties of the fly ash/slag-based geopolymers and increase the uptake of calcium that is provided from OPC. The low-calcium fly ash has a slow setting time, whereas ground granulated blast furnace slag (GGBFS) has a rapid setting time, and it is crucial in the evaluation in this study to measure the effect on the hydration time. Therefore, in the first stage of this research, the effect of OPC and Na2SiO3/NaOH (SS/SH) ratios on the setting time is a significant part of this study. Moreover, microstructural formation, gel phase identification, and elemental composition studies were evaluated besides the strength developments at certain ages to investigate the changes in the development of the binary system. For the setting time measurements, a penetrometer was modified to meet ASTM C191–Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle. A different set of equipment and tools were used for compressive strength properties, including digital caliper, precision balance, 20-quarts mixer, vibrating table, diamond saw, and compressive strength testing equipment. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were mainly used to assess microstructural characterizations and analysis of elemental compositions. Three sodium silicate to sodium hydroxide ratios were considered, and five batches were mixed in the first stage for each ratio. For each batch, a total of fifteen cylinder samples 2x4 inch were cast. In total, sixteen batches were mixed, including the geopolymer pastes and Portland cement pastes. At each testing age, five specimens were tested to record the maximum compressive strength according to ASTM C39-Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. For SEM/EDS analysis, three samples were tested for each SS/SH ratio. The three samples were prepared for the microscope following the ASTM C1723- Standard Guide for Examination of Hardened Concrete Using Scanning Electron Microscopy. In the second stage of this research, durability studies were conducted for up to 120 days. The specimens were soaked in two different environments: magnesium sulfate as the aggressive solution and regular tap water as the reference. The samples were cured for 28 days to reach sufficient maturity, and then test results were recorded at 56, 90, and 120 days. The studies concluded that the addition of Portland cement by minimal percentage as 3% and 6% content could significantly contribute to the hydration kinetics and the development of the low calcium fly ash/slag-based materials. Replacing GGBFS by 3% of OPC for R0.5 (SS/SH=0.5), R1.0 and R1.5 increased the initial setting time by 10%, 29%, and 4%, respectively. The effect of OPC on the final setting time was more pronounced using more OPC content. By increasing OPC content up to 6%, the final setting time increased by 18%, 14%, and 38% for R0.5, R1.0, and R1.5, respectively. The compressive strength results improved by 9.8%, 12.5%, and 7.0% with adding 3%, 6%, and 3% of OPC for R0.5, R1.0, and R1.0 at 120 days, respectively. In the microstructural analysis, the density of the hydrated products formed increased with increasing the SS/SH ratio. The SEM micrographs of the reaction products of R1.0 and R1.5 showed homogeneity and well-compactness. The microstructural formation improved significantly for R1.0 and R1.5 by using 6% and 3% of OPC, respectively, at 120 days. The elemental compositions analyses using EDS show significant shifts in the gel phases formed with OPC addition. The CaO/SiO2 revealed a substantial increasing trend with increasing OPC content, whereas the Al2O3/SiO2 ratio demonstrated a decreasing trend with increasing OPC content. Multiple gel phases co-existed in the final products formed, including N-A-S-H, C-N-A-S-H, C-A-S-H, and C-S-H. After the results of EDS analysis, the N-A-S-H gel phase was dominant in the control groups, whereas calcium-rich phases dominated the samples with OPC content. The durability studies showed that the increase in the SS/SH ratio from 1.0 to 1.5 increased the availability of the soluble silica. The existence of the free silica improved the dissolution of the precursors, which increased the reaction rate and improved the development of hydrated products.