Microstructure Of Metakaolin-based Geopolymer Research Articles

Early-Stage Geopolymerization Process of Metakaolin-Based Geopolymer.

The geopolymerization of aluminosilicate materials in alkaline environments is a complex physicochemical process that greatly influences the microstructure and engineering performances. This work aims to reveal the geopolymerization process of metakaolin-based geopolymer (MKG) in the first 5 d. Physicochemical characteristics of different evolution stages are disposed of in chronological order. The evolutions of electrical resistivity, dehydration process, volume deformation, and ionic concentration are comprehensively analyzed. Results show that chemical dissolution produces large dismantled fragments rather than small free monomers. The formation of a solid matrix follows the “spatial filling rule”, which means that gels grow by locking swelling fragments to form a framework, then densely filling residual space. Based on chemical models, early geopolymerization of MKG can be divided into six stages from the physicochemical perspective as dismantling, locking fixation, free filling, limited filling, second dissolution, and local mending. Those findings expand the understanding of the phase evolution of the early geopolymerization process; thus, the microstructure of MKG can be better manipulated, and its engineering performances can be improved.

Effect of Elevated Temperature on the Microstructure of Metakaolin-Based Geopolymer.

Currently, geopolymer is being considered as a future oil-well cement. For wellbore applications, geopolymers are initially tested at specific temperature conditions. However, an oil-wellbore may experience a sudden increase in temperature which may adversely affect geopolymer systems designed for low to moderate temperature conditions. In this work, the effect of elevated temperatures on the microstructure of the geopolymer was simulated. Metakaolin-based geopolymer systems cured at 163 °F for 48 h were subjected to a temperature ramp of 194 °F and 248 °F for 24 h. X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetry analysis techniques were used to study the microstructural changes. The analytical techniques show the formation of new crystalline phases when the geopolymer cured at 163 °F was suddenly exposed to higher temperatures. These crystalline phases, for instance, gobbinsite and anorthite, observed in the microstructure have the potential to cause thermal stress, weaken the system, and ultimately affect the geopolymer’s ability to effectively isolate the formation and support the casing.

Relation between drying shrinkage behavior and the microstructure of metakaolin-based geopolymer

Relation between drying shrinkage behavior and the microstructure of metakaolin-based geopolymer

Matrix wettability and mechanical properties of geopolymer cement-polydimethylsiloxane (PDMS) hybrids

Geopolymers have attracted attention as green building materials with low carbon dioxide emissions and excellent mechanical properties. Due to their hydrophilicity and porous structure, geopolymers can be infiltrated by water involving harmful ions. In this study, the effects of polydimethylsiloxane (PDMS) as an additive on the wettability, mechanical properties and microstructure of metakaolin-based geopolymer were investigated. The microstructure was characterized by MIP, μ-XCT, SEM, EDX, TIM, XRD, FTIR and 29Si NMR test. As the PDMS content increased, the geopolymer became hydrophobic, increasing its contact angle to approximately 140°. This hydrophobicity mainly results from the chemical modification and the increased roughness of the matrix. PDMS, with the content over 1.5 mL/L, can raise the porosity and pore size of the geopolymer, and promote the transformation of small gel pores into transitional and capillary pores. The fluffy pore structures and cured hybrids jointly created nano-scale and micron-scale roughness structures. No new crystalline phases will be generated by adding PDMS. The compressive strength increased with the addition of a small amount of PDMS, while it began to decrease when the PDMS content exceeded 10 mL/L. The effect of PDMS with different contents on the pore and silicate structure of geopolymers leads to changes of mechanical properties. The hybridization mechanism of PDMS molecules and N-A-S-H was summarized based on microstructure analysis.

Partial replacement of metakaolin with thermally treated rice husk ash in metakaolin-based geopolymer

The influences of rice husk ash (RHA) on the reaction kinetics, mechanical property, and microstructure of metakaolin-based geopolymer (MG) were investigated. Geopolymer pastes containing different contents of RHA were prepared by sodium silicate activation. Analysis results show that both metakaolin and rice husk ash are involved in the geopolymerization, contributing to the development of mechanical properties and formation of compact microstructure. The longer reaction duration of setting time in the group incorporated RHA was ascribed to the procedure of SiO2 dissolution. The geopolymers added with RHA have a compact pore structure, which show a relatively high compressive strength. The replacement of metakaolin by RHA did not cut the total quantity of gel phase formation, but changed the form of partial gels existence and an optimal mixed point was reached at the 20% replacement level. The abundant in gel along with the pore channel and secondary filling effects make a significant contribution to the development for refinement of pore and improvement of strength. This study can be used as an indicator for the application of RHA in geopolymer, which is of great value in urging RHA towards realization in recycling.

Deterioration in the microstructure of metakaolin-based geopolymers in marine environment

In this research, metakaolin-based geopolymers were synthesized to study their deterioration in marine environment. The geopolymers were exposed in air, seawater, dry-wet and heat-cool cycles of seawater for 30, 60 and 90 days. The mechanical property of the geopolymers was characterized through compressive strength measurements, and their microstructures were measured by X-ray diffraction (XRD), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) apparatus. It was found that seawater environment inhibited the geopolymerization reactions, so that a low amount of tetrahedral silicon of Q4(4Al), Q4(3Al) and Q4(2Al) were formed.

Effect of sodium polyacrylate on mechanical properties and microstructure of metakaolin-based geopolymer with different SiO2/Al2O3 ratio

The mechanical properties and microstructure of geopolymer are affected by the molar ratio of SiO2/Al2O3. Meanwhile, organic polymer has the effect of improving the toughness of geopolymer, which depends on the SiO2/Al2O3 ratio of geopolymer inevitably. Therefore, it is important to investigate the effect of the organic polymer on the mechanical properties and microstructure of geopolymer with varying SiO2/Al2O3 ratio for using organic polymer to modify geopolymer. In this work, the SiO2/Al2O3 ratios of metakaolin-based geopolymers are adjusted to 2.0, 2.5, 3.0, 3.5 and 4.0 by adding silica fume and β-Al2O3, with Na2O/SiO2, H2O/SiO2 being maintained at 0.2, 4.0, respectively. The geopolymers with each SiO2/Al2O3 ratios are modified by addition of 0, 0.4, 0.8, 1.2 and 1.6 wt% of sodium polyacrylate (PAAS).The mechanical properties of these samples are measured and the rate of change is used to characterize the effect of PAAS on the metakalin-based geopolymers. The mechanism is also shown by 29Si NMR, XPS and FTIR. The results show that the effects of polymer on the mechanical properties of metakaolin-based geopolymer are affected by SiO2/Al2O3 ratio and the effect becomes less obvious with SiO2/Al2O3 ratio increasing from 2.0 to 4.0. Incorporation of PAAS can reduce the degree of polymerization of [SiO]4 or [AlO]4 in geopolymer and form the Si‒O‒C bond, which are two main reasons for polymer improving the toughness of geopolymer. But these effects decrease when the SiO2/Al2O3 ratio of geopolymer increases from 2.0 to 4.0, which is corresponding to the effect on the mechanical properties. The toughening effect of organic polymer on geopolymer depends on the SiO2/Al2O3 ratio of geopolymer, and only the geopolymer with lower SiO2/Al2O3 ratio (no more than 2.5 in this work) can be significantly toughening modified by organic polymer. Therefore, it is necessary to consider the SiO2/Al2O3 ratio of the geopolymer when geopolymer modified by organic polymer is designed.

Viability of bacterial spores and crack healing in bacteria-containing geopolymer

Geopolymer is an emerging alternative green binder to Portland cement. Geopolymer is often more brittle and thus it is highly desirable to impart self-healing into geopolymer. Unlike Portland cement concrete, self-healing through hydration of cement, and leaching and carbonation of hydration products are not feasible in geopolymer. Microbially induced carbonate precipitation (MICP)-enabled sealing is therefore a potential way to engage self-healing in geopolymer. Using Sporosarcina pasteurii as a model MICP bacterium, this paper investigated viability of bacterial spores in a metakaolin-based geopolymer and crack healing in bacteria-containing geopolymer. Spores of S. pasteurii were added into geopolymer mix directly without encapsulation or immobilization. Results showed that bacterial spores did not leak out from the geopolymer matrix and the spores remained viable in the metakaolin-based geopolymer. Cracks in bacteria-containing geopolymer were sealed with CaCO3 after conditioning in precipitation medium for 3 days, which suggests bacterial spores remain viable. The microstructure of metakaolin-based geopolymer is controlled by Si/Al, Na/Al, and H2O/Na2O molar ratio and less depend on age, which allows direct addition of bacteria into geopolymer mix without encapsulation or immobilization to engage MCP-induced self-healing in geopolymer.