Showing papers on "Geopolymer published in 2019"

Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate

Abstract: Geopolymer binder and recycled aggregates are considered as the two important sustainable ingredients in concrete production. Ground granulated blast furnace slag (GGBS) and fly ash combination based geopolymeric recycled aggregates concretes (GRAC) were prepared as a substitution of normal concrete in order to reduce the consumption of ordinary Portland cement and natural aggregates as well as to make use of demolished waste concrete. The aim of this study is to investigate the combined effects of GGBS and fly ash considering water-binder (W/B) ratio on the fresh and hardened properties of alkali activated GRAC. The slump, setting time, compressive strength, stress-strain relation, elastic modulus, Poisson's ratio, toughness and failure mode of alkali activated GGBS and fly ash based GRAC containing 100% recycled coarse aggregates were experimentally examined. In addition, the hydration mechanisms were studied by XRD and SEM. Finally, the failure mechanism of this kind of novel green concrete subjected to compression was revealed. The results show that GGBS and fly ash exhibits a superior synergetic effect on the performance of GRAC, i.e. fly ash and GGBS are mainly responsible for the workability and mechanical properties, respectively. GGBS/fly ash ratio has a significant influence on the fresh and hardened properties of GRAC. Moreover, the effect of W/B ratio on GRAC strongly depends on the GGBS/fly ash ratio. GGBS content at less than 25% was found to slightly influence the consistency and compressive strength of GRAC. The combination of 50% GGBS and 50% fly ash with a 0.5 W/B ratio could provide excellent mechanical performance and workability for GRAC.

Extrusion and rheology characterization of geopolymer nanocomposites used in 3D printing

Abstract: This study focused on the suitability of geopolymer mixes for extrusion-based additive manufacturing applications. Geopolymer mixes including alkaline potassium silicate activator with different molar ratios were prepared and evaluated for their rheological responses (e.g. yield stress, viscosity and thixotropy) that influenced their extrudability and buildability. Due to the low yield stress of geopolymers, nanoclay was introduced to enhance their thixotropy, which was assessed by structural breakdown and viscosity recovery tests. An activator-to-binder ratio of 0.35 and water-to-solid ratio of 0.30 were found to work well with a clay addition of 0.5% (i.e. by mass of total binder component). The addition of nanoclay led to an improvement in the rheological properties of geopolymer nanocomposites, which was attributed to the flocculation characteristic of the clay particles. A small-scale bathroom unit was printed by using the developed geopolymer nanocomposite, indicating the suitability of the proposed mix design to be used in 3D printing applications.

Effect of slag on the mechanical properties and bond strength of fly ash-based engineered geopolymer composites

Abstract: Recently, the concept of engineered cementitious composites (ECCs) has been extended to the creation of engineered geopolymer composites (EGCs). Although showing similar mechanical characteristics (e. g., strain hardening and multiple cracking) to conventional ECC, the strength of existing EGC is generally low, and this sometimes restrains its applications. In the present study, a low-calcium (Class F) fly ash-based, polyvinyl alcohol (PVA) fiber reinforced EGC was developed and further modified by a ground-granulated blast-furnace slag (slag). The slag was used to replace the fly ash at content of 0%, 10%, 20%, and 30% (by weight). The effects of the slag on the mechanical properties (e.g., compressive strength, modulus of elasticity, uniaxial tensile behavior, flexural bending strength, and pullout bond strength) of the EGCs were investigated. The results revealed that all EGCs studied exhibited a strain/deflection hardening behavior under tension/flexure, and all slag replacements for fly ash enhanced strength-related properties but reduced ductility-related properties of the EGCs. The EGC mix with 20% slag replacement for fly ash (FA-20%S) had 102.3 MPa compressive strength, 6.8 MPa tensile strength, and 6.2 MPa bond strength, while the EGC mix with no slag (FA-0%S) had 72.6 MPa compressive strength, 4.7 MPa tensile strength, and 3.5 MPa bond strength at 28 days. These strength enhancements were mainly attributed to the improved density of the EGC matrix and the bond between the matrix and fiber. There are close relationships between the bond strength and other strengths, especially the tensile and flexural strengths, of the EGCs.

Optimum mix design of geopolymer pastes and concretes cured in ambient condition based on compressive strength, setting time and workability

Abstract: In this study, the effects of ground granulated blast furnace slag (GGBFS) content, alkaline solution to binder (Al/Bi) mass ratio, sodium silicate solution to sodium hydroxide solution (SS/SH) mass ratio, and additional water to binder (Aw/Bi) mass ratio on the compressive strength , setting time and workability of geopolymer pastes were studied. A series of mini-size specimen compression tests, setting time tests and mini-slump tests were conducted at ambient condition (23 ± 2 °C). The GGBFS and Class F fly ash (FA) were used as aluminosilicate source. The alkaline activator was a blend of sodium silicate solution and sodium hydroxide solution. Additional water was added to improve the workability and prolong the setting time. Based on the test results of compressive strength, setting time and workability, the optimum mix design was found to have GGBFS content of 40%, Al/Bi ratio of 0.5, SS/SH ratio of 2.0, and Aw/Bi ratio of 0.15. It was found that the properties of the geopolymer paste under optimum mix design were better than those of ordinary Portland cement (OPC) pastes. After that, the geopolymer concrete tests based on optimum mix design of geopolymer paste were conducted, in comparison with OPC concrete tests. It was found that the properties of geopolymer concrete were also better than the properties of OPC concrete. It is worth noting that this relatively simple and fast test methodology to obtain the optimum mix design of geopolymer concrete can help engineers save time and labour. Lastly, new mathematical models were proposed to predict the properties of geopolymer pastes , which showed high accuracy.

Silico-aluminophosphate and Alkali-aluminosilicate Geopolymers: A comparative review

Abstract: Chemically activated materials (often termed as geopolymer) have received attracting attentions in civil, material and environmental research fields as a toolkit alternative to traditional Portland cement in specific applications. This paper presents a state-of-the-art review on silico-aluminophosphate (SAP) geopolymers in terms of definition, chemistries involved during geopolymerization, mechanical and durability performance, environmental impacts, and their potentials in applications as compared to conventional alkali-aluminosilicate (AAS) geopolymers. Recommendations for future applications are also highlighted. It is found that S-A-P gels with six-coordinated aluminum environment dominate in SAP geopolymers, while the aluminum in N-A-S-H gels formed in the AAS geopolymers is characterized by four-coordinated features. Besides, the slow performance development of SAP geopolymer matrix under ambient temperature curing can be compensated through incorporating additional countermeasures (e.g., metal sources) which allow the tailored design of such geopolymers for certain in-situ applications, i.e., calcium-bearing C-(A)-S-H gels co-existed with N-A-S-H gels are dominant in AAS geopolymers, while S-A-P gels enhanced by phosphate-containing crystalline/amorphous phases are the main products in SAP geopolymers. Another advantage of SAP geopolymers is their environmental friendliness relative to the AAS geopolymers due to the utilization of phosphate activators that require lower production energy relative to silicate-containing activators. However, the higher cost of phosphate activators may confine the applications of SAP geopolymers in some exquisite or special fields.

Effect of sodium carbonate/sodium silicate activator on the rheology, geopolymerization and strength of fly ash/slag geopolymer pastes

Abstract: Reactions between the fly ash/slag composite mix and the Na2CO3/Na2SiO3 activator were monitored through Isothermal conduction calorimetry. The resulting products were analyzed with the help of XRF, XRD and FT-IR techniques. In calorimetric response, the composite pastes had more total heat release than the fly ash paste requiring ∼50% less activation energy to yield reaction products. These products were largely amorphous as observed in the XRD patterns. Rheological studies indicated that composite pastes were very stiff above 25 wt% slag addition as its yield stress was almost doubled to fly ash paste. The compressive strength of hardened pastes increased with increasing slag content and activator dosage and decreased with increasing water-binder ratio. The deposition of reaction products onto the fly ash/slag particle surfaces and also the dense microstructures as observed in FESEM supported higher strength of geopolymer pastes at higher activator and slag contents. The developed paste with standard sand at 1:2 ratio produced mortar with a compressive strength of ∼72 MPa.