Experimental Investigation on Fresh and Hardened Properties of High Calcium Flyash Based Geopolymer Concrete

The most important cause of the climate changes in the past few decades are due to the emission of CO2.It may be due to human or natural processes such as disposal of waste material from the thermal power plant, consuming natural resources or production of cement etc. Due to increase in infrastructure created the demand of more construction industries. Increasing importance of environmental protection and energy storage has led to the investigation of alternative binders to replace the cement. Geopolymers are an alternative binder for cement concrete production because of their superior mechanical properties. In the present investigation, for developing the Geopolymer concrete (GPC), high calcium fly ash is used as an alternative binder with Na2SiO3 and NaOH as alkaline liquids. Fresh and hardened properties of GPC are examined by appropriate experiments. Alkaline liquid to High calcium Fly ash ratio (AL: HCF) of 0.45, 0.55,0.6 and 0.65 are used with 8M of NaOH and the developed GPC is kept in ambient curing for 7 days, 28 days, and 56 days. It was observed that with an increase of AL to HCF ratio in the fresh GPC increased the workability of GPC. Increase of AL to HCF ratio in GPC mix increased the compressive strength, tensile strength and flexural strength up to a certain limit.

Durability of Mortars with Fly Ash Subject to Freezing and Thawing Cycles and Sulfate Attack

Destruction of cement composites occurs due to the alternate or simultaneous effects of aggressive media, resulting in the destruction of concrete under the influence of chemical and physical factors. This article presents the results of changes in the measurement of linear strains of samples and changes in the microstructure of cement after 30 freezing and thawing cycles and immersed in 5% sodium sulfate solution. The compressive strengths ratios were carried out at the moment when the samples were moved to the sulfate solution after 30 cycles and at the end of the study when the samples showed visual signs of damage caused by the effect of 5% Na2SO4. The composition of the mixtures was selected based on the Gibbs triangle covering the area up to 40% replacement of Portland cement with low and high-calcium fly ashes or their mixture. Air-entrained and non-air entrained mortars were made of OPC, in which 20%, 26.6%, and 40% of Portland cement were replaced with low and/or high-calcium fly ash. Initial, freezing and thawing cycles accelerated the destruction of non- air-entrained cement mortars immersed in 5% sodium sulfate solution. The sulfate resistance, after the preceding frost damage, decreased along with the increase in the amount of replaced fly ash in the binder. Air-entrained mortars in which 20% of cement was replaced with high-calcium fly ash showed the best resistance to the action of sodium sulfate after 30 freezing and thawing cycles.

Influence of Bentonite on Mechanical and Durability Properties of High-Calcium Fly Ash Geopolymer Concrete with Natural and Recycled Aggregates

In this study, bentonite (a naturally occurring pozzolana) was incorporated as a partial replacement (up to 20%) for high-calcium fly ash (HCFA)-based geopolymeric natural aggregate concrete (GNAC) and geopolymeric recycled aggregate concrete (GRAC). The mechanical (compressive strength and splitting tensile strength), durability (chloride migration coefficient, water absorption, and acid attack resistance), and rheological properties (slump test, fresh density, and workability) were investigated. The results revealed that incorporation of bentonite (10 wt % with ordinary Portland cement) showed appreciable improvement in the strength and durability of both the GNAC and GRAC, though its effect is more significant for GRAC than the GNAC.

Resistance to Sulfuric Acid Corrosion of Geopolymer Concrete Based on Different Binding Materials and Alkali Concentrations

Geopolymer binder is expected to be an optimum alternative to Portland cement due to its excellent engineering properties of high strength, acid corrosion resistance, low permeability, good chemical resistance, and excellent fire resistance. To study the sulfuric acid corrosion resistance of geopolymer concrete (GPC) with different binding materials and concentrations of sodium hydroxide solution (NaOH), metakaolin, high-calcium fly ash, and low-calcium fly ash were chosen as binding materials of GPC for the geopolymerization process. A mixture of sodium silicate solution (Na2SiO3) and NaOH solution with different concentrations (8 M and 12 M) was selected as the alkaline activator with a ratio (Na2SiO3/NaOH) of 1.5. GPC specimens were immersed in the sulfuric acid solution with the pH value of 1 for 6 days and then naturally dried for 1 day until 98 days. The macroscopic properties of GPC were characterized by visual appearance, compressive strength, mass loss, and neutralization depth. The materials were characterized by SEM, XRD, and FTIR. The results indicated that at the immersion time of 28 d, the compressive strength of two types of fly ash-based GPC increased to some extent due to the presence of gypsum, but this phenomenon was not observed in metakaolin-based GPC. After 98 d of immersion, the residual strength of fly ash based GPC was still higher, which reached more than 25 MPa, while the metakaolin-based GPC failed. Furthermore, due to the rigid 3D networks of aluminosilicate in fly ash-based GPC, the mass of all GPC decreased slightly during the immersion period, and then tended to be stable in the later period. On the contrary, in metakaolin-based GPC, the incomplete geopolymerization led to the compressive strength being too low to meet the application of practical engineering. In addition, the compressive strength of GPC activated by 12 M NaOH was higher than the GPC activated by 8 M NaOH, which is owing to the formation of gel depended on the concentration of alkali OH ion, low NaOH concentration weakened chemical reaction, and reduced compressive strength. Additionally, according to the testing results of neutralization depth, the neutralization depth of high-calcium fly ash-based GPC activated by 12 M NaOH suffered acid attack for 98 d was only 6.9 mm, which is the minimum value. Therefore, the best performance was observed in GPC prepared with high-calcium fly ash and 12 M NaOH solution, which is attributed to gypsum crystals that block the pores of the specimen and improve the microstructure of GPC, inhibiting further corrosion of sulfuric acid.