Mechanical and Durability Studies of Geopolymer Concrete with Combination of Alkaline Activators

Abstract

Bottom ash and fly ash both are produced as waste residue from thermal power plants. Fly ash is used partially in concrete as replacement of cement and also for block production. However, bottom ash is not as effectively utilised as fly ash and is disposed of as land fill creating an environmental hazard. As production of cement is not environmentally sustainable and due to large production of industrial waste, one of the methods to have sustainable concrete is by alkali activation. In alkali activation industrial waste and alkaline activators are reacted to produce a long chain of molecules which give strength by process of polymerisation. Activators are generally expensive and play a very pivotal role in the strength development. In first phase, behaviour of geopolymer paste with fly ash and bottom ash as source material in 50:50 proportion was studied by activating with different activator solutions. Sodium hydroxide and sodium silicate, potassium hydroxide and potassium silicate and a combination of sodium hydroxide, potassium silicate and potassium hydroxide, sodium silicate were investigated for physical properties like standard consistency, initial setting time and final setting time. Mechanical property of geopolymer paste like compressive strength was studied by varying solution ratio as 1.5, 2 and 2.5. Effect of temperature on compressive strength of geopolymer paste subjected to both ambient and temperature curing was also investigated. It was found that compressive strength was highest when the ratio of silicate to hydroxide was 2.5 for both ambient and temperature curing at 70°C. It was also observed that for geopolymer paste, both sodium hydroxide and sodium silicate and potassium hydroxide and sodium silicate gave nearly equal compressive strength while least strength was obtained for the combination of sodium hydroxide and potassium silicate. Heat curing gave more strength at 7 days but at 28 days strength of ambient and heat cured samples were approximately the same. Scanning Electron Microscope images also indicate dense polymerisation with sodium silicate activators.

In the second phase, based on the work carried out on geopolymer paste one combination of activators, potassium hydroxide and sodium silicate was selected as activator for geopolymer concrete and the trial mixes were carried out to achieve M25 target strength. It was observed that with increase in molarity, increase in amount of fly ash and increase in sodium silicate to potassium hydroxide ratio, the compressive strength of geopolymer concrete increases. After doing parametric study, the mix containing parameters 12M, solution ratio 2.5 and fly ash & bottom ash ratio 50:50 gave M25 target strength and hence was selected for further studies.

Mechanical properties like compressive strength, split tensile strength, flexural strength, modulus of elasticity and pull out tests were carried out for geopolymer concrete. Geopolymer concrete was also investigated for durability properties like, acid attack, sulphur attack, chloride attack, sorptivity, water absorption, water impermeability, accelerated corrosion, accelerated carbonation, rapid chloride penetration test (RCPT) and fire exposure at 500°C, 700°C, 900°C for 1 hour duration. To investigate flexural behaviour of reinforced geopolymer concrete beam, the beams were cast and were tested under two point load. The strain gauges and LVDTs were attached to capture strain and deflection at various stages of loadings respectively. Comparison of reinforced geopolymer beam and reinforced ordinary portland cement concrete beams was done.

Comparative study of mechanical and durability properties of fly ash and bottom ash activated with different activators was carried out. Activators used for activation were sodium based hydroxides and silicates (NN), potassium based hydroxides and silicates (KK) and combination of potassium hydroxide and sodium silicate (KN). The combination of NN gave excellent results in all mechanical tests among three combinations. Percentage loss in compressive strength under acid attack and sulphate attack was highest for KN combination. Also depth of water impermeability was highest and it recorded high rate of initial sorptivity for above combination. For chloride attack the percentage reduction in compressive strength was highest for NN. Quality of concrete derived from RCPT test is moderate for all three type of combination. Depth of carbonation for accelerated carbonation was highest for KN combination and least for NN combinations. The KN combination performed very well in accelerated corrosion test. It took more time to initiate first crack and half-cell potential readings and resistivity meter reading after accelerated corrosion were good compared to other two combinations. After the fire exposure reduction in compressive strength was more for KK combination and reduction in flexural and split tensile strength was highest for KN combination. The reinforced cement concrete beams performed well compared to reinforced geopolymer beams. The work related to sodium hydroxide and sodium silicate, potassium hydroxide and potassium silicate was carried out in past and results were taken, which were reported.