Graduate student (PhD), Missouri university of science and technology
Developing a concrete by combining high calcium fly ash with alkali (sodium (hydroxide + silicate))
Alkali activated fly ash concrete is a concrete without cement, where the cement is replaced totally with the fly ash in the concrete. The production of 1 ton of cement release about 0.8-1.0 ton CO2 to the atmosphere which present around 5 % of the total man-made CO2 emission in the world. In addition, the large amount of the consumed electricity for this production. In the other hand, the fly ash is a by product from the coal power plants. So, there are huge amount of the fly ash that are produced every year. Approximately 750 million tonnes of the fly ash is produced annually around the worldwide, with only a recycling rate of 25%. Throwing the fly ash in the landfills utilizes large areas and causes soil and groundwater contamination. Throughout my research, we are using the fly ash as the main binder in the concrete rather than the cement. Therefore, this will find a usage for the fly ash and avoid the problems that caused by the production of the cement. The fly ash types are two. First, that has high calcium content (10-25%) which is classified as class C. Second, that has low calcium content (<10%) which is classified as class F. In Missouri State, USA, fly ash class C is the major produced one by the local power plants. So, my research is focused on on fly ash class C from the different sources across the State. The sodium silicate and sodium hydroxide were used as the alkali materials. In order to producing concrete with acceptable setting time, workability, and compressive strength, four parameter were investigated in this study. The water to fly ash ratio, alkali materials to fly ash ratio, sodium silicate to sodium hydroxide ratio are studied. In addition, two different curing methods were applied, the elevated heat curing at 70 C (158 F) in an electrical oven for 24 hr, and ambient temperature curing for 7 and 28 days. The results revealed that the alkali activated concrete is a good candidate for replacing the conventional concrete.The results also revealed that the elevated and ambient curing temperatures can be used as curing methods for the alkali activated concrete which will increase the applications for that concrete. The optimum sodium silicate to sodium hydroxide ratio was 1.0.
Abstract: Publication date: Available online 18 October 2016 Source:Journal of Building Engineering Author(s): Anwar Hosan, Sharany Haque, Faiz Shaikh This paper presents the effects of sodium and potassium based activators on compressive strengths and physical changes of class F fly ash geopolymer exposed to elevated temperatures. Samples were heated at 200°C, 400°C, 600°C and 800°C to evaluate the residual compressive strength after 28 days of curing. The fly ash geopolymer were synthesized with combined sodium silicate and sodium hydroxide solutions and potassium silicate and potassium hydroxide solutions by varying mass ratios of Na2SiO3/NaOH and K2SiO3/KOH of 2, 2.5 and 3. Results show significant improvement is compressive strength in the case of Na2SiO3/NaOH ratio of 3 than 2 and 2.5, where the residual compressive strengths are increased up to 600°C. Better results on the geopolymer synthesized with potassium based activators are obtained where the residual compressive strength up to 600°C are much higher than their sodium based counterparts. It is also found that the fly ash geopolymer synthesized with potassium based activators is more stable at elevated temperatures than its sodium based counterparts in terms of higher residual compressive strengths, lower mass loss, lower volumetric shrinkage and lower cracking damage. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results of sodium and potassium activator synthesized fly ash geopolymer also corresponds to the measured residual compressive strengths.
Pub.: 28 Oct '16, Pinned: 30 Jun '17
Abstract: In this paper, the mechanical performance of fly ash and Portland cement geopolymer activated with sodium hydroxide and sodium silicate solutions was studied. The Geopolymer Mortars (GM) were made from high calcium Fly Ash (FA) and ordinary Portland Cement (PC) with FA:PC weight ratios of 100:0, 95:5, 90:10, 85:15, and 80:20. The GMs were activated with three combinations of sodium Hydroxide Solution (SH) and sodium Silicate Solution (SS) viz., SH, SH+SS (SH:SS=2) and SS. For all mixes, 10 molar SH, alkali activator liquid/solid binder ratio of 0.60 and curing at ambient temperature of 25oC were used. The result indicated that the compressive and shear bond strengths of GM depended on the alkali activators used and the amount of PC. The use of SH and SHSS resulted in the formation of additional Calcium Silicate Hydrate (CSH) which coexisted with sodium aluminosilicate hydrate (NASH) gel. Whereas, the use of SS resulted in NASH gel with only a small amount of CSH. The increasing of PC content enhanced the compressive and shear bond strengths of GMs due to the formation of additional CSH. The 15% PC mixed with SHSS gave the optimum compressive and shear bond strengths.
Pub.: 25 Oct '16, Pinned: 30 Jun '17
Abstract: Processes that maximize utilization of industrial solid wastes are greatly needed. Sodium hydroxide and sodium silicate solution were used to create alkali-activated complex binders (AACBs) from class C fly ash (CFA) and other Ca-containing admixtures including Portland cement (PC), flue gas desulfurization gypsum (FGDG), and water treatment residual (WTR). Specimens made only from CFA (CFA100), or the same fly ash mixed with 40 wt% PC (CFA60-PC40), with 10 wt% FGDG (CFA90-FGDG10), or with 10 wt% WTR (CFA90-WTR10) had better mechanical performance compared to binders using other mix ratios. The maximum compressive strength of specimens reached 80.0 MPa. Geopolymeric gel, sodium polysilicate zeolite, and hydrated products coexist when AACB reactions occur. Ca from CFA, PC, and WTR precipitated as Ca(OH)(2), bonded in geopolymers to obtain charge balance, or reacted with dissolved silicate and aluminate species to form calcium silicate hydrate (C-S-H) gel. However, Ca from FGDG probably reacted with dissolved silicate and aluminate species to form ettringite. Utilization of CFA and Ca-containing admixtures in AACB is feasible. These binders may be widely utilized in various applications such as in building materials and for solidification/stabilization of other wastes, thus making the wastes more environmentally benign.
Pub.: 22 Sep '09, Pinned: 30 Jun '17
Abstract: Publication date: December 2016 Source:Cement and Concrete Research, Volume 90 Author(s): O.G. Rivera, W.R. Long, C.A. Weiss Jr., R.D. Moser, B.A. Williams, K. Torres-Cancel, E.R. Gore, P.G. Allison This research focused on developing thermally-stable materials based on alkali-activation of slag, fly ash, and metakaolin compared to portland cement mixtures by using a hierarchical approach to material design. At lower length scales, X-ray diffraction (XRD) characterized the mineralogy that coupled to higher length scale experiments using thermogravimetric analysis (TGA) for determining the materials thermal stability. Additionally, high-energy X-ray computed microtomography (μCT) determined the best-performing material formulation that minimized thermal damage when exposed to high temperatures (650°C). The thermal loading was ramped up to 650°C from ambient temperature in 60s and then held for a total of 10min. The μCT identified that the alkali-activated fly ash mortar had less initial porosity than the ordinary portland cement mixtures, with more than 66% of the pores between 20 and 50μm in diameter. Consequently, the alkali-activated fly ash mortar was able to dissipate approximately 565°C in just 50mm of material, outperforming all the other mixes studied in this paper with μCT confirming minimal damage after the temperature exposure.
Pub.: 23 Sep '16, Pinned: 30 Jun '17
Abstract: This paper presents a comparative environmental assessment of several different green concrete mixes for structural use. Four green concrete mixes were compared with a conventional concrete mix: recycled aggregate concrete with a cement binder, high-volume fly ash concrete with natural and recycled aggregates, and alkali activated fly ash concrete with natural aggregates. All five concrete mixes were designed and experimentally verified to have equal compressive strength and workability. An attributional life cycle assessment, based on the scenario which included construction practice, transport distances, and materials available in Serbia, was performed. When treating fly ash impacts, three allocation procedures were compared: ‘no allocation’, economic, and mass allocation, with mass allocation giving unreasonably high impacts of fly ash. Normalization and aggregation of indicators was performed and the impact of each concrete mix was expressed through a global sustainability indicator. A sensitivity analysis was also performed to evaluate the influence of possibly different carbonation resistance and long-term deformational behavior on the functional unit. In this specific case study, regardless of the choice of the functional unit, the best overall environmental performance was shown by the alkali activated fly ash concrete mix with natural aggregates and the high-volume fly ash recycled aggregate concrete mix. The worst performance was shown by the recycled aggregate concrete mix with a cement binder.
Pub.: 05 Apr '17, Pinned: 30 Jun '17
Abstract: Utilization of the fly ashes is a major problem in many developing countries and in South Africa only about 7% of the fly ash produced annually by coal-fired power stations, has being utilized. Although, fly ashes can be used as an alternative binder in alkali-activated concretes, strength development of these concretes at room temperature is slow limiting application of the material. Direct electric curing is proposed for heat curing of alkali-activated fly ash concrete which will open new opportunities for in-situ applications of these concretes in the construction industry thus increasing the amount of beneficially utilized fly ash. Alkali-activated fly ash concretes containing unclassified low calcium fly ash, sodium hydroxide and sodium silicate solutions were cured at 60 ºC by means of direct electric curing. The electric resistivity and compressive strength development of the concretes were investigated. The resistivity strongly depends on the type of activator used. Compressive strength up to 33.8 MPa and 48.5 MPa at 2 and 28 days respectively, can be achieved after a short period of direct electric curing. This opens new opportunities for wider application of alkali-activated fly ash concretes and for more extensive utilization of fly ash.
Pub.: 25 May '16, Pinned: 30 Jun '17
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