A pinboard by
Eslam Gomaa

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.


Compressive behaviour of sodium and potassium activators synthetized fly ash geopolymer at elevated temperatures: a comparative study

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

Effect of elevated temperature on alkali-activated geopolymeric binders compared to portland cement-based binders

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

Environmental assessment of green concretes for structural use

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