The Relationship between Water Absorption and Porosity for Geopolymer Paste

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This paper presents an experimental study of the relationship between water absorption and porosity for geopolymer paste. In order to reduce the carbon dioxide (CO2) emissions to the environment, alternatively fly ash was used as binders in making concrete paste. Fly ash is a waste materials produced by combustion of coal at power plant. Geopolymer paste prepared from class F fly ash was obtained from coal power plant and mixed with an alkaline activator. The combination of sodium silicate (Na2SiO3) solution and sodium hydroxide (NaOH) solution were used as alkaline activator. The alkaline activator was prepared 24 hrs prior to use with the ratio of the mixture of Na2SiO3/NaOH is 2.5. Mixed fly ash and alkaline activator placed in moulds and compacted. The samples were kept at ambient temperature in the moulds until it hardened. All the samples were removed from the moulds after 24 hrs. Then, the samples were cured at 60 °C in the oven for 24 hrs. All twelve samples were prepared for water absorption and porosity measurement. The samples were examined after 7, 14, 28 and 90 days in terms of water absorption test and porosity test. It was observed that after day 90 the sample had the lowest water absorption of 3.81% and porosity 3.77%. The sample had the highest water absorption and porosity of 4.65% and 11.95% respectively at day 7. The pore size decreases and the structures became denser. The morphological structure of geopolymer paste pores can be observed by Scanning Electron Microscope (SEM). The porosity and permeability also decreased hence the durability potentially improved. Keywords: Water absorption, porosity, geopolymer, fly ash