Performance of geopolymer concrete under sulfate exposure

Synopsis: As a relatively 'new' material, geopolymer concrete offers the benefits as a construction material for sustainable development. It utilises waste materials such as fly ash and has a very low rate of green house gas emission. This paper presents the study of the performance of fly ash based geopolymer concrete to sulfate attack. Test specimens were soaked in sodium sulfate solution and sulfuric acid solution for various periods of time, and the performance of geopolymer concrete is studied by evaluating the effect on the compressive strength, change in length and change in mass. Test results show that exposure to sodium sulfate has no significant effect on geopolymer concrete, whereas exposure to sulfuric acid affects the compressive strength. Keywords: fly ash, geopolymer concrete, sodium sulfate, sulfuric acid INTRODUCTION The development of environmentally friendly concrete for sustainable development is crucial for continued use of this material. In this respect, geopolymer concrete is emerging as an alternative to Ordinary Portland Cement (OPC) concrete as a construction material for sustainable development. Geopolymer concrete that utilises waste material containing silica (Si) and alumina (Al) such as fly ash is a good alternative because it has a very low rate of greenhouse gas emissions compared to OPC concrete. The term 'geopolymer' was first introduced by Davidovits (1987) to describe a family of mineral binders based on silicoaluminates. This inorganic polymer has a wide range of applications in civil engineering Several laboratory studies have been reported to identify the parameters that influence the properties of geopolymers. Davidovits Paper for Ed Nawy Symposium, American Concrete Institute, April 2005 In the previous studies, the authors [6] reported the results of the research into engineering properties of geopolymer concrete. The concrete mixtures were made using low-calcium class F fly ash, sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) solutions as alkaline activators, and locally available aggregates. It was found that for curing temperatures up to 60 o C, there was a significant increase in compressive strength, while the increase in strength was not significant when the samples were cured above 60 o C. Also, curing for 24 hours increased the compressive strength substantially compared to a 4-hour curing. In addition, it was also shown that as the Na 2 SiO 3 -to-NaOH ratio increased the compressive strength increased significantly. Test results obtained from specimens cured at 60 o C for 24 hours revealed that the compressive strength of geopolymer concrete did not vary with the age of concrete because of the fast polymerisation process of the geopolymer gel EXPERIMENTAL PROGRAM Materials and Mixture Proportions Geopolymer concrete in this study utilised the low calcium (class F) fly ash from Collie Power Station, Western Australia as the source material. Paper for Ed Nawy Symposium, American Concrete Institute, April 2005 The mixture proportion of geopolymer concrete selected for this study was one of the mixtures used in authors' previous research Specimen Preparation The sodium hydroxide flakes were dissolved in distilled water to make a solution with a concentration of 8 M at least one day prior to use. The fly ash and the aggregates were first mixed together in a pan mixer for about 3 minutes. The sodium hydroxide and the sodium silicate solutions were mixed together with superplasticiser and then added to the dry materials and mixed for about 4 minutes. Immediately after mixing, the fresh concrete was cast into 100 x 200 mm cylindrical molds in three layers and 75x75x285 mm prismatic molds in two layers. Each layer was given 60 manual strokes using a rodding bar and vibrated on a vibrating table for 10 seconds. About 30 minutes after casting, the specimens were placed in the oven for curing at 60 o C for chosen periods. After curing, the specimens were left to air-dry in the laboratory. Test Variables and Test Procedures The specimens were soaked in sodium sulfate solution and sulfuric acid solution for intended periods of time and the changes in compressive strength, length and mass were measured. The concentration of the sodium sulfate (Na 2 SO 4 ) in the solution was 5 percent by mass and that of the sulfuric acid (H 2 SO 4 ) was 2 percent by mass [11, Test variables selected for this study are grouped into two series. In the first series, the changes in compressive strength and length of the specimens soaked in sodium sulfate solution were observed. Specimens were cured in the oven at 60 o C for the periods of 24 hours and 48 hours. For the length change tests, three specimens were made for each variable and the changes in length were measured using a horizontal length comparator one hour after removing the specimens from the immersion tank. Four specimens were made for each variable for compressive strength tests. The tests were performed one week after Paper for Ed Nawy Symposium, American Concrete Institute, April 2005 removing the test specimens from sodium sulfate solution and tested in compression in accordance with the relevant Australian Standard for testing concrete. In the second series, the changes in compressive strength and mass of the specimens soaked in sodium sulfate solution and sulfuric acid solution were observed. After each exposure period, the specimens were tested immediately after removing them from the solution. For change of mass tests, four specimens were made for each test variable and the mass was measured using a laboratory scale. Four specimens were made for each variable for compressive strength tests and the tests were performed in accordance with the relevant Australian Standard for testing concrete. For comparison, an additional set of specimens were soaked in tap water. The test variables are summarized in TEST RESULTS Visual Appearance There was no significant change in the external appearance of the surface of specimens soaked in sodium sulfate up to 12 weeks. The same was true for the specimens soaked in tap water. However, the surfaces of specimens soaked in sulfuric acid solution started to erode even after one week of exposure. Compressive Strength Figures 2 and 3 show the variation in the compressive strength of specimens tested one week after removing the specimens from sodium sulfate solution after various weeks of exposure. For both curing periods (24 hours and 48 hours), the variation in compressive strength is not significant compared to the result obtained for the companion specimens left in the laboratory ambient conditions and tested one week after casting. Change in Length Change in Mass The average unit weight of concrete in ambient conditions was 2356 kg/m 3 . This value did not change for specimens soaked in sodium sulfate solution. In the case of specimens soaked in sulfuric acid, the mass decreased less than one percent after 12 weeks. CONCLUDING REMARKS The performance of geopolymer concrete under sulfate exposure has been studied by soaking the specimens in sodium sulfate solution and sulfuric acid solution. After 12 weeks of exposure, by observing the change in compressive strength, change in mass and change in length of the specimens, the results showed that in form of sodium sulfate, sulfate attack did not have significant effect on geopolymer concrete. On the other hand, the sulfate attack in the form of sulfuric acid damaged the surface of the specimens and reduced the compressive strength of geopolymer concrete. Tests are continuing for at least one year in order to substantiate the trends observed so far. ACKNOWLEDGEMENT

Kajian Sifat Mekanik Beton Tailing Pada Pengecoran Dalam Air Dengan Menggunakan Bahan Tambah Sikacrete-w

Tailing merupakan sisa pengolahan tambang (limbah) yang tidak dikelola sehingga hanya ditampung atau dialirkan ke sungai dekat lokasi penambangan. Dampak lingkungan akibat limbah tailing ini dapat dikurangi dengan memanfaatkannya sebagai substitusi semen. Selain penggunaan pipa tremie, bucket, dan sebagainya untuk mengatasi masalah penghanyutan finer element, segregasi akibat pengecoran di bawah air, dapat juga dengan menambahkan bahan additive Sikacrete-W untuk meningkatkan daya ikat antar material penyusun beton. Dalam penelitian ini diselidiki pengaruh Sikacrete-W terhadap sifat mekanik beton tailing yaitu kuat tekan, kuat tarik belah dan kelecakan beton tailing yang dicor di dalam air, dengan kadar tailing 15%, Sikacrete-W 0%, 8%, 10%, 12%, 14%, dan 16% dari berat semen dan dibandingkan dengan nilai kuat tekan dan kuat tarik belah beton tailing pada kondisi normal. Komposisi campuran beton menggunakan metode ACI 211.1.91. Pengujian yang dilakukan adalah uji kuat tekan dan kuat tarik belah dengan menggunakan benda uji berbentuk silinder berukuran 10/20 cm yang dilakukan pada umur perawatan beton 7, 14 dan 28 hari, dan pengujian kuat tarik belah pada umur 28 hari. Perawatan benda uji yang dilakukan adalah perawatan basah dengan direndam dalam air. Hasil penelitian menunjukkan bahwa semakin besar kadar Sikacrete-W yang dipakai dalam pengecoran dalam air relatif menaikkan kuat tekan dan kuat tarik belah beton tailing, namun tidak melebihi konsentrasi optimum Sikacrete-W yaitu 12% dari berat semen. Kuat tekan beton tailing tertinggi yang dicapai pada kondisi underwater-cast concrete dengan menggunakan konsentrasi Sikacrete-W optimum BTKS-12% hanya sebesar 62,22% dari kuat tekan beton tailing pada kondisi normal (non underwater-cast concrete)

Bond strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel