Strength development of concrete with ricehusk ashWord文件下载.docx

1、1. IntroductionIn Uruguay, rice production has had a dramatic increase over the past 10years, becoming the most important crop since 2001; the main use of rice husk is as fuel in the rice paddy milling process. The use of this fuel generates a huge volume of ash. The rice-husk ash （RHA） has no usefu

2、l application, is usually dumped into water streams and causes pollution and contamination of springs. As a result, the use of rice-husk ash has aroused great interest in Uruguay.Rice-husk ash is a mineral admixture for concrete 1 and 2; the behavior of cementitious products varies with the source o

3、f RHA 3 and 4. The basic aim of this study is to investigate the influence of residual RHA from the rice paddy milling industry in Uruguay and RHA produced by controlled incineration from the United States, used for comparison, on strength development of concretes at different ages.2. Experimental p

4、rogramThe following materials were used in the preparation of the concrete specimens: fine aggregate （local natural sand） with maximum aggregate size of 4.75mm; coarse aggregate （crushed granite） with maximum aggregate size of 12.5 Portland Cement type I （normal portland cement）; and superplasticize

5、r based on a sulfonated naphthalene formaldehyde condensate. Two sources of ash were considered; a residual RHA from the unique rice paddy milling industry in Uruguay （UY RHA） and a homogeneous ash produced by controlled incineration from the United States （USA RHA）, for comparison.The residual RHA

6、used for this work was a processed waste dry-milled for the necessary time to obtain a median particle size of 8m, a defined specific surface by nitrogen adsorption 5, and with the maximum activity index according to the ASTM C311-98b. This procedure of optimization is presented in 6. Table 1 shows

7、the chemical composition, physical properties and activity index of the cementitious materials.Table 1. Physical properties and chemical analyses of the cement and RHA used Cement RHA UYUSAPhysical testsSpecific gravity3.142.062.16FinenessSpecific surface, Blaine, m2/kg309Nitrogen adsorption, m2/kg2

8、8,80024,300Setting time, minInitial145Final275Compressive strength, Mpa1-day10.13-day22.87-day33.128-day45.1Chemical Analyses, %Silicon dioxide （SiO2）21.9887.288Aluminium oxide （Al2O3）4.650.15Ferric oxide （Fe2O3）2.270.160.1Calcium oxide （CaO）61.550.550.8Magnesium oxide （MgO）4.270.350.2Manganese oxid

9、e （MnO）Sodium oxide （Na2O）0.111.120.7Potassium oxide （K2O）1.043.602.2Sulphur oxide （SO3）2.190.32Loss on ignition2.306.558.1CompoundsTricalcium silicate C3S44.0Dicalcium silicate C2S29.9Tricalcium aluminate C3A8.5Tetracalcium aluminoferrite C4AF6.9Activity indexASTM C311-98b10092.9392.4Full-size tabl

10、eView Within ArticleChemical analysis indicate that the two ashes are mainly composed of SiO2. The median particle size of the two ashes is the same, and the activity index are similar. X-ray diffraction analysis indicated that the USA RHA can be considered to be non-crystalline RHA; but the UY RHA

11、showed crystalline materials, which were identified as cristobalite. A rapid analytical method to evaluate amorphous silica in the rice husk ashes according to 7 has been used; the percentage of reactive silica contained in the USA RHA was 98.5% and in the UY RHA was 39.55%.A total of 15 concrete mi

12、xes were made; for each RHA, six concrete mixes were made, and three concretes without RHA for comparison.The different mix proportions by mass of the materials used are given in Table 2. The replacement of cement by RHA was made by volume, because the RHA presents less specific gravity than the cem

13、ent Portland, and the paste content in volume was kept the same （35% cement paste content） for the different mix proportions. The values of the slump test are also indicated in Table 2, where superplasticizer percentages are used in relation to weight of cementitious materials. Superplasticizer was

14、used in very low percentages according to the results obtained in the slumps, to allow consistency adjustments （slump=6020mm） without changing the proportion of the other materials.Table 2. Mix proportions of concrete W/（c+RHA） RHA （%） Cement （kg/m3） Fine Agg. （kg/m3） Coarse Agg. （kg/m3） Superplast

15、（%） Slump （mm） 53469010500.4047104810.200.704556204270.80486346272310180.10404160.273700.505365408758983613670.30947932767Cylindrical concrete test specimens were cast. They were compacted by external vibration and kept protected after casting to avoid water evaporation. After 24h they were demolded

16、 and stored in a moist room until the testing date.100200-mm cylinders were used to observe the compressive strength at 7, 28 and 91days. In order to obtain more information about the development of strength of the concretes, splitting tensile tests and air permeability on cylinders of 100200mm and

17、150300mm respectively, with lower and higher water/cementitious materials ratios at the age of 28days, were analysed. Air-permeability for concrete was determined with the “Torrent permeability tester” method 8 and 9. The particular features of the Torrent method are a two-chamber vacuum cell and a

18、pressure regulator, which ensures that air flows at right angles to the surface and is directed towards the inner chamber; this allows the calculation of the permeability coefficient Kt on the basis of a simple theoretical model. By comparing the results 9 of gas permeability measured by the Torrent

19、 permeability tester （Kt） and oxygen permeability obtained for the Cembureau method （K0）, the following relation is presented:where K0 and Kt are expressed in 1016m2.3. Results and discussionTable 3 shows the test results （strength and permeability）. Each value represents the average of five experim

20、ental observations. At lower ages （7days）, concretes with UY RHA present higher compressive strength that concretes with USA RHA. At higher ages （91days）, the RHA concrete had higher compressive strength in comparison with that of concrete without RHA, and the highest values of compressive strengths

21、 were achieved in concretes with 20% USA RHA. The long term compressive strength of the concretes with UY RHA is not as high as the one obtained with USA RHA, which also increases as the RHA content rises.Table 3. Test results w/（cfc （MPa） ft,d （MPa） 28dKt （m2） 28dType%7d28d91d48.455.560.63.631.0810

22、1651.160.464.33.570.2344.354.862.73.340.0539.551.464.53.620.0830.547.468.53.540.0335.842.345.641.150.454.927.940.729.740.851.523.639.457.324.632.935.92.8528.2024.131.535.52.3271.8224.934.937.92.6349.1022.734.544.42.9226.3620.852.93.0014.20Keys: fcaxial compressive strength; ft,dsplitting tensile str

23、ength; Ktpermeability coefficient.The results of splitting tensile strength and air permeability reveal the significance of the filler and pozzolanic effect for the concretes with RHA. On the one hand, the results are consistent with the compressive strength development at 28days for the USA RHA. On

24、 the other hand, in the concretes with UY RHA, lower splitting tensile strengths and less air permeability are observed, which can be due to the fact that with residual RHA, the filler effect of the smaller particles in the mixture is higher than the pozzolanic effect.4. ConclusionsThe RHA concrete had higher