Fly Ash as an Alternative Stabiliser for Road Pavement Materials: a case study in South Africa

Abstract

Fly Ash is a by-product at thermal power stations, otherwise known as residues of fine particles that rise with flue gases. Coal is pulverised and blown with air into a boiler’s combustion chamber. Fly Ash is the lighter finer ash particles that remain suspended in the flue gas. Fly Ash consists of silt sized particles which vary in size from 0,5 micron to 100 micron. The Fly Ash is spherical in shape and the size distribution makes it an acceptable mineral filler in various engineering applications. The Fly Ash varies in color and this is contributed by chemical and mineral constituents. Fly Ash has high amounts of silicone dioxide and calcium oxide which creates a by-product which is very cementitious, and thus Fly Ash has a pozzolanic property. Eskom is planning to expand its coal powered generated capacities, thus the quantity of Fly Ash dumps will increase. Replacement of natural soils or minimisation of the use is desirable. An industrial by-product may be inferior to the traditional materials used in road pavement construction. However, the lower cost makes the by-products an attractive alternative if the required performance can be achieved. By-products throughout the world are often used to enhance properties of traditional road pavement construction materials, and the combination generates a material with well-controlled and superior properties. It is in the context of this study to produce results to further motivate the use of by-products, such as Fly Ash, in road construction. Detailed design and investigations were completed to evaluate the use of Fly Ash as a suitable replacement for cement or partial replacement to reduce landfill sites and in the effect become an environmental option for road construction is South Africa. To understand Fly Ash, physical, chemical and mineralogical characteristics need to be understood before any further testing could be completed. Fly Ash is classified according to the sum of total aggregate Alumina, Silica and Ferric Oxide. Two classes are identified: Class C and Class F. If the sum is between 50% and 70%, it is a Class C. If the sum is greater than 70%, it’s a Class F. Three (3) Fly Ashes have been sourced for this study namely: DURAPOZZ, POZZFILL and Dump Ash. Two Fly Ashes are air classified Fly Ash and the other was sampled directly from the dump sites. All three Fly Ashes have been classified as Class F, as the sum was greater than 70% with DURAPOZZ 91.88%, POZZFILL 88.5% and Kendal Dump Ash 86.77%. Although the Fly Ashes have been classified as Class F, it is still known as a pozzolanic material, therefore the Fly Ash requires a reaction agent in the form of cement or lime to kick start the pozzolanic reaction. XRF tests were completed on the Fly Ashes to evaluate the elements that make up the Fly Ash particles. The study showed that all three Fly Ashes have a high Silica (SiO2) content, which forms a cementitous compound reaction, but it was found that the ratio between Lime (CaO/SiO2) is low therefore showing that a cementing agent will be required. The Loss of Ignition (LOI) was also taken into consideration as it is a critical characteristic of Fly Ash. LOI is measurement of unburnt coal and high amounts could lead to entrainment problems and affect the durability. The test results revealed that the Fly Ashes conformed to the specification and are below the required 5% allowable. The gradation of the Fly Ashes also showed that DURAPOZZ and POZZFILL will react slowly over time. The Kendal Dump Ash will also react slowly over time but also has material that will become inert, which will add to the property of the material and not to the reaction process. The Fly Ashes have shown in this study that it has enough pozzolanic material available to continue with slow reactions which will occur continuously over time. Environment is a concern when using Fly Ash in construction projects. Fly Ash is a residue generated by coal combustion, but due to potentially toxic elements found in the Fly Ash, it is considered harmful to the environment. To evaluate the potential harmful effects to the environment, the stabilised Fly Ash samples were subjected to leach tests. The chemical and physical properties of Fly Ash enabled it to be utilised to limit the environmental damage. The study revealed that using cement with Fly Ash achieves alkalinity and reduces the harmful leach elements to the environment. The pH values of the stabilised material varied between 10.29 and 10.82, which show that the material is alkaline. This will ensure continuous reactions, and the leach ability of certain elements will be decreased over a period. The pH values are also not high and this will reduce the leaching of arsenic over time. The Fly Ash leach samples were compared to the maximum allowed elements found in drinking water. The Leach results have shown that the material was “entombed” and the possibility of leachant releasing agents of a dangerous nature are minimal. Cementing agent chosen for this study were two types of cement, developed by two different suppliers specially developed for stabilisation purposes namely; LAFARGE CEM II 32,5 VA(S-V) and AFRISAM CEM II 32,5 B-M(S-V). The cement is more effective for soils of low clay content, to gain early strengths. Fly Ash has a low early strength gain but continues to gain strength slowly over a longer period. General guidelines in South Africa are utilised to obtain desired properties of soil for design purposes. Classified G5 material was used in this study, due to that the material has been found to be coarse, and this will also be able to carry much heavier loads without deformation. The material was subjected to further testing for indication of any rapid weathering materials after exposure to the atmosphere. The G5 was subjected to Ethylene Glycol and Durability Mill Index tests. The material was found to be durable and suitable for the purpose of design and lifespan. Basic design steps were used as normal, followed during a construction program. Rapid methods of testing was completed, which are currently used in a construction project, and the reactions of the Fly Ash can be evaluated to see if it conforms to the current standards. Reference samples were completed by stabilizing the G5 with 1% cement respectively. All stabilised Fly Ash tests were compared to the standards and to the reference samples. This was to evaluate the impact Fly Ash has during a stabilisation project. Initial Consumption of Cement (ICC) was completed, but due to the poor quality of Fly Ash, it was expected that no constant results will be shown. Material stabilised with cement alone showed that the material will stabilise between 4% cement and 5% cement for both types of cement. This indicated that substantial amount of Fly Ash will be required to meet the demand of the material and to ensure proper cementation and durability. The G5 material was thus stabilised with the Fly Ash mixtures containing 18%, 20%, 22% and 24% respectively. Unconfined Compressive Strength (UCS) and Indirect Tensile Strength (ITS) tests were completed on the mixtures and compared to the reference samples and to the required standard. All the results have shown an improvement to the UCS which varied from 1759kPa to 3830kPa when compared to the reference samples. The same can be seen for the ITS, even though the Dump Ash mixtures decreased the ITS values. The evaluation of the UCS and ITS results showed that lower Fly Ash percentages could be used for projects. The total amount of samples was thus reduced to, namely; 1% LAFARGE mixed with 16% Dump Ash, 1% LAFARGE mixed with 16% POZZFILL, 1% LAFARGE mixed with 16% DURAPOZZ, 1% AFRISAM mixed with 18% Dump Ash, 1% AFRISAM mixed with 18% POZZFILL, 1% AFRISAM mixed with 18% DURAPOZZ. Further additional testing is required to ensure that the stabilised material will be adequate for the design life of the pavement. The main aim of durability testing is to ensure that the material durability will last for the duration of its design life, and to indicate whether the material will withstand natural forces if the top layers have failed, thus leaving stabilised layers exposed to the natural elements. The additional testing is evaluated by 3 tests, namely; Wet Dry Durability (WDD), CSIR Erosion Test, and Triaxial Tests. WDD is a specified test and can be found in most contract documents while the CSIR Erosion test is not. The referenced samples conformed to C4 classification with % loss of 20.1% and 23.7% respectively. Out of the 6 samples that were mixed with Fly Ash, 2 conformed to C4, 3 conformed to C3 and 1 totally failed the WDD test. The results indicate that the mixtures will have sufficient durability. The CSIR Erosion Test is not seen as a proper durability test for gravel road testing and is mostly used by researches and designers on asphalt surfacing, seal, and concrete overlays. The results do confirm this, as the results do not conform to the required specification. Triaxial tests were completed to calculate the safety factor of the material to deformation. The safety factor concept developed form the Mohr-Coulomb theory represents the ration of shear strength divided by the applied stress causing shear. Safety factor of above 1 was required to ensure deformation of a gradual nature and not a rapid failure in a short time. These were achieved well above the norm when compared to reference samples. The poor quality of Fly Ash has shown that no uniformity of the results could be seen nor can it be predicted, thus all testing needs to be completed thoroughly including durability testing. Most of the results conform to the standard testing available for durability and normal design process testing. The standards followed in this study showed the effectiveness of the Fly Ash mixtures when used as a stabilisation option.