Effects of high LOI fly ash with different calcium oxide contents on properties of mortars

In 2015, Vietnam had around 25 coal-fired thermal power plants with an annual coal consumption of about 39 million tons and emitting 11 million tons of ash and slag, which would continue to increase in the future. The amount of fly ash dumped is one of the main reasons for the polluted environment, especially for the lives of people around coal power plants. One of the solutions for this waste is applying fly ash as a mineral admixture in mortar or concrete. However, the quality of Vietnamese fly ash is not uniform due to the various sources, and it mainly has a high LOI content. High LOI fly ash has been treated before using it in concrete to reduce the unburned carbon content. The process of eliminating the unburned carbon (UC) in fly ash wastes time, effort, and money. For that reason, the study of the efficient use of untreated high LOI fly ash in concrete is necessary. The literature review of previous research about the influences of LOI amount in fly ash on concrete showed outstanding results in improving some properties of concrete with proper fly ash replacement. Nevertheless, the type of fly ash used had CaO content higher than 10%, which is type 2b in TIS 2135 standard and type C in the Vietnamese standard (TCVN). The influences of high LOI fly ash on the characteristics of concrete with different CaO contents are still unclear. When analyzing and synthesizing the CaO content of Vietnamese fly ash, the results show that the range of CaO in Vietnamese fly ash is very diverse, from low (0.7%) to very high (27.1%). For that reason, this thesis aims to research the effects of high LOI fly ash with various CaO contents on the properties of mortars, including basic, mechanical, and durability. The artificial high LOI fly ash with the desired LOI content was prepared by adding powdered activated carbon (PAC) to the original low LOI fly ashes from BLCP and Mae Moh power plants in Thailand. The PAC used in this study has particle shape, pore structure, and particle size distribution similar to the unburned carbon (UC) in the original high LOI Vietnamese fly ash. The artificial high LOI fly ashes have various CaO contents from low (1.31%) to very high (28.08%) according to ASTM standards and have a %LOI of 6, 12, 18, and 30. The amount of fly ash to substitute cement is 20% and 30%. The influences of LOI content in fly ash on characteristics of fresh mortar, such as flow, water requirement, and superplasticizer amount used, were conducted. The research focused on the strength index and durability as well. The effects of high LOI fly ash with different CaO contents on mechanical properties were performed in three cases: controlled flow by adjusting water, controlled flow by using a superplasticizer, and controlled water-to-binder ratio at 0.5. The durability characteristics of mortars investigated in the research are autogenous shrinkage, total shrinkage, and rapid chloride penetration test (RCPT). Because the high LOI fly ash applied in this thesis contains porous and irregular particles due to the added PAC particles, it increases the water requirement of the mixtures and superplasticizer amount used; in contrast, its flow decreases. The water requirement of Mae Moh fly ash mortar is more sensitive than that of BLCP fly ash mortar when the LOI content increases. The strength index of high LOI fly ash mortars decreases in the case of controlled flow by modifying water. However, in the case of the controlled flow using a superplasticizer and controlled water-to-binder ratio, when the LOI increases, the strength index increases, reaches its highest value, and decreases when LOI becomes too high. The LOI content that produces the highest strength index differs according to fly ash’s CaO contents. The largest values of LOI content in fly ash which makes a strength index not lower than that of the original fly ash (maximum allowable LOI) differs for fly ash with different CaO contents. The maximum allowable LOIs of the fly ash having low, high, and very high CaO contents are 25, 12, and 18%, respectively, in case of controlled flow using a superplasticizer; and 18, 6, and 9% in case of controlled water-to-binder ratio. This proved that in term of strength, high LOI fly ash with a %LOI higher than 6% can be used without treatment.The effects of LOI content on the chloride resistances of fly ash mortars show worse performance when the LOI content increases. Nevertheless, they are still better than the OPC-only mixture. One of the findings demonstrating the high LOI fly ash’s capacity for internal curing is the drastically reduced autogenous shrinkage. Higher CaO content has a higher autogenous shrinkage due to faster reactions. With a growth in fly ash’s %LOI, the total shrinkage of high LOI fly ash mortar steadily rises. However, we also found the maximum values of LOI content in fly ash to produce lower total shrinkage than the OPC mix. The maximum allowable LOIs of the fly ash having low, high, and very high CaO contents are 15, 15, and 7%, respectively. The sensitivity to change in LOI content in fly ash on total shrinkage of high LOI fly ash mortar increases when the CaO content in fly ash increases. Based on the results of the maximum LOI that produces an equal strength index and durability to the original fly ash and OPC mixtures, the highest LOI contents for low CaO, high CaO, and very high CaO fly ashes allowed to be used in mortar when controlled flow by using superplasticizer are 15, 12, and 7%, respectively. In conclusion, lower CaO content fly ash has higher allowable LOI contents.