Main Participants: KONG Xiangzhi, ZHANG Sijia, LI Wenwei, JI Guojin, LIU Yanxia, WU Kui, ZHU Zhenggui, ZHENG Xu, LIU Chen, LIU Chenxia, MA Xiaoxu and ZHANG Yangsan

Fly ash is a solid waste from coal-fired power plants with an annual emission of about 600 million tons. It has been widely used in industrial and civil buildings as a recycled material. High-content fly ash， as one of the necessary components of hydraulic concrete，is often adopted in hydropower projects to solve the thermal cracking of mass hydraulic concrete. . In recent years, with the raising of the national emission standards for nitrogen oxides, denitrification was added to the production process of coal-fired power plants， which led to the residual ammonium salts in fly ash, commonly referred to as "denitrated fly ash". The chemical reaction of concrete mixed with denitrated fly ash would release ammonia gas that adversely affects the engineering quality and endangers the health of operators in severe cases. However, there was still no test method and quality control standard for denitrated fly ash at home and abroad, and its impact on concrete performance is not completely clear, so it was imperative to carry out fly ash application technology and related theoretical research. This project aimed to study the characteristics of denitrated fly ash and the technical problems of its safe application in hydraulic concrete.

·   To reveal the characteristics and reaction mechanism of denitrated fly ash by testing chemical constituents, physical properties, particle gradation and morphology of fly ash from power plants.

·   To test the properties of fresh concrete, hydration products and microstructures of hardened concrete and analyze the mechanical, thermal, deformation properties and durability of concrete  prepared with fly ash of different ammonium content levels.

·   To study the law of ammonia releasing and reveal the influence and mechanism of ammonium salt in fly ash on concrete performance.

·   To study and determine the safety limit of ammonium salt affecting concrete performance, and determine the safety limit of ammonium content to ensure the occupational health of constructors and operators.

·   To establish the quality control system of concrete mixed with denitrated fly ash.

·   Having expounded the mechanism and influencing factors of ammonium salt in denitrated fly ash of thermal power plants, and revealed the influence of residual ammonium on the quality of fly ash, macro- and micro-properties of cement mortar and concrete.

·   Having proposed two test methods of ammonium content in fly ash, which are respectively applicable for indoor accurate test and on-site rapid test, and established the national standard and the enterprise standard of China Three Gorges Corporation.

·   Having developed a device and a method for measuring the whole process of ammonia releasing from concrete mixed with denitration fly ash, and revealed the law and influencing factors of ammonia releasing.

·   Having proposed the safety limits of ammonium content in fly ash: Class A, ammonium content ≤ 200 mg/kg, applicable for any part; Class B, ammonium content ＞ 200 mg/kg, limited use, on-site demonstration to be conducted and necessary measures to be taken.

·   Having established the quality control technology system of concrete mixed with denitrated fly ash, including the test method of ammonium content in fly ash, safety limit and key parameters of concrete quality control.

The research results have been applied to Baihetan hydropower station, Wudongde hydropower station and SETH water control project in Xinjiang. Based on the research results, inspections are required to be done at the exit and entry of fly ash. After on-site inspection of fly ash performance and concrete pouring, it has been confirmed that the concrete meets the design requirements and thus ensures the occupational health of operators. The period from 2018 to 2020 was the peak construction period  of Baihetan hydropower station. In this period, by reducing concrete waste and reducing the loss of parking waiting time for supply vehicles, this technology helped to cut the construction cost of Baihetan Project by about 372 million yuan. The national standard “Limit and test method of ammonium ion content in fly ash” (GB/T 39701-2020) composed based this research has been officially implemented since November 1st, 2021.

Figure 1 Hydration products of concrete mixed with denitrated fly ash

a. Distillation — titration method

b. Electrode method

Figure 2 Test method of ammonium content in denitrated fly ash

Figure 3 Ammonia releasing law of concrete mixed with denitrated fly ash

Figure 4 Quality control system of concrete mixed with denitrated fly ash