IntroductionAlkali-activated fly ash cementitious materials are characterized by several advantageous properties, including corrosion resistance, high-temperature resistance, and the ability to solidify heavy metals. Additionally, these materials demonstrate a high utilization rate of fly ash and are produced through an environmentally friendly process. Despite these benefits, a significant limitation is their slow solidification and hardening at room temperature, which restricts their practical engineering applications. Consequently, alkali-seed composite activated solutions were used to prepare alkali-seed composite excitation fly ash (ASAFA) specimens, and their properties and structural evolution were explored. The results indicated that N-A-S-H gel and zeolite (sodalite and chabazite-Na) were the main components of alkali-seed composite activated solution, and their proportion played a crucial role in determining the excitation effect. Through adjustments in the activity of raw materials, reaction temperature and duration, it is possible to increase the total content and proportion of N-A-S-H gel and zeolite in the seed, thereby encouraging accelerated and uniform growth of the reaction products within the ASAFA system, and reducing the proportion of harmful pores in the hardened structure. As a result, the setting and hardening process and compressive strength of ASAFA was promoted.MethodsThe study utilized first-class fly ash (FA1) with a specific surface area of 430 m²/kg as the primary raw material. FA1 was subjected to sorting-roller grinding to achieve specific surface areas of 715 m²/kg and 1014 m²/kg, resulting in the production of FA2 and FA3, respectively. The activity of the three types of fly ash was assessed in accordance with GB/T 1596–2005 "Fly Ash Used in Cement and Concrete." The results indicated that the mechanical activation from the grinding process significantly enhanced the activity of FA2 and FA3 at all ages compared to FA1, with activity values at 28 days reaching 92% and 114%, respectively. An 8 mol/L NaOH solution was employed as the alkali activator for the synthesis process.The alkali-seed composite activated solution was prepared by treating fly ash with the NaOH solution at elevated temperatures. During the preparation, four technological parameters were systematically adjusted: ① Type of fly ash used; ② Mass ratio of fly ash to NaOH solution; ③ Synthesis temperature; And ④ synthesis time. The ASAFA specimen was created using the alkali-seed composite activated solution in conjunction with FA1. For comparative purposes, a reference specimen (GB) was prepared using a mass ratio of 0.5:1.0 of NaOH solution and FA1. Cubic specimens measuring 40 mm × 40 mm × 40 mm were fabricated and cured at a temperature of (20 ± 3) ℃ and 60% relative humidity. Following the initial curing period, the specimens were demoulded and subjected to additional curing for 7 and 28 days prior to the compressive strength testing.Results and discussionIn the preparation of the alkali-seed composite activated solution, a significant amount of sodium hydroxide solution is utilized, with a mass ratio of 100:4 to fly ash. The simultaneous application of heating and stirring notably accelerates the geopolymerization process. After a synthesis duration of 0.5 h, a compound activator is produced, characterized by a zeolite phase that includes sodalite and sodium-chabazite, exhibiting a markedly higher content than that observed in conventional polymerization methods. As the synthesis time is extended to 1 hour, the sodalite content within the composite activator continues to rise, with the zeolite phase predominantly existing at the nanometer scale. However, upon further extending the synthesis time to 2 hours, a slight decrease in sodalite content is observed, accompanied by an increase in the N-A-S-H gel content. This phenomenon may be attributed to the interaction between the zeolite phase and the strong alkali present in the sodium hydroxide solution, which facilitates the dissolution of aluminum ions from the zeolite phase into the alkali solution. Consequently, the N-A-S-H gel is generated through a polymerization reaction, a process further evidenced by the blurring of grain corners of the zeolite phase. Overall, in the sample designated as AD0.5, a greater coexistence of N-A-S-H gels and zeolite is observed. In contrast, AD1 contains only a limited number of unreacted fly ash particles, while the N-A-S-H gel undergoes a secondary reaction, resulting in the formation of smaller zeolite grains. In AD2, the content of zeolite particles decreases slightly, yet the structure becomes denser, and the particle size increases, alongside a rise in the N-A-S-H gel phase content. The seeds present in the alkali-seed composite activator primarily consist of N-A-S-H gel and zeolite crystal phases. These nano-products serve as nucleation sites that facilitate the geopolymerization of fly ash. Additionally, they fill the spaces between fly ash particles, promoting uniform growth of the induced polymerization products throughout the system. Notably, an increased content of crystal seeds in the composite activator, particularly a higher proportion of small zeolite crystal phases, significantly enhances their positive impact on the advancement of geopolymerization.ConclusionsThe alkali-seed composite activated solution primarily consists of N-A-S-H gel and zeolite phases, with their content and proportions being influenced by several factors, including the activity and dosage of fly ash, as well as the synthesis temperature and duration. By optimizing these parameters, it is possible to achieve the final setting of ASAFA pastes within as little as 2 h. Furthermore, these optimized specimens exhibit significant compressive strength, reaching 4.5 MPa at 7 d and 13 MPa at 28 d. In contrast, alkali-activated fly ash paste demonstrates a considerably longer final setting time of nearly 100 h, along with a much lower compressive strength of only 0.2 MPa at 7 d. This comparison underscores the advantages of using alkali-seed compounds in enhancing the performance of ASAFA pastes.