###### Abstract  
The alkali-activated composites technique is a promising method for the in situ preparation of cavity filling/grouting materials from engineering waste soil. To investigate the feasibility of engineering waste soil utilization by the alkali activation process, the macroscopic and microscopic properties of the fly ash/slag-based alkali-activated composites, after solidification/stabilization (S/S) with sandy clay excavated at Baishitang Station of Shenzhen Metro, were studied. The unconfined compressive strength (UCS) test was conducted to evaluate the S/S effect of alkali-activated composites. The results show that the optimum quality ratio of slag and fly ash correspond to 7:3, the modulus of alkaline activator to 1.3, and the alkalinity of alkaline activator to 10%. The alkali-activated composite's strength under these parameters can reach 45.25 MPa at 3 days, 49.85 MPa at 7 days, and 62.33 MPa at 28 days. A maximum 3-day UCS of 21.71 MPa, 75% of the 28-day UCS, was achieved by an engineering waste soil and alkali-activated composites mass ratio of 5:5, slaked lime content of 4.5%, and a water-to-solid ratio of 0.26, and it can also meet the required fluidity and setting time for construction well. Fluidity is primarily affected by the soil-to-binder ratio, which decreases as the ratio decreases, while the water-to-solid ratio increases fluidity. Saked lime has minimal impact on fluidity. The setting time is mainly influenced by the soil-to-binder ratio, followed by slaked lime content and water-to-solid ratio, with setting time shortening as the soil-to-binder ratio and slaked lime content increase, and lengthening as the water-to-solid ratio increases. Through Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) tests, microscopic analysis showed that loose granular units are firmly cemented by alkali-activated composites. Based on the results of on-site grouping tests in karst caves, the alkali-activated grout materials reached a strength of 5.2 MPa 28 days after filling, which is 162.5% of the strength of cement grouting material, satisfying most of the requirements for cavity filling in Shenzhen. g 2024 2024 22 May 2024 20  
resource depletion due to waste. Consequently, the effective recycling and reuse of such waste have been crucial concerns, garnering significant attention and exploration across various societal sectors [3; 4; 5]. Alkali-activated composites, as an emerging green, inorganic, composite cementitious material, are primarily composed of aluminosilicate compounds. They exhibit characteristics similar to those of traditional Portland cement [6; 7; 8; 9; 10]. Alkali-activated composites not only exhibit rapid hardening, but also demonstrate low energy consumption and minimal pollution during production. The manufacturing costs are significantly lower, and the process is considerably streamlined [11; 12; 13; 14; 15; 16]. [PERSON] and [PERSON] [17; 18] activated slag using an alkali activation method, endowing it with cementitious properties. Experimental validation demonstrated that the slag-based alkali-activated composites possess excellent mechanical properties. [PERSON], [PERSON], and [PERSON] [19; 20; 21; 22] utilized fly ash as the raw material for alkali-activated composite production and conducted experiments on the mechanical properties of fly ash-based alkali-activated composites. The results indicated that the strength could reach up to 39 MPa. Additionally, they investigated the gelation mechanism through microscopic tests, including scanning electron microscopy. [PERSON] [23] prepared alkali-activated composites by incorporating both slag and fly ash, achieving a maximum strength of 43.1 MPa through alkali activation. Based on previous research and theoretical analysis, it is understood that the silicon-aluminum components in fly ash and slag can undergo an alkali-activated composites reaction when exposed to alkaline activators.