Investigation and Utilization of Alkali-Activated Grouting Materials Incorporating Engineering Waste Soil and Fly Ash/Slag

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. Slaked 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 grouting 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.