Optimization and characterization of GGBFS-FA based alkali-activated CLSM containing Shield-discharged soil using Box-Behnken response surface design method

With the rapid growth of shield-discharged soil (SDS), there is an increasing demand for effective recycling and transformation methods. This study aims to develop an alkali-activated controlled low-strength material (CLSM) by utilizing ground granulated blast furnace slag (GGBFS) and fly ash (FA) as precursors, SDS as fine aggregate, and sodium hydroxide (NaOH) solution as an activator. The Box-Behnken design (BBD) within the response surface methodology (RSM) framework was employed, considering liquid-to-solid ratio, alkali equivalent, aggregate-to-binder ratio, and foam agent content (FC) in SDS as key factors. Regression models were constructed to analyze the effects of these factors on flowability, bleeding rate, setting time, compressive strength, elastic modulus, and water absorption. The results confirmed the effectiveness of RSM in determining optimal conditions for material performance. In addition, microscopic analyses were conducted to explore hydration products, microstructural characteristics, and pore distribution. The findings revealed that the fresh density of the CLSM ranged from 1460 to 1740 kg/m(3), classifying it as a low-density material. The 28-day compressive strength varied from 1.837 to 7.884 MPa, while the setting time ranged between 1.2 and 5.6 hours. These properties comply with the ACI 229 standard and are suitable for practical applications. Interestingly, when the aggregate-to-binder (A/B) ratio was between 0.2 and 0.4, increasing the ratio did not lead to a consistent reduction in mechanical properties. Instead, the properties initially decreased and then improved. Moreover, an increase in foam agent content (FC) extended the setting time and reduced mechanical strength. The correlation coefficients of all models exceeded 0.98, with a coefficient of variation below 10 % and a signal-to-noise ratio greater than 4, demonstrating strong reliability and accuracy of the models. Additionally, the average relative error between predicted and experimental values in six scenarios was under 6 %, validating the feasibility of optimizing the design of alkali-activated CLSM using RSM. The formation of Ca(OH)(2) crystals facilitates early strength development, resulting in final cementitious materials reticular, fibrous C-S-H, C-A-H, and other gel-like hydration products. Calcium promotes the formation of gels such as C-S-H, shortening the setting time and enhancing microstructural density. This study provides valuable insights for optimizing the design of alkali-activated CLSM containing SDS, thereby expanding methods for utilizing construction and demolition waste.