Mix design and performance characterization of one-part alkali activated recycled aggregate concrete

This paper aims to develop one-part alkali activated recycled aggregate concrete (OPAARAC) using solid alkaline activators, and systematically investigate its workability, mechanical properties and microstructural characteristics. Compared with conventional two-part alkali activated recycled aggregate concrete, the developed material avoids the inconveniences associated with liquid alkaline activators during transportation, storage and construction, thereby promoting the commercialization and practical application of alkali activated materials. An orthogonal experimental design was first employed to optimize the one-part alkali activated mortar (OPAAM) by considering ground granulated blast furnace slag (GGBS) content (30%, 50% and 70%), anhydrous-Na 2 SiO 3 modulus (1.0, 2.0 and 2.3) and alkali-to-binder ratio (0.045, 0.060 and 0.075) as key factors. The results indicated that anhydrous-Na 2 SiO 3 modulus exerted the most significant influence on the 28 d compressive and flexural strengths, and lower modulus led to superior mechanical performance. The optimal values of the three factors were 70%, 1.0 and 0.075 respectively. On this basis, 15 mixtures of OPAARAC were designed by varying recycled aggregate replacement ratio (0, 20%, 50%, 70% and 100%), alkali-to-binder ratio (0.045, 0.060, 0.075 and 0.090), GGBS content (30%, 50% and 70%), water-to-binder ratio (0.35, 0.40 and 0.43) and Na 2 CO 3 content (0, 20%, 50% and 70%). Slump, setting time, cube compressive strength, splitting tensile strength, flexural strength and microstructure were then systematically investigated. The results indicated that OPAARAC exhibited satisfactory workability and a pronounced early-strength characteristic. Increasing recycled aggregate replacement ratio and Na 2 CO 3 content reduced mechanical strength, whereas GGBS incorporation promoted the reaction and enhanced strength. Both alkali-to-binder ratio and water-to-binder ratio showed optimum values of 0.075 and 0.40, respectively. A moderate Na 2 CO 3 dosage could regulate workability without significantly compromising mechanical performance. Finally, scanning electron microscopy was adopted to observe the microstructural features of the matrix and to clarify the mechanisms by which the investigated variables governed the macroscopic mechanical performance.