The treatment of excavated soil using the dry sieving method to produce recycled sand is an effective approach for resource utilization. Currently, the hot-air drying process used in this method exhibits high energy consumption. To address this issue, this study proposes a microwave drying technology to dry the excavated soil. Comparative experiments on microwave (1-6 kW) and hot-air (105-205 degrees C) drying of the excavated soil were conducted. The drying behavior and specific energy consumption of the excavated soil were investigated. The Weibull-Fick combined method was recommended for the segmental determination of the effective moisture diffusion coefficient, and the question of whether microwave drying adversely affects sand particles in the excavated soil was answered. The results revealed the following: Compared with hot-air drying, microwave drying demonstrated shorter drying time (3.5-38 min vs 75-1200 min), lower specific energy consumption (6.2-11.5 MJ/kg vs 22.3-55.4 MJ/kg), and a higher range of effective moisture diffusion coefficient (10-8-10-7 m2/s vs 10-9-10-8 m2/s). With increasing microwave power (3-6 kW), the time required for complete drying of the sample was reduced by up to 56 %. Under microwave drying, relaxing the termination moisture content criterion from 0 to 0.01 resulted in a 17 %-32 % reduction in specific energy consumption, accompanied by a 24 %-36 % decrease in drying time. Microwave drying did not damage sand particles within the excavated soil.

The existing literature suggests that natural aggregate concrete demonstrates the least shrinkage, followed by recycled aggregate concrete (RAC) prepared using natural sand, with RAC prepared using recycled sand (RS) from the weathered residual soil of granite demonstrating the greatest shrinkage. Internal incorporation of a MgO expansion agent (MEA) effectively compensates for the excessive shrinkage of the latter; however, the influence of the MEA on the strength development of RAC prepared using RS after natural curing, rather than accelerated carbonation curing, remains unclear. In this study, compression tests of RAC prepared using RS at different stages of natural curing were performed and the corresponding material compositions of RAC were determined and quantified via X-ray diffraction and thermogravimetry-differential thermogravimetry. The soluble carbonate content in RS was determined by ion chromatography, and the morphology of RAC was observed using scanning electron microscopy. The mechanism of strength development of RAC during aging was determined. Furthermore, compressive tests of recycled lump-aggregate concrete (RLAC) were performed to investigate the influencing degree of RAC as fresh concrete on the compressive properties of RLAC. The following key results were noted: (a) the MEA impairs the compressive strength of concrete, but the degree of impairment decreases with curing, and this is attributed to the transformation of Mg(OH)2 to MgCO3. (b) The presence of soluble carbonates in RS (7.2 %) is the main source of carbonate in the conversion of Mg(OH)2 to MgCO3. Mg(OH)2 particles adhere to the surface of RS particles and react with soluble carbonate to generate MgCO3. (c) At 56 days of curing, the addition of 6 % MEA or increasing the replacement ratio of RS impaired the compressive strength of RLAC to a certain extent. However, even with 100 % RS, the compressive strength and elastic modulus of RLAC were impaired by only 7.4 % and 5.8 %, respectively. With 6 % MEA, the impairments were even smaller and negligible.