Study on Calcium Ion Dissolution Law of Shotcrete Under Contact Corrosion Condition

The precipitation of Ca2+ ions in the concrete and other products during the crystallization of calcium carbonate primarily causes drain blockage within the tunnel drainage system. To study the contact corrosion law of calcium ion precipitation in sprayed concrete, a fixed number of concrete blocks were prepared and subjected to a 30-day deionized water immersion test. The variations in calcium ion concentration, precipitation amount, and pH value in solution were measured during the test. The results showed that when the samples were taken out after standard curing for 28 d were soaked immediately, the dissolution amount of calcium ions in single samples remained below 20 mg·L-1 throughout the soaking cycle. The samples were immersed in deionized water immediately after demolding, and the pH value of aqueous solution was maintained at 12.5-12.8 during the soaking cycle. The maximum calcium ion dissolution concentration of single samples was 164.2 mg·L-1. After demolding, the test block was immediately soaked in deionized water. If the fixed volume of 1 L deionized water was changed every 24 hours, while detecting the change in calcium ion concentration of the water solution and calculating calcium ion weight each time, it can be concluded that the pH value of the water solution gradually decreases from 12.4 to 11.0 during the soaking period. The average weight of calcium ions dissolved from a single block was 1 092.78 mg, while the highest weight was 1149.21 mg. After stripping, the test block was cut into three parts having equal height and then soaked. Under the condition of no water change, the maximum amount of calcium ion dissolution was 227 mg·L-1. Under the condition of water change, the accumulated weight of calcium ion dissolution was 1 289.35 mg within 30 days, and the pH value of the solution decreased from 12.1 to 11.0. The results show that when concrete is exposed to external water, insufficient hydration and increased contact area with water lead to continuous precipitation of free calcium ions, resulting in an increase in pores and cracks and a decrease in its compactness. However, when concrete undergoes complete curing and hydration, it achieves satisfactory compactness, and the precipitation rate and amount of calcium ions are significantly reduced under conditions of contact dissolution. © 2023 Xi'an Highway University. All rights reserved.