When a cement-based material experiences humidity for a long time period, water may penetrate into the material, causing leaching of calcium hydroxide. Taiwan is located in a subtropical and rainy zone, and thus, cement-based material structures are always exposed to high temperature, humidity, and a salty environment. This circumstance guarantees that the ground water and/or surface rainwater permeates into cement-based materials with time. Penetrating water in cement-based materials along the path of capillary pores renders a ubiquitous ion concentration unbalanced. High concentration ions sequentially move forward to low concentration ions, inducing leaching of hydration products [1
]. In addition, underground oil storage composed of cement-based materials also requires care against leaching of calcium ions because concentration ions leach out from cement-based materials to soil or rock, which is in contact with ground water. Leaching of calcium hydroxide was found to be a common case in the underground environment as well. For example, Yokozeki et al.
(2004) carried out leaching experiments to construct a model to evaluate the effects of various factors on the degradation rate, which is considered to be important for evaluating the long-term performance of cement-based materials [2
During the leaching process, calcium hydroxide was found to be the first hydration product leached from the cement-based material due to its solubility. Calcium hydroxide is slightly soluble, and can be leached out because of enlarging capillary pores [3
]. Leaching of calcium ions increases the porosity of cement-based material, thus resulting in degradation including damage to the pore structure. Increasing porosity results in a weakened matrix and lower compressive strength of the cement-based material. Leaching of calcium ions also has a detrimental effect on durability, since the occurrence provides an entry for aggressive harmful ions into the cement-based material, causing reinforcing steel corrosion. While cement-based material is degraded by the leaching of calcium ions, compressive strength drops dramatically. Furthermore, the degradation volume of the cement-based material can be increased with the leaching progression of calcium ions, and has a linear relationship with the loss in compressive strength [4
Cement-based materials inherit low tensile strength, are brittle, and have other shortcomings. When cement-based materials are exposed to harsh environments, insufficient structural durability is always a common problem. Based on the previous research data, the loss in durability caused structural damage, and repair costs increased yearly in the United States [5
]. For example, the repairing costs for reinforcing steel corrosion in U.S. highway concrete bridges caused by salt attacks have been more than $150 billion USD up to the year of 2007 [6
]. Studies focusing on lowering the cause of the durability degradation mechanism and exploring influential factors have been extensive. Based on their results, these studies proposed to increase the durability of the structures. However, the indicator of durability has not yet been clearly defined. Thus, establishing objective indicators of durability for proper assessment is necessary to ensure a sustainable design or maintenance [7
The previous studies [9
] reported that the rate of calcium leaching may be very slow. The leaching depth of concrete submerged in still water for 100 years was about 5 to 10 mm [10
]. To speed up and investigate the state of leaching out of calcium ions over time, accelerated leaching test methods are usually adopted, namely, electrochemical and chemical acceleration methods [4
]. Hiroshi Saito [1
] performed leaching tests on different mortars by employing an electrochemical method, which accelerates the dissolution of cement hydrate from mortar in contact with water to apply a potential gradient across the specimen. Jain [11
] has dealt with calcium leaching from cement pastes incorporating glass powder, silica fume or fly ash by deionized water medium method. Planel [12
] has continuous monitored the calcium loss in pure deionized water and performed the calcium-depleted part of the specimens by microstructure observation. The chemically accelerated method was chosen with the use of a concentrated ammonium nitrate solution. The similarity of water and leaching has been established chemically, mineralogically and mechanically by Carde [13
]. Agostini [3
] applied a 6 M concentration of ammonium nitrate solution to accelerate the calcium ions leaching out, which increased the calcium ions leaching out rate 300 times compared with natural calcium leaching [14
]. Fundamentally, an ammonium nitrate solution to accelerate the leaching process of the cement-based materials could be seemed as an easily, quickly and quantitatively chemical acceleration method.
This study utilized an ammonium nitrate solution to accelerate the leaching process of the cement-based materials. Leaching durations were set to 56 days, 91 days, and 140 days. Scanning electron microscopy, X-ray diffraction analysis, and thermogravimetric analysis were employed to analyze and compare the cement-based material compositions prior to and after calcium ions leaching out. In addition, leaching depth, initial surface absorption test (ISAT), mercury intrusion porosimetry (MIP), and ultrasonic pulse velocity were measured to analyze the effects of calcium ions leaching on the pore structures of the cement-based materials. The effect of calcium ions leaching on the compressive strength and durability of cement-based materials with mineral admixtures can thus be investigated via chemical analysis and pore structure analysis.
This study investigated the effects of leaching behavior of calcium ions on compressive strength and durability of the cement-based materials. Since the parameters influencing the leaching behavior of cement-based materials are unclear and diverse, this study focused on the influence of adding mineral admixtures (fly ash, slag, and silica fume) on the leaching behavior of calcium ions regarding compression and durability of cemented-based materials. The effects of calcium ion leaching on the compressive strength and durability of cement-based materials with mineral admixtures were investigated via chemical analysis and pore structure analysis.