Long-term behavior of mortars produced with sulfide-bearing aggregates

ABSTRACT The variety of sulfides (inorganic compounds formed by combining sulfur with a metal element) presented in earth crust is not small, which makes it difficult to avoid the use of sulfide-bearing aggregates in the concrete, especially in places where another source of material is non-existent or economically impracticable, as is the case of dams. Within this scenario, this work presents an evaluation of the long-term behavior of mortar produced with aggregates containing pyrite and pyrrhotite. For the study, rock samples from the Irapé Hydroelectric Power Plant area in Minas Gerais were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost 1300 days, subjected to three different exposed conditions: 1-temperature of 23° ± 2°C and relative humidity of 95 to 100%; 2- submerged in calcium hydroxide solution diluted in water during almost 850 days and then kept in water up to 1300 days, both at room temperature; 3- submerged in calcium hydroxide solution diluted in water at 50° C during almost 850 days and then kept in water (50° C) up to 1300 days. The expansion caused by the iron sulfide oxidation as well as the content of each sulfide were evaluated over time. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content over time. The results showed that the calcium hydroxide solution speeds up the expansion just like the temperature. It was also observed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not only by the total amount of sulfur.摘要 地壳中赋存的硫化物（sulfides，即硫与金属元素结合形成的无机化合物）品类丰富，这使得混凝土生产中难以完全规避含硫化物骨料的使用，尤其在无其他材料来源或经济上不可行的场景中，例如大坝工程。在此背景下，本研究针对以黄铁矿（pyrite）和磁黄铁矿（pyrrhotite）为骨料制备的砂浆的长期性能开展评估。本研究采用巴西米纳斯吉拉斯州伊拉佩水电站区域的岩石样本，该电站建于岩石硫化物含量至少达3%的地质场地。研究首先将上述岩石样本研磨后作为骨料制备砂浆，并将砂浆试样置于三种不同暴露条件下养护近1300天：1. 温度23±2℃，相对湿度95%~100%；2. 先在室温下的稀释氢氧化钙（calcium hydroxide）溶液中浸泡近850天，随后继续置于水中养护至1300天；3. 先在50℃的稀释氢氧化钙溶液中浸泡近850天，随后继续置于50℃水中养护至1300天。研究定期评估硫化铁氧化引发的体积膨胀量以及各类硫化物的含量。通过将骨料或砂浆试样在盐酸（hydrochloric acid）溶液中浸提，可获取磁黄铁矿的存在形式与含量，该方法同时可用于追踪黄铁矿含量随时间的变化。结果表明，氢氧化钙溶液与升温均可加速砂浆体积膨胀。此外研究还观察到，在三种暴露条件下，黄铁矿的含量几乎未随时间发生变化。该发现说明，骨料中的硫含量限值应根据所赋存的硫化铁类型进行设定，而非仅依据总硫量。