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) present 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 in Brazil. 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 (Brazil) 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 almost 850 days and then kept in water up to 1300 days, both in 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 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. 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 地壳中存在的硫化物（sulfides，即硫与金属元素结合形成的无机化合物）种类繁多，这使得混凝土生产中难以避免使用含硫化物的骨料；尤其在缺乏替代原料来源或原料获取经济上不可行的地区，如巴西的大坝工程场景中，这一问题尤为突出。 针对这一现状，本研究对采用含黄铁矿（pyrite）与磁黄铁矿（pyrrhotite）骨料制备的砂浆的长期性能开展了评价。本研究采用了取自巴西米纳斯吉拉斯州伊拉佩水电站（Irapé Hydroelectric Power Plant）区域的岩石试样。该电站建于地质条件为岩石硫化物含量不低于3%的场址。 将上述岩石试样先进行磨细处理，随后作为骨料制备砂浆试样。在近1300天的试验周期内，砂浆试样分别置于三种不同暴露环境中养护：① 温度23℃±2℃、相对湿度95%~100%；② 先置于室温下的氢氧化钙稀释水溶液中养护近850天，随后转入室温水环境中养护至1300天；③ 先置于50℃的氢氧化钙稀释水溶液中养护近850天，随后转入50℃水环境中养护至1300天。 通过采用盐酸溶液对骨料或砂浆试样进行浸提试验，可获取磁黄铁矿（pyrrhotite）的存在状态与含量信息，同时该方法也可用于黄铁矿含量的测定。 试验结果表明，氢氧化钙溶液与温度均会加速砂浆的膨胀变形。同时观察到，在三种暴露环境下，黄铁矿含量随时间推移基本保持不变。该发现意味着骨料中的硫含量限值应结合所存在的硫化铁种类进行设定，而非仅依据总硫含量确定。