Effect of metakaolin addition on capillary absorption and carbonation of concrete

ABSTRACT This study evaluates the effects of metakaolin (MK) addition to concrete with compressive strength of 30MPa and 40MPa, on capillary absorption and depth of carbonation. For the capillary absorption test, the samples were oven dried, weighed, immersed in water for periods of 1, 2, 3 and 24 hours. After each period, the samples were dried and weighed, and these values were used to obtain the Abrams curve. The carbonation test was carried out after curing the specimens in a moist chamber, standardizing the humidity for two hours and remaining in the carbonation chamber for 30 days with 5% CO2 at 23±2°C. After this period, the specimens were cut in half, sprayed with a solution of phenolphthalein and an image analysis was performed. For samples prepared with CPIII cement with one or two aggregates, the MK addition increased the capillary absorption. For the samples manufactured with CPV cement, the sample without MK addition showed a higher capillary absorption. The lowest value of carbonation depth was observed for samples manufactured with CPV cement and two aggregates. The highest carbonation depth was observed for the sample with CPIII cement, crushed stone 1, with metakaolin addition.摘要 本研究评估了偏高岭土（metakaolin，MK）掺入抗压强度为30MPa与40MPa的混凝土后，对其毛细吸水性能与碳化深度的影响。针对毛细吸水试验，试验样品经烘箱烘干并称量后，分别置于水中浸泡1、2、3及24小时。每完成一轮浸泡后，将样品烘干并再次称量，基于上述测试值绘制艾布拉姆斯曲线（Abrams curve）。碳化试验则为先将试件置于保湿养护室（moist chamber）中养护，将湿度标准化处理2小时后，移入二氧化碳体积分数为5%、温度为23±2℃的碳化箱（carbonation chamber）中养护30天。养护周期结束后，将试件对半切开，喷涂酚酞（phenolphthalein）溶液，并开展图像分析。结果表明：对于采用CPIII型水泥（CPIII cement）、掺加单一种类或两种集料制备的试样，掺入偏高岭土会提升其毛细吸水量；而采用CPV型水泥（CPV cement）制备的试样中，未掺加偏高岭土的试件毛细吸水量更高。采用CPV型水泥与双集料制备的试样碳化深度最低，而采用CPIII型水泥、掺加1号碎石并掺入偏高岭土的试件碳化深度最高。