Effects of mineral powders on hydration process and hydration products in normal strength concrete

摘要

The use of fine material in concrete manufacture has received an increased attention during the last 10-20 years. A number of studies have been published on the influence of fillers on special types of concretes, however, the influence of fine material on ordinary concrete is less well examined. In this paper the effects of mineral powders on hydration process and hydration products of normal strength concrete were studied. Tests were carried out on concretes and cement pastes with quartz and limestone mineral powders. According to the results, heat evolution appeared to be affected by the use of mineral powders. Greater hydration heat and thus higher maturities, degrees of hydration and compressive strengths were obtained for concretes containing up to 40 wt% of added mineral powder. However, increasing the amount of mineral powder did not proportionally increase the amount of hydration heat. TGA tests on cement pastes revealed that the amount of Ca(OH)_2 was slightly lower in the mineral powder pastes but the degree of hydration was greater. The use of mineral powder to replace 10 wt% of the cement had no effect or only a small one on the amount of Portlandite and compressive strength of the pastes. However, replacing a higher amount of cement reduced the amount of Portlandite and compressive strengths notably. In addition, the results from the TGA also suggested that some of the CaCO_3 originating from the limestone was incorporated into the hydration phases. The XRD study confirmed the results obtained from the TGA. A 10 wt% cement replacement appeared to have almost no effect on the amount of Portlandite formed at the age of 28 days. However, using mineral powders to replace 30 wt% or more of the cement reduced the amount of Portlandite notably. In many of the cement pastes in which mineral powders replaced cement, the net intensity of C_3S measured by XRD was lower than could be expected to happen as a result of mere cement replacement. It was related to a higher degree of hydration of cements with mineral powders which was additionally supported by the amount of Portlandite and the compressive strength results.