A Preliminary Study on Cracking Tendency of Cement Paste Incorporating High Calcium Fly Ash

摘要

Fly ash is a waste material from burning coal that can be used to reduce the amount of cement in making concrete and to improve the characteristics of concrete. Besides being able to improve the flowability of fresh concrete, fly ash can also serve to reduce cracking of concrete. But in certain cases, cement paste incorporating fly ash type C (high calcium fly ash) experiences cracks, right after being released from formwork. The purpose of this study were to investigate the causes of cracking of cement paste incorporating fly ash type C, the influence of fly ash variations, and the countermeasures. The evaluation conducted for this experiment were based on visual inspection and compressive strength test of cement paste at 28 days. Test specimens were made in the form of cement paste with fly ash content of 50%, by mass. Fly ash used was of type C taken from three different batches from one source; with fly ash type F from three different sources used for control specimens. Superplasticizer, silica fume, and calcium carbonate were used as additives to evaluate their effect in mitigating cracks. The results show that the use of fly ash type C may cause cracks on the surface of hardened paste. Fly ash content, especially CaO and MgO, are the key factors affecting the cracks tendency on the surface of cement paste, due to expansion. Superplasticizer and silica fume can be used to mitigate cracks of cement paste.