Early-Age Autogenous Shrinkage Of High Performance Cement Pastes

For over 100 years, concrete is a worldwide leading engineering material. According to development tendency of concrete technology, high performance concrete (HPC) will become a dominant product in the future, which incorporates with a high volume of mineral admixtures and superplasticizers to obtain high strength, high fluidity and excellent durable performance. HPC occupies a low w/c ratio. It may differ in hardening characteristics and microstructures from normal concrete. Crackings often occur on surfaces of HPC structures, which may be caused by autogenous shrinkage (AS) during initial hardening and other shrinkages. Cracking of concrete should be eliminated because it may affect mechanics performance and durable performance of concrete structures.AS of cement pastes may be main contribution to that of concrete. This paper focuses on AS of high performance cement pastes added with high range water reducers and a serious of mineral admixtures.This paper summarized the current situation of AS at home and abroad systematically. Through comparison of testing methods for AS, a new device was developed to evaluate AS of cement pastes. The device is consisted of a corrugated pipe equipped with slugs at each end. Major advantage of this method was to obtain AS in 1d. Pore size distributions of hardened cement pastes are measured by mercury intrusion porosimeter. SEM was employed to reveal structures of materials used. Hydration degree of cement pastes was studied by examination of combined water by DTA and ignition. Theory of prior adsorbed water of mineral admixtures and capillary depression was applied to explain experimental conclusions.In this study, effects of w/c ratio, fly ash (PFA) content, blast furnace slag powder (S) content, coal gangue(G) content and super-plasticizer on the AS of cement pastes were experimentally studied. The w/c ratio of cement pastes studied was in the range of 0.25 to 0.40 and content of mineral admixtures was in the range of 10% to 40% by weight of cement.