Effects of temperature rising inhibitor and shrinkage reducing admixture on pore structure of early-age cement paste

摘要

High-strength and high-performance concrete (HSHPC) is frequently used in practical engineering, but HSHPC tends to crack at an early age because of the large temperature drop (temperature shrinkage) and autogenous contraction. Therefore, a temperature rising inhibitor (TRI) and a shrinkage reducing admixture (SRA) are adopted to improve the properties of HSHPC. The macroproperties of concrete are closely linked to its pore structure. In this paper, the effects of the water-to-cement ratio (w/c) and TRI and SRA contents on the pore size distribution (PSD) and porosity of early-age cement paste were tested by low-field nuclear magnetic resonance spectroscopy (LF-NMR). The test results showed that the PSDs of samples generally contain two peaks. The most probable pore diameter (MPPD) increases with the w/c and reduces with the TRI content and age. However, the MPPD changes only slightly with SRA content. For the same SRA or TRI content, the MPPD changes in samples containing both SRA and TRI are similar to those of samples with TRI or SRA only. However, the combined use of SRA and TRI has an apparent influence on the PSDs of samples. The porosity becomes large with the w/c and SRA content but declines with TRI content. The porosities of samples with both SRA and TRI addition are significantly lower than those of samples without the SRA and/or TRI at 28 days. A modified two-peaks model, which is adapted from the Shimomura and Maekawa model, is developed for cement pastes with the TRI and/or SRA. The curves of the modified two-peaks model coincide with the experimental data.