NMR study on bleeding properties of the fresh cement pastes mixed with polycarboxylate (PCE) superplasticizers

This study investigated the bleeding properties of fresh cement paste mixed with various polycarboxylate superplasticizers (PCEs) using low-field nuclear magnetic resonance (NMR). The PCE superplasticizers were synthesized with various acid to ether ratio, molecule weight of methyl allyl alcohol polyoxyethylene ether (HPEG) and functional groups. The bleeding process of fresh cement paste mixed with the PCE superplasticizers was monitored by low-field NMR and the bleeding rate was obtained with the calculation of transverse relaxation time (T2) peaks. Results showed that the proposed NMR method can not only evaluate the bleeding extent, probe the microstructure of fresh cement paste but also provide information about the PCEs concentration of bleeding water. It was found that PCE superplasticizers synthesized with high acid to ether ratio would reduce the bleeding, and PCE superplasticizers synthesized with the HPEG of molecule weight 4400 and functional group of methyl methacrylate (MMA) would lead to less bleeding when the fresh cement paste reached the same fluidity. The T2 relaxation time of bleeding water increased with bleeding rate, which implies the PCEs molecules in bleeding water was diluted during the bleeding process. Furthermore, the results suggest more bleeding water coming out from the pore water results in larger microstructure changes due to sedimentation.