Research On Early Shrinkage And Crack Resistance Of Carbon Nanotubes Reinforced Cement Composites

In engineering practice,the early shrinkage deformation of concrete is easy to cause serious cracking problem,which greatly reduces the safety and durability of the structure.Many studies have shown that the addition of fiber in the cement material can significantly inhibit its early contraction deformation.Carbon nanotubes(CNTs)is a new type of carbon fiber material with excellent mechanical properties,in this paper,the CNTs admixture is added to cement to improve its early shrinkage deformation and enhance its early anti-cracking performance.In order to quantitatively study the improvement effect of CNTs,this paper studied the early shrinkage of cement paste with CNTs by linear measurement method,and studied the effect of CNTs admixture,water cement ratio and fine aggregate addition on the early shrinkage of cement paste.At the same time,the effect of cnts content and water cement ratio on the early crack resistance of cement mortar was studied by using the ring-restrained test method to study the early crack resistance of cement mortar doped with CNTs.The main conclusions of this paper are as follows:(1)The autogenous shrinkage curve of cement paste is "three-stage" law,namely fast contraction stage,slight expansion stage and smooth contraction stage;The addition of CNTs can inhibit the shrinkage deformation of the cement paste specimen,and the 0.10 wt.% of the cement paste with water cement ratio 0.3 and 0.4 is the best,and the cnts dosage shrinkage inhibition effect is better than that of the cement paste with a ratio of 0.35,and the 0.15 wt.% CNTs dosage is the best.After the cement initial coagulation 20 h,the smaller the water cement ratio,the larger the shrinkage deformation of the cement paste;the addition of fine aggregates has a strong inhibitory effect on the shrinkage of cement paste,which can reduce the shrinkage deformation by more than 20%.The early expansion of cement paste shrinkage curve is mainly caused by the hydration temperature rise and the volume increase of hydration products in the initial stage of cement hydration.(2)The annular strain curve of cement mortar also shows a "three-stage" rule,that is,slight expansion stage,contraction stage and instantaneous cracking stage.The addition of CNTs is beneficial to improve the early crack resistance of the cement mortar specimen,and the strain of the cnts after mixing with the cement mortar with water/water ratio of 0.3 is reduced,and the compressive strain of the specimen is the smallest.Under the condition of semi-closed,due to the existence of dry shrinkage,the cracking time of each group was similar,but under the condition of total sealing,the cracking time of mortar specimen with 0.05 wt.% content was the longest,its early anti-cracking performance was the best,and with the increase of water/cement ratio,the strain of CNTs with CNTs was decreased and the cracking time increased.Early cracking resistance is better.