石墨烯的导热性能及分布效应对水泥浆体早期收缩的影响机制研究

Cement hydration reaction is an exothermic process with the characteristics of changing volume. When the slurry sets and hardens, an artificial stone that have many micro pores and water in it will form. Because of the poor heat conducting feature of air, water and hydrates, the heat released during the hydration cannot be effectively delivered. The distribution of in the cement will not be uniform, leading to early shrinkage cracking. In order to reduce cracking, some technologies such as low/ moderate thermal cement, expansive agent, water-cooled tube, etc have been devoloped. Graphene is a nano material with the high strength and good thermal conductivity,which can enhance cement-based materials.However, the effect of graphene on heat conduction for cement has rarely been reported. In this project, we intend to study the heat transfer and volumetric contraction of the harden slurry, taking advantage of the good thermal conductivity of graphene and the strong interface contacting with the hydration reaction. With the aid of surface modification, different dispersing methods and media, the graphene space distribution degree and slurry micro pores structure will be adjusted and tailored. The mechanism of the dispersion and heat conduction and the shrink resistance will be clarified. The optimum condition for heat transfering and diffusion controlling will be studied. The role of grapheme as "heat-conducting nanosheet"will be investigated. Using the high strength of graphene, the crack resistance of cement material will be improved, reducing the early shrinkage and crack damage induced by temperature. This project will provide a new method for improving internal structure of the cement concrete materials and increasing their service life.

水泥水化是一个化学放热、体积变化的过程，并凝结硬化成富含孔隙和水的石状体。由于水化物、空气和水均是热的不良导体，会产生温变应力，如控制不当，易造成混凝土早期收缩开裂等导致的危害。目前中低热水泥、膨胀剂、埋设冷却水管等技术均是为了减少这些危害。石墨烯是目前强度最高、导热性最好的纳米材料，它能增强水泥基材料，但如何影响水泥热传导却鲜有报道。本项目拟借助石墨烯的导热性能，利用其与水化反应界面接触大的特点，采用表面改性、不同分散手段和介质等，调节石墨烯分布程度与浆体孔隙结构，使其空间分布及连接状态可控，研究其对热量传递和体积收缩的影响规律，阐明分散及导热与抗收缩的机理，明确热量传递与扩散的控制条件，发挥石墨烯的“纳米导热片”作用，使水泥硬化浆体保持温变均匀，导热平衡；并利用石墨烯的高强度，提高水泥材料的抗裂性，降低早期收缩和温变带来的裂缝损伤。为改善水泥混凝土内部结构，提高其服役寿命提供新的思路。

本项目主要研究了石墨烯对水泥材料的强度、导热、收缩开裂、耐腐蚀等性能影响。通过采用石墨烯不同分散手段和方法，使加入石墨烯的水泥硬化浆体强度增加，导热平衡，并利用石墨烯提高水泥材料的抗裂性，降低早期收缩和温变带来的裂缝损伤。研究成果如下：.研究发现GO在水泥中易团聚的机理：GO表面-COOH等官能团与二价阳离子之间的络合作用以及强碱性条件下的还原反应是导致其团聚的主要原因，GO团聚物的横向尺寸可达125 μm，纵向厚度12 μm。借助SEM和X-CT分析GO的空间分布发现：高速搅拌无法阻止GO的团聚行为； PCE分散法可以显著改善GO的分散性；包覆法和球磨法均可防止GO团聚。当PCE/rGO的质量比为0.5时，rGO在水中的分散效果最好。.阐明了rGO对水泥材料导热能力的影响规律。随rGO掺量增加，硬化水泥石的导热能力逐渐提高，导热系数和热扩散系数最大可提高7.80%和29.00%。rGO改性砂浆表层、中间层、底层的最高温差分别为1 ℃, 4 ℃和1.25 ℃。同时微应变在69～76之间，有效缩减了内外温差及温度应变。.进一步研究了rGO改性水泥砂浆因湿度因素诱发的收缩应变。rGO能够增大砂浆的塑性收缩，抑制干燥收缩和自收缩。其中，2.00 wt.%的rGO能够使塑性收缩峰值增加约11倍，使相应的干燥收缩和自收缩降低38.25%。rGO显著降低了塑性收缩裂缝的数量、长度及宽度，2.0 wt.% rGO改性的试样表面基本无宏观可视裂纹，最大宽度下降了79.68%。.发现rGO对砂浆的保水作用导致内部结构孔中形成了更大的弯月面半径，缩减了表面张力，降低了自收缩和干燥收缩。此外，rGO加剧了砂浆表面水分蒸发速率与内部水分渗出速率之间的不平衡关系，导致塑性收缩增大。rGO在水化产物中发挥桥接作用，分散毛细管应力，限制不均匀的收缩变形。rGO提高了砂浆的抗裂能力，抵消了因塑性收缩增加引起的负面影响。.探究了rGO对砂浆中钢筋腐蚀电位及电流的影响。未浸泡NaCl溶液前，rGO改性砂浆的抗腐蚀性相比对照组提高。0.6 wt.% rGO在短时间内可以提高钢筋的抗腐蚀性，当rGO掺量过高或浸泡龄期较长时，均会加速钢筋腐蚀。