基于NMR技术的胶结钙质砂热特性机理分析

钙质砂含有丰富的内、外孔隙,且颗粒具有易破碎性,因此,钙质砂的微观孔隙结构变化是引起其热传导特性演变的本质原因。为了探究钙质砂微观孔隙结构对其热特性演变的影响机理,首先基于热针法探讨了5个不同水泥掺量(P_s=5.0%、7.5%、10.0%、12.5%、15.0%)胶结钙质砂在不同养护龄期(t=1d、3d、7d、14d、28d)的导热系数,分析了水泥掺量P_s和养护龄期t对胶结钙质砂热传导特性的影响规律；利用核磁共振(NMR)技术揭示了上述过程中胶结钙质砂微观孔隙结构变化的本质特征,在此基础上阐释了各影响因素作用下胶结钙质砂热特性演化的微观机理。试验结果表明：(1)胶结钙质砂的导热系数λ随养护龄期t的增加先急剧增大而后缓慢减小,随水泥掺量P_s递增而增加,但增长幅度递减；(2)NMR试验曲线反映了胶结钙质砂内部孔隙大小和数量的变化,根据反演的孔径d可将曲线划分为微孔隙(＜66.24nm)、小孔隙(66.24nm~6.048×10^₃nm)和大孔隙(6.048×10^₃nm~1.92×10^₅nm)3个区域；其中,微孔隙的变化情况反映了胶结钙质砂中水泥水化过程,中、大孔隙的变化情况反映了钙质砂中孔隙被水化胶凝产物分割、填充过程；(3)胶结钙质砂内微观孔隙大小、数量的变化从本质上决定了其宏观导热系数的变化规律,可利用NMR曲线得到合理的微观机理解释。

Analysis on Thermal Characteristics Mechanism of Cemented Calcareous Sand Based on NMR Technology

Calcareous sand contains abundant internal and external pores, and its particles are easily broken. Therefore, the change of microscopic pore structure of calcareous sand is the essential reason for the evolution of its heat conduction characteristics. In order to explore the influence mechanism of calcareous sand micro-pore structure on the evolution of its thermal characteristics, the thermal conductivity of cemented calcareous sand with 5 different cement contents (P_S =5.0%, 7.5%, 10.0%, 12.5%, 15.0%) at different curing ages (t =1d, 3d, 7d, 14d, 28d) was studied based on hot needle method and the effects of cement content P_s and curing age t on the thermal conductivity of cemented calcareous sand were analyzed. Then, nuclear magnetic resonance (NMR) technique was used to reveal the essential characteristics of the microscopic pore structure changes of cemented calcareous sand during the above process. On this basis, the microcosmic mechanism of the evolution of the above thermal characteristics was explained. The results show that the thermal conductivity of cemented calcareous sand increases sharply firstly and then decreases slowly with the curing age increasing. However, the thermal conductivity increases with the increase of cement content, but the increase ranges decrease in turn. It is also found that the NMR curves reflect the change of the size and number of pores in cemented calcareous sand. According to pore size, the NMR curves mentioned above can be divided into three regions: micro-pores (<66.24nm), small pores (66.24nm~6.048×10^₃nm) and macro-pores (6.048×10^₃nm~1.92×10^₅nm); Among them, the change of micro pores reflects the hydration process of cement in cemented calcareous sand, and the change of medium and large pores reflects the process that pores are divided and filled by hydration cementitious products in calcareous sand. The variation of the size and number of micro-pores in the cemented calcareous sand essentially determines the variation of the macroscopic thermal conductivity, which can be explained by the NMR curves.