利用建筑垃圾制备再生混凝土是实现建筑垃圾资源化利用、解决我国砂石资源匮乏的重要途径，但是目前在废弃混凝土和废弃砖破碎、分离和强化过程中会不可避免地产生再生混凝土粉(Recycled Concrete Powder, RCP)和再生砖粉(Renewable Brick Powder, RBP)，故而使得建筑垃圾利用率低。因此，将RCP和RBP混合使用制备再生复合微粉混凝土(Recycled Composite Micro-Powder Concrete, RCMPC)能有效增加建筑垃圾利用率，同时减少对水泥的使用，具有良好的经济性和工程适用性。在我国北部，混凝土经常受到冻融循环作用的影响，但是目前针对RCMPC的研究较少，对其在冻融循环作用下性能劣化规律和劣化机理的研究则更少。

本文在课题组前期RCMPC配合比研究的基础上，通过将RCP:RBP按2:8比例混合，分别以0%、10%、20%、30%的取代率替代水泥，并基于20%掺量组设置0.35、0.45、0.55水胶比制备RCMPC。采用抗压强度试验、劈裂抗拉强度试验、XRD试验、NMR试验、SEM试验和显微硬度试验，对冻融循环作用下RCMPC的宏观性能和微观结构进行研究，建立了基于损伤层的冻融损伤演化模型和微观结构与宏观表征之间的定量关系，探究了RCMPC的性能劣化规律及机理。具体研究结论如下：

根据相对动弹性模量、质量损失率、立方体抗压强度、劈裂抗拉强度和损伤层厚度变化可知，RCMPC的抗冻性能随再生复合微粉(Recycled Composite Micro-Powder, RCMP)掺量、水胶比和冻融循环次数的增加而降低，当RCMP取代率为20%，水胶比为0.45时抗冻性能最佳。且未考虑残余强度修正时，基于损伤层的冻融损伤度误差较大，引入修正系数后，损伤度评估精度显著提高。且随着冻融循环次数的增加，损伤层厚度修正系数β逐渐增大，反映了损伤层范围内材料残余强度不断减小，实际损伤度不断增大的行为。

通过NMR试验可知，随着RCMP取代率的提高，RCMPC胶凝孔和有效孔的孔级占比降低，宏孔和无效孔的孔级占比增大。其中有效孔的孔级占比为63.09%、58.49%、57.03%，分别低于普通混凝土孔级占比的0.07%、0.14%、0.16%。无效孔的孔级占比为36.91%、41.51%、41.51%，分别高于普通混凝土孔级占比的0.15%、0.30%、0.34%。引入孔体积分形维数表征RCMPC的劣化特性，建立了宏-微观关系式，回归系数均大于0.95，表明RCMPC的孔体积分形维数与孔隙大小之间存在紧密的联系。其中随着RCMP掺量、水胶比和冻融次数的不断增加，孔体积分形维数相比于基准组分别下降16.05%、54.94%、82.72%；50.98%、68.63%；11.84%、24.34%、32.89%、51.97%。

通过SEM试验和显微硬度试验可知，随着RCMP取代率的增加，天然骨料与RCMP砂浆的界面过渡区谷底硬度值呈现出逐步下降的趋势，界面过渡区厚度值呈现出逐步上升的趋势。随着RCMP取代率从0%依次增加到10%、20%、30%，界面过渡区厚度值较普通混凝土分别提高了12.84%、39.26%、60.74%，界面过渡区谷底硬度值较普通混凝土分别提高了降低了11.50%、25.24%、42.11%。分别建立RCMPC的相对动弹性模量、抗压强度、劈裂抗拉强度、修正后损伤层厚度与界面过渡区厚度的宏-微观关系式，发现其均线性相关，且相关系数大部分大于0.9，说明这些指标之间存在着紧密的关联性。

研究成果对“高值化”利用建筑垃圾，进一步研发适合寒冷地区工程应用的高抗冻性RCMPC提供了参考。

Using construction waste to prepare recycled concrete is an important way to realize the resource utilization of construction waste and solve the shortage of sand and gravel resources in China. However, at present, it is inevitable to produce recycled concrete powder (RCP) and recycled brick powder (RBP) in the process of crushing, separation and strengthening of waste concrete and waste brick, so the utilization rate of construction waste is low. Therefore, the preparation of Recycled Composite Micro-Powder Concrete (RCMPC) by mixing RCP and RBP can effectively increase the utilization rate of construction waste and reduce the use of cement, which has good economy and engineering applicability. In northern China, concrete is often affected by freeze-thaw cycles, but there are few studies on RCMPC, and even fewer studies on its performance degradation law and degradation mechanism under freeze-thaw cycles.

Based on the previous research on the mix ratio of RCMPC in the research group, RCP:RBP was mixed at a ratio of 2:8, and the cement was replaced with substitution rates of 0%, 10%, 20%, and 30%, respectively. RCMPC was prepared by setting 0.35, 0.45, and 0.55 water-binder ratios based on the 20% dosage group. The macroscopic properties and microstructure of RCMPC under freeze-thaw cycles were studied by compressive strength test, splitting tensile strength test, XRD test, NMR test, SEM test and microhardness test. The freeze-thaw damage evolution model based on damage layer and the quantitative relationship between microstructure and macroscopic characterization were established, and the performance degradation law and mechanism of RCMPC were explored. The specific research conclusions are as follows :

According to the changes of relative dynamic elastic modulus, mass loss rate, cube compressive strength, splitting tensile strength and damage layer thickness, the frost resistance of RCMPC decreases with the increase of Recycled Composite Micro-Powder (RCMP) content, water-binder ratio and freeze-thaw cycles. When the replacement rate of RCMP is 20% and the water-binder ratio is 0.45, the frost resistance is the best. When the residual strength correction is not considered, the freeze-thaw damage degree error based on the damage layer is large. After the correction coefficient is introduced, the accuracy of damage degree evaluation is significantly improved. With the increase of the number of freeze-thaw cycles, the thickness correction coefficient β of the damage layer increases gradually, which reflects the behavior that the residual strength of the material in the damage layer decreases continuously and the actual damage degree increases continuously.

Through NMR experiments, it can be seen that with the increase of RCMP substitution rate, the proportion of pore size of RCMPC gel pores and effective pores decreases, and the proportion of pore size of macro pores and ineffective pores increases. Among them, the proportion of effective pores is 63.09%, 58.49% and 57.03%, respectively, which is lower than 0.07%, 0.14% and 0.16% of ordinary concrete pores. The proportion of invalid holes is 36.91%, 41.51% and 41.51%, respectively, which is higher than 0.15%, 0.30% and 0.34% of ordinary concrete holes. The fractal dimension of pore volume is introduced to characterize the deterioration characteristics of RCMPC, and the macro-micro relationship is established. The regression coefficients are all greater than 0.95, indicating that there is a close relationship between the fractal dimension of pore volume and the pore size of RCMPC. With the increase of RCMP content, the fractal dimension of pore volume decreased by 16.05%, 54.94% and 82.72% respectively compared with the reference group. With the increase of water-binder ratio of RCMP, the fractal dimension of pore volume decreased by 50.98% and 68.63% respectively compared with the reference group. With the increasing number of freeze-thaw cycles of RCMP, the fractal dimension of pore volume decreased by 11.84%, 24.34%, 32.89% and 51.97%, respectively, compared with the reference group.

Through SEM test and microhardness test, it can be seen that with the increase of RCMP replacement rate, the hardness value of the valley bottom of the interfacial transition zone between natural aggregate and RCMP mortar shows a gradual downward trend, and the thickness value of the interfacial transition zone shows a gradual upward trend. With the increase of RCMP replacement rate from 0% to 10%, 20% and 30%, the thickness of the interfacial transition zone increased by 12.84%, 39.26% and 60.74% respectively compared with that of ordinary concrete, and the hardness of the bottom of the interfacial transition zone increased by 11.50%, 25.24% and 42.11% respectively compared with that of ordinary concrete. The macro-micro relationship between the relative dynamic elastic modulus, compressive strength, splitting tensile strength, modified damage layer thickness and interfacial transition zone thickness of RCMPC were established respectively. It was found that they were linearly correlated, and most of the correlation coefficients were greater than 0.9, indicating that there was a close correlation between these indicators.

The research results provide a reference for the ‘high-value’ utilization of construction waste and the further development of high frost resistance RCMPC suitable for engineering applications in cold regions.