Cement based materials are the most widely used and widely used engineering materials, and their usage environments are complex and diverse. During actual service, cement-based materials deteriorate under factors such as wet dry cycles, temperature changes, and mechanical loads, which affect structural safety. Mortar is a commonly used repair material, and the bonding quality between the repair mortar and the original substrate is crucial for the stability and durability of the repair system. Its influencing factors include the composition of the mortar and the pre-treatment of the substrate.

The team led by Professor Zhang Peng from the School of Civil Engineering at Qingdao University of Technology studied the influence of the initial moisture content of old mortar on the microstructure of the composite formed by the bonding between old and new mortar by combining X-ray CT and mercury intrusion porosimetry (MIP). Based on the three-dimensional reconstruction images of the SanYing precision X-ray CT equipment (nanoVoxel-3000 series), the microstructure of the interface between new and old mortar and the spatial distribution characteristics of internal pores and microcracks after the repair mortar has hardened were analyzed. The types and formation mechanisms of pores in the repaired mortar were analyzed using MIP results. Based on neutron transmission imaging, a visual water absorption test was conducted on the composite, and the water content distribution curve over time was obtained. The water absorption characteristics of the composite and its relationship with the microstructure were analyzed.

Sanying Precision NanoVoxel-3000 Micro CT

Two composite materials were designed by changing the initial moisture content of the old mortar. The initial moisture conditions before the old mortar specimen comes into contact with the newly mixed repair mortar are set as follows:

a) Dry matrix - The old mortar sample is dried at 105 ℃ for 24 hours and cooled at room temperature.

b) Saturated matrix - old mortar soaked in water for 5 hours, with a measured moisture content of 20.4%.

Figure 1 Schematic diagram of new and old mortar composite specimens

Combining X-ray CT and MIP testing to characterize the microstructure of the studied complex. Firstly, the sample was subjected to X-ray CT scanning using nanoVoxel-3502E produced by Tianjin Sanying Precision Instrument Co., Ltd. in China. Perform gap segmentation on the original data through image binarization, as shown in Figure 3, and analyze the gap size (height ± 2.2 mm) around the interface after segmentation.

Due to the inability of X-ray CT testing to detect pores smaller than the voxel size in the image, the microstructure of the repair mortar was jointly analyzed through MIP testing. Samples D06-R and S06-R were cut from D06 and S06, respectively, and moisture was removed by drying before MIP testing. Finally, the neutron imaging test platform was used to conduct imaging research on the water absorption process of the new and old mortar composite.

Figure 2: Reconstructing three-dimensional images of the specimen based on X-ray CT data and extracting regions of interest for analysis

Figure 3 Extraction of Pore and Crack Segmentation in the Region of Interest

The internal microstructures of the two specimens are shown in Figure 4. D06 exhibits significant shrinkage cracking, while S06 has a denser microstructure. The area fraction of voids (pores and microcracks) with dimensions greater than 44.2 µ m on different cross-sections (length x width) in the region of interest (ROI) was calculated. In Figure 5, plot the area score of each slice according to height. The results showed that the fraction of large void area in the old mortar of both specimens was around 0.03. The area fraction of large voids in the S06 new mortar is concentrated around 0.03, while the area fraction of large voids in the D06 new mortar is 0.06, which is about twice that of the S06 new mortar specimen.

Figure 4 Internal microstructure of new and old mortar composite specimens

Figure 5: Variation of Large Gap Area Score with Height

Figure 6: Color labeling of gaps based on equivalent diameter

By analyzing the microstructure, it is inferred that the formation mechanism of large-sized voids in the interface area of D06 and S06 is different. For the D06 sample, due to the gradient of moisture content, placing the newly mixed repair mortar on the pre dried old mortar will extract a large amount of moisture from the repair mortar, causing severe shrinkage and cracking, resulting in large voids; When preparing S06 specimens, the old mortar is pre saturated with water. The loss of moisture from the repaired mortar and the possible backflow of water in the saturated old mortar can lead to the accumulation of moisture at the interface, resulting in a large discontinuous area around the interface and the formation of large gaps.

Using the cyclic mercury injection method to characterize the microstructure of repair mortar with "ink bottle like" pores. According to the accumulated mercury intrusion data, the effective connectivity porosity and "ink bottle" porosity of the repaired mortar were calculated. The effective connectivity porosity of D06-R (0.074) is very close to the volume fraction of small pores (0.076). It can be inferred that the shrinkage cracks are not directly interconnected, but are connected through capillary pores. The effective connected porosity of S06-R (0.095) is lower than the small pore volume fraction (0.129). It is speculated that only some small pores can be well connected in S06-R.

Figure 7 Pore size distribution curves obtained after the first and second mercury injection of new mortar

By using neutron imaging technology to track the water absorption phenomenon of new and old mortar composites in real time, the dynamic moisture content distribution is obtained, as shown in Figure 9. Among them, the moisture content is calculated by the ratio of the thickness of neutron permeability to the thickness of the specimen, and the position of the interface is indicated by a dashed line, with the bottom being new mortar. Set a ROI (yellow rectangle) with a width of 5 mm on the image, monitor the changes in moisture content profile along the water flow direction, determine the position of the wetting front based on the dynamic moisture content profile in Figure 10, and use a linear regression function to fit the experimental data to obtain the water absorption coefficient.

The water absorption coefficient of the new mortar in D06 is 1.5 times that of the new mortar in S06. There are many cracks in the new mortar of D06, which makes its water absorption coefficient higher. The newly mixed repair mortar in S06 has less water loss, which can ensure the normal hydration of cement and form a denser pore structure. Therefore, the new mortar in S06 has lower water absorption.

Figure 9 Dynamic Moisture Content Changes of New and Old Mortar Composite during Water Absorption Process

Figure 10 Determination of dynamic moisture content curve during water absorption process of new and old mortar composite (a) D06; (b) S06

Figure 11: Changes in the wetting front position during the water absorption process of the new and old mortar composite (a) D06; (b) S06

Through X-ray CT imaging observation, a large amount of moisture is extracted when the freshly mixed mortar comes into contact with pre dried old mortar, resulting in significant shrinkage and cracking. When new mortar and saturated old mortar stick together, the bleeding of the new mortar and the potential backflow of moisture in the saturated old mortar may lead to the accumulation of moisture at the interface, resulting in a large discontinuous area around the interface.

The new mortar bonded with pre dried old mortar and saturated old mortar has similar intrusive porosity, but there are significant differences in pore size distribution characteristics. The capillary pore sizes of the two new mortars are similar, while the new mortar bonded to the saturated old mortar has more capillary pores. Therefore, pre wetting the old mortar can slow down the water loss of the newly mixed repair mortar bonded to it, which is beneficial for the normal hydration of cement.

The water absorption of the old mortar matrix has a significant impact on the microstructure and water absorption characteristics of the new and old mortar composite. When designing a concrete structure repair plan, in addition to considering the interfacial bonding strength, the transmission performance of the repaired mortar after hardening should also be considered from the perspective of durability.

X-ray micro CT scanning has achieved three-dimensional visualization of the pore and crack structures inside the specimen, effectively characterizing the microstructure of cement-based materials and displaying the real situation inside the material.