Magnesium oxide (MgO) was prepared by calcining natural magnesite ore in an electric furnace at a temperature above 1500℃, and then ground using an SM-500 test mill for 30 min. The particle size range of MgO was 10-60μm (18). Table 1 shows its chemical composition and basic physical properties of MgO powders.

Potassium dihydrogen phosphate (KH₂PO₄) is a colourless or white rhombic crystal with lustre. It was prepared through a neutralization reaction of potassium hydroxide or potassium carbonate with thermal phosphoric acid. The relative density is 2.338, the melting point is 252.6℃, and the particle size range of KH₂PO₄ was 40-60 mesh. The technical parameters are shown in Table 2.

The retarding admixture is a mixture of borax, disodium hydrogen phosphate dodecahydrate and chlorides in a mass ratio of 1:1.5:1, which can be used to retard the coagulation and improve the fluidity (Note that in case of causing durability problem, applying this kind of retarder to reinforced concrete should be very careful.). The main performance indexes of the borax used met the requirements of GB/T632-1993. Disodium hydrogen phosphate dodecahydrate is a colourless and transparent columnar crystal, insoluble in ethanol and easy to weather in dry air. The details are given in Table 3 and Table 4.

Nickel slag is a type of industrial waste residue produced during the reduction smelting of nickel by an electric furnace. Table 5 shows the chemical composition of this nickel slag. It can be seen from the table that the main components of the slag are SiO₂, MgO, and FeO, which form part of the FeO-MgO-SiO₂ ternary slag system. The main mineral composition of nickel slag is 2MgO·SiO₂, FeO·SiO₂ and MgO·SiO₂. Nickel slag powder is obtained by grinding the crushed nickel slag through a test mill for 15 minutes and then passing it through a 200-mesh sieve. In this paper, ultrafine powder of nickel slag was used, the specific surface area of which was 450 m²/kg.

Silica fume is a greyish-white powder with a density of 1600-1700 kg/m³. More than 80% of the silica fume particles are smaller than 1 μm, and the average particle size is 0.1-0.3 μm. Silica fume is a kind of ultrafine hollow spherical powder with a very large specific surface area (about 30-50 times greater than that of fly ash), reaching 20-28m²/g. The main component of silica fume is active SiO₂, which exhibits a high pozzolanic effect and dissolves in water quickly, filling pores in the mortar, improving its density, and enhancing its strength. More details are given in Table 5.

The fine aggregate used in this study was ordinary natural river sand with an apparent density of 2.628 g/cm³ and fineness modulus of 2.65; the size distribution of fine aggregate is shown in Table 6. It was classified as a medium sand with good grading and could be used after washing and drying.

A copper-plated micro-filament steel fibre was used in this study. Its single-wire shape was linear. Table 7 shows its technical indexes.

P. I 42.5 Portland cement, supplied by Xuzhou Zhong-lian Cement Corporation in China was used in this study, which conformed to the Chinese National Standard GB175 (equivalent to European CEM I 42.5). The specific gravity of the cement was 3100 kg/m³, its mean size was 26.6 µm, and its specific surface area was 348 m²/kg. Table 8 shows the chemical composition of Portland cement. Selection of fine aggregates as shown in section 1.1 (6) of this manuscript. Crushed limestone coarse aggregates with a maximum nominal size, saturated surface dry (SSD) specific gravity, and water absorption of 40 mm, 2780 kg/m³, and 1.52%, respectively are used.

The specimens to be repaired were C30 concrete specimens made from ordinary cement, and the concrete weight ratio was cement: water: sand: stone= 0.5:1:1.24:2.52. The specimens were classified into groups: A, B, C, D, E and F (with six pieces in each group); details are given in Table 9. In groups A and B, the size of the specimens was 100 × 100 × 200 mm; no treatment was done on the back end of specimens after mould removal, and the ends of specimens were unprocessed interface. In group C, the size of the specimens was 100 × 100 × 200 mm, and the cement mortar on the end surface of the test piece was immediately removed with a steel wire brush to expose the coarse aggregate, forming a rough bonding interface. In group D, the specimens were in the form of poured long prisms with dimensions of 100 × 100 × 400 mm; after 28 days of standard curing, the specimen was split equally, and two specimens - with sizes of 100 × 100 × 200 mm, to be repaired - were formed with a natural fracture bond interface at the fracture. In group E, the size of specimens was 100 × 100 × 200 mm; wood strips of different shapes were inserted at one end of the mould during moulding, and the wood plates were pulled out after the concrete reached a certain strength, so as to form a bonding interface with different depths reserved at the end of the specimen; in group F, a matrix specimen was used, with a size of 100 × 100 × 400 mm.

MKPC mortar was used as the repair material, and nickel slag, silica fume, and steel fibre were used to modify MKPC. Subsequently, the nickel slag modified mortar (NMKPC), interfacial agent (SMKPC) and nickel-powder steel fibre composite modified mortar (FNMKPC) were prepared. Table 10 shows the mixed proportion of MKPC mortar and modified mortar.

The preparation process was as follows: first, the retarding admixture, KH₂PO₄, and water were poured into the mortar mixer according to the mass ratio of 1:6.25:3.13, and the mixture was stirred slowly (rotation 140 ± 5 rpm, revolution 62 ± 5 rpm) for 1 min until it was fully dissolved. Second, magnesium oxide and its modifier (silica fume, nickel slag or steel fibre) were added and slowly stirred for 2 min. Next, the mixture was stirred for 1 min (rotation 285 ± 10 rpm, revolution 125 ± 10 rpm) until it was fully mixed. Finally, the fine aggregate was added in a rapid mixing state and mixed until it was uniform.

The specimens were repaired and spliced after 28 days of standard curing. First, the two repair surfaces of the specimen to be repaired were put into an iron mould, and the distance between them was measured and adjusted by the scale, finally being fixed. Second, the repair material was poured in and the specimens were vibrated to ensure compactness. After vibration, the repair area of the specimen was cured through film mulching. The repair method of the specimen is shown in Figure 1. According to the specimen scheme, several groups of splicing specimens were fabricated, with three specimens in each group.

MKPC mortar and its modified mortar were prepared according to the mix proportion given in section 1.3. The working performance and mechanical strength of MKPC mortar, nickel slag modified mortar and nickel-powder steel fibre composite modified mortar were determined according to the Test Method for the Fluidity of Cement Mortar GB/T 2419-2016 and Method of Testing Cements-Determination of Strength GB/T17671-2020. The strength test method of cement mortar is shown in Figure 3.

Analysis of compressive and flexural properties of 40mm × 40mm × 160mm mortar specimens. Curing of mortar specimen, ① After curing for 5 hours in natural environment, the mortar specimen was taken out and the specimen was demoulded. ② The labeled specimen is placed horizontally in the curing chamber, and the specific curing method is: temperature 20±1ºC, relative humidity controlled at about 50 %.

(1) The MKPC mortar splicing thicknesses are 1.00cm, 2.00 cm, 3.00 cm, 4.00 cm, 5.00 cm, and 6.00 cm, when repairing and splicing the group A specimens. (2) The best thickness from (1) is then selected, and the test pieces of group B are repaired and spliced using nickel powder modified mortar. (3) Next, four specimens with different interface morphology-from groups B, C, D, and E-are selected to be repaired and spliced with nickel powder modified mortar. (4) The best splicing interface morphology from step (3) is selected and 1-2 mm silica-fume modified mortar (interface agent) is smeared on the interface; then, nickel powder modified mortar is used to repair the splicing of the test piece. (5) On the basis of step (4), the steel fiber is added to the nickel powder modified mortar to continue to repair and splice the test piece. The above-mentioned steps are used to prepare the samples for measurement of the flexural bearing capacity of the test piece through four-point bending tests, observing the cross-sectional morphology of the test piece, and studying the effect of repair mortar thickness, modified mortar, interface morphology, and the interface agent on the flexural bearing capacity of the repaired sample.

The mortar fluidities of MKPC, NMKPC, and FNMKPC are given in Table 11. It can be seen that the fluidity of MKPC mortar is 150 mm, with high viscosity and relatively poor fluidity. The addition of nickel slag improves the fluidity of MKPC mortar - which increases from 150 mm to 180 mm - an increase of 20%. This is because nickel-iron slag is a regular shaped vitreous body after grinding and has a smooth surface, such that friction with the particles is reduced and the fluidity is enhanced (22-24). The fluidity of nickel-powder steel fibre composite modified mortar is 170 mm, which is slightly lower than that of nickel slag modified mortar, but its work performance is still greatly improved in comparison to MKPC mortar.

The mechanical strengths of MKPC mortar, nickel slag modified mortar and nickel-powder steel fibre composite modified mortar are shown in Table 12. It can be seen from Table 7 that the 3-day flexural strength and compressive strength of MKPC mortar are 6.15 MPa and 41.50 MPa, respectively. The addition of nickel slag reduces the mechanical strength of MKPC mortar, and its flexural and compressive strength decreases by 5.70% and 4.82%, respectively. With the addition of steel fibre, the flexural strength is increased by 81%, while the compressive strength decreased by 1.45%.

Since nickel slag has a smooth surface, dense structure and high hardness, the fluidity of MKPC mortar increases and the interactions between MgO particles and aggregates is reduced (20-22). Further, the potential pozzolanic activity of nickel slag is low, the hydration reaction is relatively slow and the addition of nickel slag replaces part of the MgO, which reduces the cementitious products generated by the hydration of MgO and KH2PO4, consequently decreasing the strength of MKPC mortar. The addition of steel fibre limits the lateral expansion of the component under compression, delays the failure process and the good adhesion between the steel fibre and matrix effectively inhibits the germination and expansion of matrix cracks (23, 24). As a result, the flexural strength of the matrix is enhanced.

The effect of the repair layer thickness on the bending capacity is shown in Figure 4. It is clear that the flexural capacity of the matrix specimen is 16.23 kN. When the thickness of the repair layer is 1.00 cm, the 3-day flexural capacity of repairing the long prism is 4.57kN, which is only 28.16% of that of the matrix specimen. With the increase in the thickness of the repair layer, the flexural capacity increases gradually. When the thickness of the repair layer is 3.00 cm, its 3-day bending capacity reaches the maximum value of 5.26kN, which represents an increase of 32.41% in comparison to the matrix specimen. With a further increase in the thickness of the repair layer, the flexural capacity of the prism changes only slightly.

The failure mode and section morphology of the splicing component are shown in Figure 5. Figure 5 (a) shows that the failure of the specimen occurs at one side of the interface between the repair mortar and the cement matrix. In Figure 5 (b), the fine aggregate distributed on the surface can be clearly seen on the repair mortar section, and some MKPC paste is attached to the cross-section of the cement matrix. In comparison to Figure 5 (b), the MKPC paste adhered to the cement matrix section in Figure 5 (c) is more uniform and continuous, and the interface is almost entirely covered with MKPC mortar.

According to the results of the influence of the thickness of the repair layer on the flexural capacity, the failure mode and section morphology of the spliced components, it can be concluded that when the thickness of the repair layer is less than 3 cm, the MKPC mortar has high viscosity, poor fluidity and fast setting and hardening time, which is disadvantageous for construction applications (25-27); the repair mortar at the interface cannot fully infiltrate the bonding surface, resulting in discontinuous repair.

When the thickness of the repair layer is greater than or equal to 3 cm, the working face for the injected vibration is relatively large, and operation is more convenient. The repair mortar at the interface can almost fully infiltrate the bonding surface, which increases the bonding strength between the repair mortar and the cement matrix (25). However, the thicker the bonding layer, the less sufficient the reaction between the repair mortar and the cement matrix, and the greater the internal stress caused by expansion and contraction at the bonding layer (26, 27). As a result, the bonding performance is affected at the bonding position.

Therefore, to ensure uniform continuity and cohesiveness of interface repair, a value of 3 cm can be used as the optimum thickness of the repair layer. However, it is important to further analyse and modify the performance of repair mortar to enhance its working performance.

The influence of nickel slag modified MKPC mortar on flexural capacity is shown in Table 13. The flexural capacity of MKPC mortar with 10% nickel slag is 1kN higher than that of MKPC mortar alone. This is because nickel slag is glassy and its surface is smooth. The addition of nickel slag can improve the fluidity of MKPC mortar, which facilitates the subsequent injection and operation. Thus, the bonding area between the repair mortar and cement matrix is increased (28, 29). Further, the water requirement of the MKPC repair mortar with nickel slag is reduced, its setting time is longer and its working performance is improved, enhancing bonding performance (30).

The failure mode and section morphology of the nickel slag modified MKPC mortar are shown in Figure 6. It is clear that the MKPC paste uniformly adheres to the damaged section of the cement matrix in Figure 6 (b), and the MKPC paste is no longer uneven in thickness as the repair mortar on the fracture surface of the cement matrix in Figure 6 (a). This is because nickel slag, as an addition, has the effect of reducing water content and plasticization. Nickel-slag can be used to improve the work performance of MKPC repair mortar, increase the bonding area with the cement matrix and improve the joint working performance of the repair mortar and cement matrix. However, the bonding strength between the repair mortar and cement matrix does not increase significantly. In other words, the bonding performance between the repair mortar and cement matrix should be increased by other means, such as changing the interface morphology.

As shown in Figure 7, the flexural capacity of the roughened interface is 0.72kN higher than that of the unprocessed interface, indicating that the bonding strength between the interfaces is improved. This is because surface slurry and dust can be removed by chiselling untreated interfaces, such that concrete crushed stone is exposed and the roughness of the concrete surface increases. Then, the repair material penetrates into the uneven groove or gap in the substrate. After curing, a biting force is produced in the interface area, which rivets the cement matrix and enhances the bonding force.

It can be seen from Figure 7 that the flexural capacity of the natural section (C) in the repair specimen is 7.27kN, which represents the largest increase of all interface conditions. This is because the natural section is rougher than the roughened interface, and the concave-convex fracture surface is more obvious, which increases the effective contact area between the repair mortar and the old matrix specimen. After hydration of the MKPC mortar, the hydration product K-struvite has strong gelling characteristics, infiltrates into the pits of the natural section, and rivets more closely with the matrix concrete, such that the bonding performance of the two is maximized. Therefore, the greater the filling degree of the repair mortar for interface defects, the stronger the bonding performance.

As shown in Figure 7, the flexural capacity of the cement matrix specimen with the reserved groove after repair increases with increases in groove depth. When the groove depth is 4 cm, the flexural capacity is 7.46kN. The bond strength of the two can be enhanced by using a toothed pattern connection. However, constructing the reserved groove section is labour-intensive, the injection of the repair mortar becomes difficult and the increase in bending bearing capacity is not significant. Therefore, this section form is not recommended.

The influence of interface morphology on the fracture surface can be seen in Figure 8. When the unprocessed interface is made rough, the repair mortar in the upper part of the section is pulled out from the hole, while the mortar in the lower part is broken along the repair surface. In the case of the natural section, coarse and fine aggregate become exposed on the surface, which can increase the mechanical tooth force and promote the Van der Waals force of the MKPC mortar (21, 22). In the case of the reserved groove, no matter how deep the groove is, the failure surface occurs at the outer edge of the groove, and the bonding effect of the groove is not obvious. According to the aforementioned results, it can be seen that the bonding properties of the natural cross-section are superior to those of other cross-section forms; only changing the interface morphology cannot change the weak position of the interface. Therefore, it is necessary to strengthen the interface on the basis of the natural section.

The effects of the interfacial agents on flexural capacity can be seen in Table 14. The flexural capacity of nickel slag modified MKPC mortar coated with an interfacial agent is increased by 2.36kN in comparison to that without an interfacial agent, and its 3-day flexural capacity is 10.83kN. This is because silica fume has a small size and a better wetting effect on the concrete surface. It can effectively fill the gap on the surface and improve the homogeneity and compactness of the cement matrix. As a result, the structure of the interface zone is obviously improved and the bonding performance of the interface is enhanced, such that the nickel slag repair mortar is well bonded to the surface of the old matrix specimen (31-33).

The effect of the interfacial agent on the failure position is shown in Figure 9. It can be seen from the figure that the cross-section position of the specimen coated with the interfacial agent is no longer at the bonding interface, but is transferred to the place of the repair mortar. The 3-day failure location of the repaired specimen is an oblique crack along the repaired mortar. The damage position of the specimen was still found to be at the repair mortar, and the mortar in the compression zone was crushed in 28 days. This is because silica fume is a kind of ultra-fine hollow spherical powder with a very large specific surface area. The incorporation of silica fume is conducive to filling the pore size of mortar, improving the interface performance of the cement matrix and enhancing the adhesion of the matrix interface, such that the bonding interface is no longer weak. However, the failure location still occurs within the repair area. If the strength of the repair mortar is enhanced, the absolute repair value of the repair material for the matrix can be realized. Therefore, the following section is aimed at enhancing the strength of the repair mortar.

The influence of steel fibre modified mortar on flexural capacity is shown in Table 15. It is clear that, with the addition of steel fibre, the flexural capacity of specimen II is increased by about 3.33kN in comparison to that of specimen I; with all other conditions unchanged, when the curing age increases to 28 days, the flexural capacity of specimen III is increased by 2.73kN, accounting for 98% of the flexural capacity of the matrix specimen. This is due to the strong tensile properties of steel fibre, which can effectively inhibit the generation and expansion of cracks in the repair mortar. Hence, the flexural strength of repair mortar is greatly improved, and its bearing capacity and toughness are increased.

The influence of steel fibre modified mortar on the failure morphology of specimens is shown in Figure 10. It can be seen from the figure that the fracture position of the repaired specimen occurs in the cement matrix. This indicates that the application area of repair mortar is no longer the weak position that leads to failure; the ultimate failure position changes owing to the enhancement of the flexural capacity of the repair mortar (34-36).

The effect of steel fibre modified mortar on the load-displacement curve is shown in Figure 11. It can be seen from the figure that when the load of the steel fibre modified MKPC mortar approaches the initial crack load, the specimen does not fail immediately, but experiences a certain strain hardening process. At this stage, the mechanical properties of steel fibre are fully exerted. The longer the stage, the better the effect of fibre connection, and the higher the toughness and energy absorption capacity of the composite. When the external load is applied, it is necessary to overcome the chemical binding force and friction force between the steel fibre and MKPC mortar. Therefore, the steel fibre cannot be pulled out easily, but will be in the stage of repeated loading and unloading. In the load-displacement curve, dense ripples are generated in the curve after the ultimate load is reached. The more stable this stage is, the more stable the formation and propagation of cracks are, and the smaller the width of the microcracks formed (37-40).