Study on the Effect of Ultra-fine Limestone Powder on the Resistance of Cement-slag Powder System to Chloride Ion Permeability

The effect of different limestone powder fineness on the chloride ion migration coefficient of the cement paste was tested using the Rapid Chloride Migration (RCM) method. The changes in the phase composition and pore structure of the paste were studied using hydration kinetics analysis, XRD and pore structure analysis. The results showed that before the chloride ion erosion, the limestone powder mainly exhibited filling effect, microcrystalline nucleation effect, and chemical activity effect. It reacted with the hydration products of cement to form calcium aluminate carbonate, which improved the strength of the hardened paste. After chloride ion erosion, the diffraction peak of calcium aluminate carbonate weakened, and Friedel’s salt was formed. With the increase of the specific surface area of limestone powder, the chemical activity of the powder became more obvious, and the generation of Friedel’s salt increased, leading to stronger resistance to chloride ion penetration. Limestone powder can reduce the porosity of the hardened paste and optimize the pore structure. When the particle size of the powder increased from 800 to 5000 mesh, the total porosity of the hardened paste decreased from 20.7% to 14.7%, and the most probable pore size decreased from 450nm to 75nm. Limestone powder with a particle size larger than 1500 mesh can fully exert its activity and play a role in solidifying chloride ions, thus improving the resistance to chloride ion penetration of limestone powder cement-based materials.


Introduction
Corrosion of steel reinforcement is an important factor affecting the durability of reinforced concrete structures, and chloride ion penetration into concrete is the main cause of steel reinforcement corrosion. [1]Chloride ions that penetrate into the interior of concrete generally exist in two forms [2] : one is the reaction with the hydration products of 3CaO• Al2O3(C3A) in cement to form low-solubility monochloroaluminate calcium, also known as Friedel's salt, or adsorbed into the cement hydration products as a component of the hydration product; the other is in the form of free ions in the pore solution of concrete.However, only free chloride ions will cause corrosion to the steel reinforcement in concrete.This means that the less the amount of free chloride ions, the less harm it will cause to the concrete.Studies [3,4] have shown that replacing a large amount of cement clinker with mineral admixtures is an important measure to improve the compactness of concrete and solve the problem of chloride ion corrosion.
Limestone powder is recognized as a mineral admixture in cement and concrete due to its technical and economic advantages.GB 175-2007 allows the addition of a certain amount of limestone powder as a non-active admixture in cementitious materials, which can be used in conjunction with cement to form composite cementitious materials in concrete, supplementing the lack of fine particles in concrete, reducing water bleeding and segregation, and improving the workability of concrete.Limestone powder in concrete mainly exhibits filling, nucleation, and chemical effects [5][6][7] .When the particle size of limestone powder is large, the filling effect is more obvious, and this effect can optimize the pore size of concrete, reduce the porosity of concrete, and thus improve the concrete's resistance to chloride ion penetration.With the increase of the specific surface area of limestone powder, limestone powder particles can serve as nucleation sites for the dissolved state C-S-H gel, causing an increase in its precipitation probability.At the same time, the aluminum in concrete can react with limestone powder to generate calcium aluminate carbonate [8] , and the nucleation and chemical reaction effects gradually become prominent.However, the interaction between chloride ions and limestone powder and aluminum in limestone powder concrete in a chloride salt environment is still unclear, and it is urgent to clarify the chemical reaction between chloride ions and limestone powder and aluminum in order to explore their impact on the phase transformation and microstructure of limestone powder cementbased materials.Based on this, this paper prepares mortar using limestone powder of different fineness, tests the influence of limestone powder on the compressive strength of mortar, uses the rapid chloride migration coefficient method (RCM method) to test the effect of different limestone powder fineness on the migration coefficient of net slurry chloride ions, and uses hydration kinetics analysis, XRD and pore structure studies to analyze the phase and pore structure changes of the mortar.

Experimental Materials
The experiment used Jidong 42.5 ordinary Portland cement with initial setting time of 175 minutes, final setting time of 285 minutes, specific surface area of 385m 2 /kg, and compressive strength of 28.4MPa and 57.5MPa at 3 days and 28 days, respectively.Its density is 2.96g/cm 3 .The S95 slag powder used had a whiteness of 89, density of 2.87g/cm 3 , specific surface area of 443m 2 /kg, and activity indices of 84% and 106% at 7 days and 28 days, respectively, with a flowability ratio of 97.3%.400 mesh, 800 mesh, 1500 mesh, and 5000 mesh limestone powder were selected.The chemical composition of the cementitious materials is shown in Table 1, and the particle size distribution of the limestone powder is shown in Figure 1.

Experimental Program
The fixed cement content accounted for 70% in the composition of the binder materials, with 30% slag content for the control group and 20% for the experimental group, and 10% limestone powder content for different particle sizes.The water-to-binder ratio was fixed at 0.4, and the composition of the mortar is shown in Table 2.A net slurry of Φ100mm×100mm was cast, demolded after 24 hours of curing, and then standard-cured for 28 days in a curing chamber.In the middle of the hardened slurry, Φ100mm×50mm cylinders were cut and tested for chloride ion migration coefficient using the rapid chloride ion migration coefficient method (RCM method).The changes in the phase and pore structure of the mortar were studied through hydration kinetics analysis, XRD, and pore structure analysis.

Effect of Limestone Powder Fineness on Compressive Strength of Mortar
To test the influence of limestone powder fineness on the mechanical strength of cementitious materials, according to the "Strength Testing Method of Cement Mortar" (GB/T 17671-1999), mortar specimens were formed according to the proportions of cementitious materials in Table 2, with a sandcement ratio of 2.5.The specimens were cured to the designated age for unconfined compressive strength testing.The test results are shown in Figure 2. Taking the 28-day age as an example, the unconfined compressive strength of the CSL0 group is 66.3 MPa, while the strengths of the CSL1, CSL2, CSL3, and CSL4 groups are 62.8 MPa, 63.4 MPa, 64.5 MPa, and 64.8 MPa, respectively.The strength of the control group with slag powder is higher than that of the group with limestone powder, mainly because the slag powder has higher activity than the limestone powder, and more hydration products are generated during the hydration process, resulting in stronger bonding.As the limestone powder fineness increases and the specific surface area of the powder increases, the limestone powder fills the gaps between the cement particles in the early stage of cement-slag hydration, improving the particle size distribution of the powder material and increasing the density of the hardened cement paste.Additionally, limestone powder serves as a nucleation substrate for the hydration of calcium silicate hydrate (C-S-H), which can reduce the nucleation energy barrier and accelerate cement hydration.After 28 days, the limestone powder reacts with the hydration products to generate hydrated calcium aluminate cement products, further improving the strength of the mortar.The finer the limestone powder, the more obvious the reaction effect, which can be seen from the strength growth rate.Relative to the 28-day strength, the strength growth rates of the CSL0-4 groups are 13.0%, 4.5%, 9.9%, 14.6%, and 13.4%, respectively.

Effect of fineness of limestone powder on hydration heat of slurry
Figure 3 and Figure 4 show the impact of limestone powder fineness on the hydration heat release rate and total heat release of cementitious materials, respectively.Generally, the hydration heat release rate curve of the cement-slag system can be divided into four stages: initial reaction period, induction period, hydration acceleration period, and hydration deceleration period [9] .The initial reaction period corresponds to the first few minutes of hydration heat release after the slurry is mixed, mainly corresponding to the early dissolution heat release of cement.The period from the end of the initial reaction period to the lowest point of the early hydration heat release curve is the induction period.
After the induction period, the hydration heat release rate increases significantly until it reaches a peak, which is the hydration acceleration period.After the peak of hydration heat release, the heat release rate of cement slows down again, which is the hydration deceleration period.
From the graphs, it can be seen that the influence of limestone powder on the hydration heat release curve mainly has two aspects [10] : on the one hand, due to the lower activity of limestone powder compared to cement-slag powder, it reduces the hydration heat release rate and decreases the total heat release.It is worth noting that as the fineness of limestone powder increases, the hydration process accelerates, and some of the limestone powder gradually reacts with the hydration products, resulting in an increase in the total heat release of the cementitious material, which is consistent with previous studies [6] ; on the other hand, the cementitious system prepared using 5000-mesh limestone powder shortened the induction period of hydration, and the second heat release peak, namely the acceleration period of hydration, occurred earlier.However, CSL1-3 groups had a longer induction period of hydration and a lower rate of acceleration period of hydration due to the lower activity of the limestone powder.

Effect of Limestone Powder Fineness on Chloride Ion Transport Coefficient of Slurry
Table 3 shows the effect of different limestone powder fineness on the chloride ion migration coefficient of the cement-slag powder system.From the table, it can be seen that the chloride ion migration coefficient of CSL0 group is 21.4×10 -12 m 2 /s, while that of CSL1 group increases to 23.6×10 - 12 m 2 /s.The addition of limestone powder increases the chloride ion migration coefficient of the sample, mainly because the slag reacts actively under the stimulation of Ca(OH)2 and CaSO4 to generate gel products such as C-S-H, forming a dense paste structure.Although limestone powder reduces the pore volume of the paste to a certain extent through filling effect, its effect on the binding of slag hydration products is not as significant as that of slag itself [8] .However, as the specific surface area of limestone powder increases from CSL1 to CSL4, the chloride ion migration coefficient of the cement-slag powder paste decreases from 23.6×10 -12 m 2 /s to 15.4×10 -12 m 2 /s, which is significantly lower than that of the CSL0 group, indicating that the filling effect and micro-aggregate effect of limestone powder become more and more significant with the increase of its specific surface area, promoting the hydration rate of cement and slag while filling the pore structure [11] .This enhances the resistance to chloride ion penetration [12,13] .In addition, previous studies have shown that limestone powder can undergo pozzolanic reaction in cement paste to generate calcium aluminate silicate hydrate, which can be converted to Friedel's salt under the action of chloride ions, further improving the resistance of the paste to chloride ion penetration [7] .

Effect of Fineness of Limestone Powder on the Phase Composition of Slurry
To verify the effect of limestone powder on the solidification of chloride ions in the cement-slag powder system, XRD tests were performed on the specimens after the rapid chloride ion migration coefficient test (RCM) to observe the influence of limestone powder fineness on the composition of the cement paste phase.Figure 5 shows the XRD spectra of cement slurries with different finenesses of limestone powder after RCM testing.According to previous studies, the XRD spectra of cement slurries containing limestone powder exhibit clear peaks of calcium carbonate aluminate.However, after RCM testing, the diffraction peaks of calcium carbonate aluminate weakened or disappeared, and new peaks of Friedel's salt (2θ=11.3°)were generated.This indicates that some of the calcium carbonate aluminate formed in the limestone powder-containing cement-based material was transformed into Friedel's salt in the chloride environment, increasing the chemical binding of chloride ions and enhancing the resistance of the cement slurry to chloride ion penetration.Studies have shown [7] that chloride ions can replace carbonate ions in calcium carbonate aluminate and form Friedel's salt.This means that before the erosion of chloride ions, limestone powder reacted with the hydration products of cement to form the diffraction peak of calcium carbonate aluminate, and after the erosion of chloride ions, the diffraction peak of calcium carbonate aluminate weakened and Friedel's salt was formed.As the specific surface area of limestone powder increased, the amount of calcium carbonate dissolved in the pore solution increased, enhancing the chemical effect of limestone powder and generating more calcium carbonate aluminate, which was transformed into more Friedel's salt under the action of chloride ions.Therefore, this is also an important reason why the chloride ion migration coefficient of the cement slurry continuously decreases as the specific surface area of limestone powder increases.At the same time, it should be noted that the Ca(OH)2 peak weakened, indicating the occurrence of the pozzolanic effect [10] .

Effect of fineness of limestone powder on the pore structure of slurry
Furthermore, the pore structure distribution of the hardened paste after RCM test was tested.Figure 6 and Figure 7 show the cumulative porosity and pore size distribution curves of cement paste with different fineness of limestone powder after RCM test, and the pore structure characteristic parameters are shown in Table 4.It can be seen from Table 4 that the CSL0 group has the highest porosity of 22.1%, and the porosity and most probable pore size of cement paste decrease with the addition of limestone powder.When the limestone powder fineness increases from 800 to 5000, the total porosity decreases from 20.7% to 14.7%, and the most probable pore size decreases from 450nm to 75nm.
Observing the pore size distribution data, it can be found that the pores with a diameter greater than 200nm account for more than 50% of the CSL0 group paste, while the pores with a diameter greater than 200nm in the CSL4 group account for only 2.8%, and the pore size is mainly concentrated in the range of 50-200nm, indicating that the addition of limestone powder can reduce the porosity of the hardened paste and optimize the pore structure.The improvement of the pore structure by limestone powder is mainly reflected in its filling effect and chemical activity.On the one hand, most of the particle size of cement particles is larger than 10μm, and the filling is poor due to the large interparticle voids.Limestone powder fills between cement particles, improves the particle size distribution of powder materials, and increases the compactness of the hardened cement paste.On the other hand, the main component of limestone powder is CaCO3.In cement-based materials, its chemical activity is low but not completely inert.With the continuous hydration reaction, CaCO3 can

Conclusion
This paper investigates the effect of different particle sizes of limestone powder on the compressive strength of mortar.The researchers also used the rapid chloride migration (RCM) method to test the effect of limestone powder particle size on the chloride migration coefficient of the mortar.They analyzed the phase composition and pore structure of the mortar using hydration kinetics, XRD, and pore structure analysis.The main findings are: (1) During the early hydration of cement-slag binder, the strength of mortar mixed with limestone powder is lower due to the higher reactivity of slag powder compared to limestone powder.Limestone powder mainly plays an inert filling role and a microcrystalline nucleation role.After 28 days of age, limestone powder reacts with the hydration products to form hydrated calcium aluminate carbonate, which improves the strength of the mortar.The finer the limestone powder, the more significant the reaction effect.
(2) After the addition of limestone powder into cement slurry, the hydration heat release rate was reduced, and the total heat release was decreased.As the fineness of the limestone powder increases, the hydration process accelerates and the total heat release of the cementitious material increases accordingly.The use of cementitious material systems prepared with limestone powder of 5000 mesh size shortened the hydration induction period and caused the second heat release peak, which is the acceleration period of hydration, to appear earlier.(3) When the limestone powder has a fineness of 800 mesh, its addition increased the chloride ion migration coefficient of the specimen.As the fineness of the powder increases, the specific surface area also increases, making the filling effect and micro-aggregate effect more significant.At the same time, the calcium aluminate hydrate formed by the reaction of limestone powder in the cement slurry can be transformed into Friedel's salt under the action of chloride ions, further enhancing the resistance of the slurry to chloride ion penetration.
(4) Prior to chloride ion erosion, limestone powder reacts with the hydration products of cement to form a diffraction peak of calcium aluminate carbonate.After chloride ion erosion, the diffraction peak of calcium aluminate carbonate weakens and Friedel's salt is formed.With the increase of the specific surface area of limestone powder, the generation of Friedel's salt is greater, and the resistance to chloride ion penetration is stronger.
(5) When the limestone powder fineness increases from 800 to 5000 mesh, the total porosity of the hardened slurry decreases from 20.7% to 14.7%, and the most probable pore size also decreases from 450nm to 75nm.The addition of limestone powder can reduce the porosity of the hardened slurry and optimize the pore structure through filling and chemical activity effects, significantly reducing pores with a diameter greater than 200nm.
Therefore, it is recommended to use limestone powder with a fineness greater than 1500 in chloride salt environments.At this fineness, limestone powder can fully exert its filling effect, micro-aggregate effect, and chemical activity effect, solidifying chloride ions, thereby improving the chloride ion penetration resistance of limestone powder cementitious materials.

Figure 2 .
Figure 2. Mortar strength at different fineness of limestone powder

Figure 3 .Figure 4 .
Figure 3.Effect curve of limestone powder fineness on hydration heat release rate of cementitious materials

Table 2 .
The cementitious material composition of the slurry

Table 3
Effects of limestone powder fineness on chloride ion mobility coefficient of cement-slag powder system

Table 4 .
and C4AF in cement to form carbon aluminate composite with certain cohesiveness.This hydration product overlaps with other hydration products, increases the density of the hardened paste, and after RCM test, the hydration products such as calcium carbonate aluminate continue to react with chloride ions to generate Friedel's salt, which improves the resistance to chloride ion penetration and increases the compactness of the paste.1Characteristic parameters of limestone powder cement slurry pore structure after RCM test