# Effect of Curing Time on the Surface Permeability of Concrete with a Large Amount of Mineral Admixtures

Authors: Zhenguo Liu; Guorong Liu; Guosheng Zhang
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

In order to explore the influence of curing time on the surface permeability of concrete, 10 groups of high content fly ash and slag concrete with two kinds of water-binder ratio were designed. There were cured under five curing methods, respectively. The influence of curing time on the surface permeability of concrete was studied. The mechanism was explained by hydration heat and mercury intrusion porosimetry. The results show that inadequate curing time leads to insufficient hydration of cementitious materials in concrete with large amount of mineral admixtures and significant increase in the diffusion of chloride ions on the surface of concrete. Standard curing for 14 days can basically ensure the chloride ion penetration resistance of concrete with large amount of mineral admixture. When the standard curing time is less than 7 days, the chloride ion penetration resistance deteriorates seriously. Under the condition of insufficient standard curing time, adding a small amount of limestone powder can effectively enhance the chloride ion permeability of concrete with a large amount of fly ash and slag.


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## Body

## 1. Introduction

Reinforced concrete structure is the most widely used structural form in the world, and its structural durability has become one of the most concerned hotspots in the field of civil engineering. The concrete surface has a protective effect on the interior of the structure and its reinforcement and is the first barrier to prevent external erosion [1, 2]. However, studies have shown that there is a rich slurry effect and boundary effect with the formwork when concrete is poured and hardened, which results in large differences in the microstructure between the concrete surface and its internal body [3–5]. Therefore, the study of concrete surface permeability is of great significance to the exploration the durability of concrete structures.Concrete based on cement binder is the basic building material. The Portland clinker as the basic component of cement has the characteristics of high production energy consumption [6]. In order to reduce carbon dioxide emissions and protect nonrenewable resources, industrial waste is used as much as possible to reduce the content of silicate clinker in cement and concrete components [7, 8]. Fly ash is one of the residues produced by coal combustion for power generation and heating. It is captured by an electrostatic precipitator or a bag filter before flue gas emission. Slag is a byproduct of pig iron production by a large blast furnace at 1300–1500°C [9–11]. Modern concrete often uses a large amount of mineral admixtures to replace part of the cement, which reduces the heat of hydration of concrete, improves the later strength, and reduces carbon emissions in the building materials industry [12–15].According to previous studies, after adding fly ash and slag into concrete, fly ash and slag can react with calcium hydroxide to form calcium silicate hydrate, resulting in a finer pore structure, hindering chloride ion penetration, reducing permeability, and improving structural durability [16–21]. Silica fume can reduce the segregation and bleeding of concrete mixture and effectively reduce the permeability under proper curing. However, compared with concrete without silica fume, concrete containing silica fume needs a larger water cement ratio to ensure the same slump [22–24]. The durability of modern concrete depends not only on the composition of materials but also on the maintenance mode of concrete.Generally speaking, the longer the moisturizing curing time of concrete, the more sufficient hydration, the denser the microstructure, and the better the development of macro performance [25, 26]. However, in actual projects, there are cases of improper maintenance from time to time. In order to shorten the construction period, the maintenance time is often shortened and the molds are removed in advance. When the concrete surface is in direct contact with the external environment, the water loss is the fastest. So the curing time has an obvious impact on the permeability of the concrete surface [27–29]. Therefore, this article discusses the influence of curing time on the surface permeability of concrete with high-volume mineral admixtures by setting up different curing methods.


## 2. Raw Materials and Test Methods

### 2.1. Raw Materials

The following raw materials were used for the test: P.O 42.5 ordinary Portland cement; Grade II fly ash with a specific surface area of 346 m2/kg; S95 slag with a specific surface area of 454 m2/kg; and limestone powder with a specific surface area of 627 m2/kg. The chemical composition of all the above cementitious materials is shown in Table 1. The coarse aggregate used was 5–25 mm continuous graded limestone crushed stone and the fine aggregate used was river sand with a particle size of less than 5 mm.Table 1
Chemical composition of cementitious materials (w/%).
Cementitious materialsCaOSiO2Al2O3MgOFe2O3MnOSO3CaCO3Cement60.4520.264.872.562.322.412.74-Slag38.2130.1315.458.200.760.431.86-Fly ash2.3152.1328.751.535.32--0.35-Limestone powder-7.352.112.781.120.040.1284.26


### 2.2. Test Method

In the experiment, different proportions of concrete were designed based on admixtures such as fly ash, slag, and limestone powder. The water-binder ratio (W/B) was designed to be 0.45 and 0.35, respectively. Under the same water-binder ratio, the mixing amount of cement, fly ash, slag, and limestone powder was changed. The mixing amount of fly ash and slag was set to 35% and 45% of the total mass of the cementitious material, respectively. Moreover, a trace of limestone powder was mixed. The specific mix proportion designs of concrete with water-binder ratio of 0.45 and 0.35 are shown in Tables2 and 3, respectively. Different mix proportions were numbered by the initial letters of fly ash, slag, and limestone powder and water-binder ratio. For example, the number “F35–0.45″ indicates that the cementitious material in the concrete contains 35% fly ash and the water-binder ratio is 0.45. The number “SL 45–0.45″ means that the cementitious material in this mix proportion concrete contains 45% slag and 5% limestone powder by mass, with the water-binder ratio of 0.45. Other numbers can be deduced by analogy.Table 2
Concrete mix design with a water-binder ratio of 0.45 (kg/m3).
NumberCementWaterSandCoarse aggregateFly ashSlagLimestone powderF35–0.45247171832101713300F45–0.45209171827102217100FL45–0.452091718271022152019S35–0.45247171840100901330S45–0.45209171835101401710SL45–0.452091718351014015219Table 3
Concrete mix design with a water-binder ratio of 0.35 (kg/m3).
NumberCementWaterSandCoarse aggregateFly ashSlagLimestone powderF35–0.35312168700105116800F45–0.35264168695105621600FL45–0.352641686951056192024S35–0.35312168710104101680S45–0.35264168706104502160SL45–0.352641687061045019224Different curing conditions were adopted. The standard curing environment is that the relative humidity is above 95% and the temperature is in the range of 20 ± 2°C. The dry air curing environment is that the relative humidity is within the range of 30%–45% and the temperature is within the range of 20 ± 2°C. Five maintenance methods were designed: dry air curing to 60d after standard curing for 1d, which is recorded as C-1; dry air curing to 60d after standard curing for 3d, which is recorded as C-3. After standard curing for 7d, dry air curing for 60d, which is recorded as C-7. After standard curing for 14d, dry air curing for 60d, which is recorded as C-14. After 28 days of standard curing, dry air curing for 60 days, which is recorded as C-28. The summary of curing methods is shown in Table4.Table 4
Curing methods.
Curing methodsNumberDry air curing to 60d after standard curing for 1dC-1Dry air curing to 60d after standard curing for 3dC-3Dry air curing to 60d after standard curing for 7dC-7Dry air curing to 60d after standard curing for 14dC-14Dry air curing to 60d after standard curing for 28dC-28When the curing age was 60 days, the chloride ion diffusion coefficient of the concrete surface was obtained based on the permit test method, so as to characterize the permeability of the concrete surface. The permit test method is based on the principle of chloride ion electromigration, which can facilitate and accurately test the ion permeability onsite. The schematic diagram of the test principle is shown in Figure1 [30, 31]. The tester consists of two concentric cylinders isolated from each other. During the test, the tester is placed on the concrete surface. The cylinder with smaller diameter is the inner chamber as the stainless steel cathode, which is filled with chloride ion solution. The cylinder with a larger diameter is an outer chamber, which is used as a mild steel anode and filled with deionized water. Under the action of an electric field, chloride ions in the inner chamber migrate asymmetrically through the concrete surface to the outer chamber. The chloride diffusion coefficient was determined by monitoring the chloride concentration in the outer chamber.Figure 1
Schematic diagram of the permit test method [30].In addition, several representative mix proportions were selected to test the hydration heat and mercury intrusion and to analyze the hydration process and pore characteristics. The hydration heat was tested using an isothermal calorimeter (TAM air 8-channel standard volume calorimeter). First, the calorimeter was equilibrated at 25°C for 12 h. 6 g of the binder was mixed in situ with 1.8 g of deionized water in a 20 ml glass ampoule agitator for 1 minute, then sealed and placed on a calorimeter to record the development of hydration heat over time for 72 h. The mercury injection was tested using the Auto Pore V 9600 instrument produced by microelectronics company.


## 2.1. Raw Materials

The following raw materials were used for the test: P.O 42.5 ordinary Portland cement; Grade II fly ash with a specific surface area of 346 m2/kg; S95 slag with a specific surface area of 454 m2/kg; and limestone powder with a specific surface area of 627 m2/kg. The chemical composition of all the above cementitious materials is shown in Table 1. The coarse aggregate used was 5–25 mm continuous graded limestone crushed stone and the fine aggregate used was river sand with a particle size of less than 5 mm.Table 1
Chemical composition of cementitious materials (w/%).
Cementitious materialsCaOSiO2Al2O3MgOFe2O3MnOSO3CaCO3Cement60.4520.264.872.562.322.412.74-Slag38.2130.1315.458.200.760.431.86-Fly ash2.3152.1328.751.535.32--0.35-Limestone powder-7.352.112.781.120.040.1284.26


## 2.2. Test Method

In the experiment, different proportions of concrete were designed based on admixtures such as fly ash, slag, and limestone powder. The water-binder ratio (W/B) was designed to be 0.45 and 0.35, respectively. Under the same water-binder ratio, the mixing amount of cement, fly ash, slag, and limestone powder was changed. The mixing amount of fly ash and slag was set to 35% and 45% of the total mass of the cementitious material, respectively. Moreover, a trace of limestone powder was mixed. The specific mix proportion designs of concrete with water-binder ratio of 0.45 and 0.35 are shown in Tables2 and 3, respectively. Different mix proportions were numbered by the initial letters of fly ash, slag, and limestone powder and water-binder ratio. For example, the number “F35–0.45″ indicates that the cementitious material in the concrete contains 35% fly ash and the water-binder ratio is 0.45. The number “SL 45–0.45″ means that the cementitious material in this mix proportion concrete contains 45% slag and 5% limestone powder by mass, with the water-binder ratio of 0.45. Other numbers can be deduced by analogy.Table 2
Concrete mix design with a water-binder ratio of 0.45 (kg/m3).
NumberCementWaterSandCoarse aggregateFly ashSlagLimestone powderF35–0.45247171832101713300F45–0.45209171827102217100FL45–0.452091718271022152019S35–0.45247171840100901330S45–0.45209171835101401710SL45–0.452091718351014015219Table 3
Concrete mix design with a water-binder ratio of 0.35 (kg/m3).
NumberCementWaterSandCoarse aggregateFly ashSlagLimestone powderF35–0.35312168700105116800F45–0.35264168695105621600FL45–0.352641686951056192024S35–0.35312168710104101680S45–0.35264168706104502160SL45–0.352641687061045019224Different curing conditions were adopted. The standard curing environment is that the relative humidity is above 95% and the temperature is in the range of 20 ± 2°C. The dry air curing environment is that the relative humidity is within the range of 30%–45% and the temperature is within the range of 20 ± 2°C. Five maintenance methods were designed: dry air curing to 60d after standard curing for 1d, which is recorded as C-1; dry air curing to 60d after standard curing for 3d, which is recorded as C-3. After standard curing for 7d, dry air curing for 60d, which is recorded as C-7. After standard curing for 14d, dry air curing for 60d, which is recorded as C-14. After 28 days of standard curing, dry air curing for 60 days, which is recorded as C-28. The summary of curing methods is shown in Table4.Table 4
Curing methods.
Curing methodsNumberDry air curing to 60d after standard curing for 1dC-1Dry air curing to 60d after standard curing for 3dC-3Dry air curing to 60d after standard curing for 7dC-7Dry air curing to 60d after standard curing for 14dC-14Dry air curing to 60d after standard curing for 28dC-28When the curing age was 60 days, the chloride ion diffusion coefficient of the concrete surface was obtained based on the permit test method, so as to characterize the permeability of the concrete surface. The permit test method is based on the principle of chloride ion electromigration, which can facilitate and accurately test the ion permeability onsite. The schematic diagram of the test principle is shown in Figure1 [30, 31]. The tester consists of two concentric cylinders isolated from each other. During the test, the tester is placed on the concrete surface. The cylinder with smaller diameter is the inner chamber as the stainless steel cathode, which is filled with chloride ion solution. The cylinder with a larger diameter is an outer chamber, which is used as a mild steel anode and filled with deionized water. Under the action of an electric field, chloride ions in the inner chamber migrate asymmetrically through the concrete surface to the outer chamber. The chloride diffusion coefficient was determined by monitoring the chloride concentration in the outer chamber.Figure 1
Schematic diagram of the permit test method [30].In addition, several representative mix proportions were selected to test the hydration heat and mercury intrusion and to analyze the hydration process and pore characteristics. The hydration heat was tested using an isothermal calorimeter (TAM air 8-channel standard volume calorimeter). First, the calorimeter was equilibrated at 25°C for 12 h. 6 g of the binder was mixed in situ with 1.8 g of deionized water in a 20 ml glass ampoule agitator for 1 minute, then sealed and placed on a calorimeter to record the development of hydration heat over time for 72 h. The mercury injection was tested using the Auto Pore V 9600 instrument produced by microelectronics company.


## 3. Results and Discussion

### 3.1. Hydration Characteristics of Representative Mix Proportion

The hydration heat of the representative mix proportion of large amount of fly ash and slag is shown in Figure2. It can be seen that after a large amount of mineral admixture is added, the hydration heat release rate and total heat release of cement system decrease significantly. The S45–0.45 system showed the second exothermic peak at 7.3 h, which was 2.3 h earlier than that of the pure cement system at 9.6 h, but the peak of the exothermic peak was 6.94 J/(g h), which was half lower than that of pure cement system at 13.92 J/(g h). This is due to the fact that slag is less reactive than cement and has a lower peak hydration exothermic rate. The second exothermic peak of the F45–0.45 system appears later and lasts longer. The peak value of the exothermic peak is about 1/10 of that of pure cement system, and the duration is greatly prolonged to about 24h. This is because the activity of fly ash is low. After adding a large amount of fly ash, the hydration heat release rate decreases obviously. As shown in Figure 2(b), the cumulative heat release curves of PC-0.45 and S45–0.45 systems rise rapidly around 12h, after which the rise rate slows down. The cumulative heat release of F45–0.45 system begins to rise slowly at 12h. At 72h, the heat release of the S45–0.45 system and the F45–0.45 system are 71.7% and 25.5% of that of the pure cement system, respectively.Figure 2
Hydration heat of the representative mix proportion of large amount of fly ash and slag (a) hydration heat release rate curve, (b) cumulative heat release curve.
(a)(b)The hydration heat of mix proportion mixed with a trace amount of limestone powder is shown in Figure3. The numbers “PC-0.45″ and “L05–0.45″ represent, respectively, the pure cement and cementitious material mixed with 5% limestone powder, in which the water-binder ratio is 0.45. It can be seen that after adding a trace amount of limestone powder, the hydration heat release rate and the total heat release of the cement system have little change. However, the second exothermic peak of the L05–0.45 system is about 1 hour earlier than that of the pure cement system, which promotes the early hydration of the cement. This is mainly due to the nucleation of limestone powder: limestone powder provides additional growth sites for hydration products and accelerates the nucleation growth of hydration products [32]. Figure 3(b) shows that the cumulative heat release curves of the two systems of PC-0.45 and L05–0.45 are almost coincident. Since the initial hydration heat release rate of the L05–0.45 system is slightly higher, the cumulative heat release curve of the L05–0.45 system is above the curve of pure cement before 30h and then below it.Figure 3
Hydration heat of the mix proportion mixed with a trace amount of limestone powder (a) hydration heat release rate curve, (b) cumulative heat release curve.
(a)(b)The 3d pore structure of the composite cementitious material mixed with trace limestone powder is shown in Figure4. The numbers “PC-0.3″ and “L05–0.3″ represent, respectively, the pure cement and cementitious material mixed with 5% limestone powder, in which the water-binder ratio is 0.3. The pore structure of the cement system is slightly refined after adding a small amount of limestone powder. The pore distribution less than 50 nm for L05–0.3 system is more than that of pure cement system, and the pore distribution of 200 nm to 700 nm becomes less. Figure 4(b) displays that the cumulative pore volume of L05–0.3 system is also lower than that of pure cement system, that is, the total porosity is a little smaller.Figure 4
3 d pore structure of the composite cementitious material mixed with trace limestone powder: (a) differential distribution curve; (b) cumulative distribution curve.
(a)(b)


### 3.2. Chloride Ion Permeability Test Results

The 10 groups of concrete in Tables2 and 3 were cured by the C-28 curing method, and the chloride ion diffusion coefficient was tested for 60 days. The test results of concrete with water-binder ratios of 0.45 and 0.35 are shown in Figures 5(a) and 5(b), respectively. It can be seen that the chloride ion diffusion coefficient of concrete with a water-binder ratio of 0.45 is about 2.5 × 10−12m2s−1. The chloride ion diffusion coefficient of concrete with a water-binder ratio of 0.35 is about 1.0 × 10−12m2s−1. When the water-binder ratio decreases from 0.45 to 0.35, the chloride ion diffusion coefficient is greatly reduced by about 60%. For two kinds of concrete with water-binder ratio of 0.45 and 0.35, the chloride ion diffusion coefficient of the F45 group is 18% lower than that of F35 group. Compared with the S35 concrete, the chloride ion diffusion coefficient of the S45 group concrete is reduced by 18%. It means that increasing the content of fly ash and slag from 35% to 45% can reduce the chloride ion permeability of concrete. This is because after sufficient curing, the hydration of fly ash and slag can be better developed reducing the porosity of concrete while improving the resistance to chloride ion penetration [33]. The chloride diffusion coefficient of the FL45 group concrete is higher than that of the F45 group, which means that the permeability of concrete mixed with trace limestone powder is higher after 28 days of full standard curing.Figure 5
60d chloride ion diffusion coefficient of concrete under C-28 curing mode: (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)After the concrete was cured by different curing methods, the 60d chloride ion diffusion coefficient was tested, and the results are shown in Figure6. Figures 6(a) and 6(b) are the test results of concrete with a water-binder ratio of 0.45 and 0.35, respectively. It can be seen that for the concrete with two water-binder ratios, the chloride ion diffusion coefficient increases greatly with the reduction of the standard curing time from 28d to 1d. For example, the chloride ion diffusion coefficient of F45–0.45 group increases from 2.31 × 10−12m2s−1 under C28 curing mode to 6.35 × 10−12m2s−1 under C1 curing mode. The diffusion coefficients of the S35–0.35 group are 1.06 × 10−12m2s−1 under C28 curing mode and 3.01 × 10−12m2s−1 under C1 curing mode respectively. The diffusion coefficients of the two increased by 175% and 184%, respectively. This is due to the slower hydration rate of fly ash and slag, as shown in Figure 2. Once the curing is inadequate, there will be more water loss, resulting in insufficient cement hydration. Consequently, the fly ash and slag cannot fully react, the admixture cannot play its due role. It means that the curing method has a great influence on the chloride ion diffusion coefficient of concrete. If the standard curing of concrete is not sufficient, it will significantly increase the chloride ion diffusion and affect the durability of the project.Figure 6
60d chloride ion diffusion coefficient of concrete under different curing mode (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)In addition, the diffusion coefficients of fly ash and slag concrete with different contents have different sensitivities to curing methods. For example, under the C14 and C28 curing modes, the diffusion coefficient of the F45 group is lower than that of the F35 group, while the diffusion coefficient of the B45 group is lower than that of the S35 group. On the contrary, under the C1 curing mode, the F45 group has a higher diffusion coefficient than the F35 group, while the diffusion coefficient for S45 group is lower than that of S35 group. It means that when the standard curing is relatively sufficient, a larger amount of fly ash and slag can reduce the permeability of concrete. However, when the curing is insufficient, the admixture cannot play its role. The concrete with a larger amount of fly ash and slag has greater permeability and is more sensitive to the curing method.Taking the chloride ion diffusion coefficient of concrete under the C28 curing mode as a reference, the chloride ion diffusivity under other curing methods is comparatively studied. Figure6 shows that for all the groups of concrete, the diffusion coefficient under the C14 curing mode is very close to that under the C28 curing mode, and the increase in the diffusion coefficient is basically within 10%. It can be seen that the standard curing for 14 days can basically guarantee the chloride ion penetration resistance of the concrete. Compared with the permeability under C28 curing mode, the increase in permeability coefficient of C7 curing mode is basically in the range of 20% to 55%. Among them, the increase of F45 group concrete has reached 55%. The standard curing time of 7d is relatively short, which cannot guarantee the chloride ion penetration resistance of concrete. Compared with the C28 curing method, the chloride ion diffusion coefficient of concrete is greatly increased under the C1 and C3 curing methods. Therefore, removing the mold after standard curing for 1 or 3 days and switching to dry air curing will cause serious deterioration of the resistance to chloride ion penetration. Consequently, it is not recommended to remove the mold prematurely in the actual project and sufficient curing is required.Taking the chloride ion diffusion coefficient of concrete under the C28 curing mode as a reference, the chloride ion diffusivity under other curing modes was comparatively studied. It can be seen that the diffusion coefficient of concrete mixed with limestone powder increases slightly under C14 curing mode, within 10%. Under C7 curing mode, the increase of diffusion coefficient of concrete mixed with limestone powder is less than 15%, while the concrete without limestone powder increases by 20%–55% under C7 curing mode. Therefore, adding a small amount of limestone powder can effectively reduce the chloride ion diffusivity under C7 curing mode. In summary, when the concrete cannot be fully cured in the actual project, it can be considered to add 5% limestone powder by mass, which can effectively reduce the chloride ion diffusion. However, even if limestone powder is added, the mold can be removed only after standard curing for at least 7 days.Before and after adding limestone powder, the differences of 60d chloride ion diffusion coefficient of concrete under different curing methods were compared, respectively. Figures7(a) and 7(b) show the comparison results of concrete with water-binder ratio of 0.45 and 0.35, respectively. It can be seen that under C1, C3, and C7 curing modes, adding 5% limestone powder can effectively reduce the diffusion coefficient of chloride ions. That is, under the condition of insufficient standard maintenance, the addition of a small amount of limestone powder can enhance the penetration resistance of chloride ions. This is because the limestone powder is relatively fine, which can play a role in nucleation in promoting the early hydration of cementitious materials and improving the pore structure, as shown in Figures 3 and 4, respectively. So, limestone powder can prevent the outward diffusion of water to a certain extent. In the C14 and C28 curing modes, the diffusion coefficient of chloride ions increases slightly after adding a small amount of limestone powder.Figure 7
60d chloride ion diffusion coefficient of concrete under different curing mode (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)


## 3.1. Hydration Characteristics of Representative Mix Proportion

The hydration heat of the representative mix proportion of large amount of fly ash and slag is shown in Figure2. It can be seen that after a large amount of mineral admixture is added, the hydration heat release rate and total heat release of cement system decrease significantly. The S45–0.45 system showed the second exothermic peak at 7.3 h, which was 2.3 h earlier than that of the pure cement system at 9.6 h, but the peak of the exothermic peak was 6.94 J/(g h), which was half lower than that of pure cement system at 13.92 J/(g h). This is due to the fact that slag is less reactive than cement and has a lower peak hydration exothermic rate. The second exothermic peak of the F45–0.45 system appears later and lasts longer. The peak value of the exothermic peak is about 1/10 of that of pure cement system, and the duration is greatly prolonged to about 24h. This is because the activity of fly ash is low. After adding a large amount of fly ash, the hydration heat release rate decreases obviously. As shown in Figure 2(b), the cumulative heat release curves of PC-0.45 and S45–0.45 systems rise rapidly around 12h, after which the rise rate slows down. The cumulative heat release of F45–0.45 system begins to rise slowly at 12h. At 72h, the heat release of the S45–0.45 system and the F45–0.45 system are 71.7% and 25.5% of that of the pure cement system, respectively.Figure 2
Hydration heat of the representative mix proportion of large amount of fly ash and slag (a) hydration heat release rate curve, (b) cumulative heat release curve.
(a)(b)The hydration heat of mix proportion mixed with a trace amount of limestone powder is shown in Figure3. The numbers “PC-0.45″ and “L05–0.45″ represent, respectively, the pure cement and cementitious material mixed with 5% limestone powder, in which the water-binder ratio is 0.45. It can be seen that after adding a trace amount of limestone powder, the hydration heat release rate and the total heat release of the cement system have little change. However, the second exothermic peak of the L05–0.45 system is about 1 hour earlier than that of the pure cement system, which promotes the early hydration of the cement. This is mainly due to the nucleation of limestone powder: limestone powder provides additional growth sites for hydration products and accelerates the nucleation growth of hydration products [32]. Figure 3(b) shows that the cumulative heat release curves of the two systems of PC-0.45 and L05–0.45 are almost coincident. Since the initial hydration heat release rate of the L05–0.45 system is slightly higher, the cumulative heat release curve of the L05–0.45 system is above the curve of pure cement before 30h and then below it.Figure 3
Hydration heat of the mix proportion mixed with a trace amount of limestone powder (a) hydration heat release rate curve, (b) cumulative heat release curve.
(a)(b)The 3d pore structure of the composite cementitious material mixed with trace limestone powder is shown in Figure4. The numbers “PC-0.3″ and “L05–0.3″ represent, respectively, the pure cement and cementitious material mixed with 5% limestone powder, in which the water-binder ratio is 0.3. The pore structure of the cement system is slightly refined after adding a small amount of limestone powder. The pore distribution less than 50 nm for L05–0.3 system is more than that of pure cement system, and the pore distribution of 200 nm to 700 nm becomes less. Figure 4(b) displays that the cumulative pore volume of L05–0.3 system is also lower than that of pure cement system, that is, the total porosity is a little smaller.Figure 4
3 d pore structure of the composite cementitious material mixed with trace limestone powder: (a) differential distribution curve; (b) cumulative distribution curve.
(a)(b)


## 3.2. Chloride Ion Permeability Test Results

The 10 groups of concrete in Tables2 and 3 were cured by the C-28 curing method, and the chloride ion diffusion coefficient was tested for 60 days. The test results of concrete with water-binder ratios of 0.45 and 0.35 are shown in Figures 5(a) and 5(b), respectively. It can be seen that the chloride ion diffusion coefficient of concrete with a water-binder ratio of 0.45 is about 2.5 × 10−12m2s−1. The chloride ion diffusion coefficient of concrete with a water-binder ratio of 0.35 is about 1.0 × 10−12m2s−1. When the water-binder ratio decreases from 0.45 to 0.35, the chloride ion diffusion coefficient is greatly reduced by about 60%. For two kinds of concrete with water-binder ratio of 0.45 and 0.35, the chloride ion diffusion coefficient of the F45 group is 18% lower than that of F35 group. Compared with the S35 concrete, the chloride ion diffusion coefficient of the S45 group concrete is reduced by 18%. It means that increasing the content of fly ash and slag from 35% to 45% can reduce the chloride ion permeability of concrete. This is because after sufficient curing, the hydration of fly ash and slag can be better developed reducing the porosity of concrete while improving the resistance to chloride ion penetration [33]. The chloride diffusion coefficient of the FL45 group concrete is higher than that of the F45 group, which means that the permeability of concrete mixed with trace limestone powder is higher after 28 days of full standard curing.Figure 5
60d chloride ion diffusion coefficient of concrete under C-28 curing mode: (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)After the concrete was cured by different curing methods, the 60d chloride ion diffusion coefficient was tested, and the results are shown in Figure6. Figures 6(a) and 6(b) are the test results of concrete with a water-binder ratio of 0.45 and 0.35, respectively. It can be seen that for the concrete with two water-binder ratios, the chloride ion diffusion coefficient increases greatly with the reduction of the standard curing time from 28d to 1d. For example, the chloride ion diffusion coefficient of F45–0.45 group increases from 2.31 × 10−12m2s−1 under C28 curing mode to 6.35 × 10−12m2s−1 under C1 curing mode. The diffusion coefficients of the S35–0.35 group are 1.06 × 10−12m2s−1 under C28 curing mode and 3.01 × 10−12m2s−1 under C1 curing mode respectively. The diffusion coefficients of the two increased by 175% and 184%, respectively. This is due to the slower hydration rate of fly ash and slag, as shown in Figure 2. Once the curing is inadequate, there will be more water loss, resulting in insufficient cement hydration. Consequently, the fly ash and slag cannot fully react, the admixture cannot play its due role. It means that the curing method has a great influence on the chloride ion diffusion coefficient of concrete. If the standard curing of concrete is not sufficient, it will significantly increase the chloride ion diffusion and affect the durability of the project.Figure 6
60d chloride ion diffusion coefficient of concrete under different curing mode (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)In addition, the diffusion coefficients of fly ash and slag concrete with different contents have different sensitivities to curing methods. For example, under the C14 and C28 curing modes, the diffusion coefficient of the F45 group is lower than that of the F35 group, while the diffusion coefficient of the B45 group is lower than that of the S35 group. On the contrary, under the C1 curing mode, the F45 group has a higher diffusion coefficient than the F35 group, while the diffusion coefficient for S45 group is lower than that of S35 group. It means that when the standard curing is relatively sufficient, a larger amount of fly ash and slag can reduce the permeability of concrete. However, when the curing is insufficient, the admixture cannot play its role. The concrete with a larger amount of fly ash and slag has greater permeability and is more sensitive to the curing method.Taking the chloride ion diffusion coefficient of concrete under the C28 curing mode as a reference, the chloride ion diffusivity under other curing methods is comparatively studied. Figure6 shows that for all the groups of concrete, the diffusion coefficient under the C14 curing mode is very close to that under the C28 curing mode, and the increase in the diffusion coefficient is basically within 10%. It can be seen that the standard curing for 14 days can basically guarantee the chloride ion penetration resistance of the concrete. Compared with the permeability under C28 curing mode, the increase in permeability coefficient of C7 curing mode is basically in the range of 20% to 55%. Among them, the increase of F45 group concrete has reached 55%. The standard curing time of 7d is relatively short, which cannot guarantee the chloride ion penetration resistance of concrete. Compared with the C28 curing method, the chloride ion diffusion coefficient of concrete is greatly increased under the C1 and C3 curing methods. Therefore, removing the mold after standard curing for 1 or 3 days and switching to dry air curing will cause serious deterioration of the resistance to chloride ion penetration. Consequently, it is not recommended to remove the mold prematurely in the actual project and sufficient curing is required.Taking the chloride ion diffusion coefficient of concrete under the C28 curing mode as a reference, the chloride ion diffusivity under other curing modes was comparatively studied. It can be seen that the diffusion coefficient of concrete mixed with limestone powder increases slightly under C14 curing mode, within 10%. Under C7 curing mode, the increase of diffusion coefficient of concrete mixed with limestone powder is less than 15%, while the concrete without limestone powder increases by 20%–55% under C7 curing mode. Therefore, adding a small amount of limestone powder can effectively reduce the chloride ion diffusivity under C7 curing mode. In summary, when the concrete cannot be fully cured in the actual project, it can be considered to add 5% limestone powder by mass, which can effectively reduce the chloride ion diffusion. However, even if limestone powder is added, the mold can be removed only after standard curing for at least 7 days.Before and after adding limestone powder, the differences of 60d chloride ion diffusion coefficient of concrete under different curing methods were compared, respectively. Figures7(a) and 7(b) show the comparison results of concrete with water-binder ratio of 0.45 and 0.35, respectively. It can be seen that under C1, C3, and C7 curing modes, adding 5% limestone powder can effectively reduce the diffusion coefficient of chloride ions. That is, under the condition of insufficient standard maintenance, the addition of a small amount of limestone powder can enhance the penetration resistance of chloride ions. This is because the limestone powder is relatively fine, which can play a role in nucleation in promoting the early hydration of cementitious materials and improving the pore structure, as shown in Figures 3 and 4, respectively. So, limestone powder can prevent the outward diffusion of water to a certain extent. In the C14 and C28 curing modes, the diffusion coefficient of chloride ions increases slightly after adding a small amount of limestone powder.Figure 7
60d chloride ion diffusion coefficient of concrete under different curing mode (a) concrete with a water-binder ratio of 0.45; (b) concrete with a water-binder ratio of 0.35.
(a)(b)


## 4. Conclusion

In this paper, the chloride ion diffusivity of concrete with a large amount of mineral admixtures under different curing methods was studied. The influence of curing time on the permeability of concrete surface was explored. The following conclusions are obtained:(1)
The hydration rate of cementitious system with large amount of mineral admixture is slow. When the curing time is short and the water loss is large, the hydration of cementitious materials in the concrete with large amount of admixtures will be insufficient. It will significantly increase the diffusion of chloride ions on the concrete surface, thus affecting the durability of the project. Moreover, the admixture cannot play its due role.(2)
Taking the C28 full curing mode as a reference, the increase of diffusion coefficient under C14 curing mode is basically less than 10%. That is, standard curing for 14 days can basically ensure the anti-chloride ion permeability of concrete. When the standard curing time is less than 7 days, the chloride ion penetration resistance will deteriorate seriously. Therefore, in actual projects, it is not recommended to remove the concrete mold prematurely and sufficient maintenance is required.(3)
Under the condition of insufficient standard curing, adding a small amount of limestone powder can effectively enhance the anti-chloride ion penetration performance. This is because the limestone powder is relatively fine, which can play a role in nucleation in promoting the early hydration of cementitious materials, improving the pore structure, and preventing the outward diffusion of water to a certain extent. Therefore, when the concrete cannot be fully cured in actual projects, it can be considered to add a small amount of limestone powder, which can effectively reduce the chloride ion diffusion.



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*Source: 1034388-2022-03-15.xml*