chloride diffusion of alkali-activated fly ash/slag concrete

CHLORIDE DIFFUSION OF ALKALI-ACTIVATED FLY ASH/SLAG CONCRETE

Jingxiao Zhang (1)(2), Yuwei Ma (1)(2), Jiazheng Zheng (1)

(1) Guangzhou University - Tamkang University Joint Research Centre for EngineeringStructure Disaster Prevention and Control, Guangzhou University, China

(2) Centre for Future Materials, University of Southern Queensland, Australia

(3) School of Materials Science and Engineering, South China University of Technology, China

Abstract The widespread application of alkali-activated fly ash/slag (AAFS) concrete requires

satisfaction of a series of performance criteria both from its early age properties (e.g. workability, strength) and long-term stability. In this study, long-term (till 180 days) natural chloride diffusion tests were conducted to evaluate the chloride diffusion in AAFS concretes prepared with different slag content, water-binder (w/b) ratio, alkali content, and sand-aggregate ratio. The results revealed that the free chloride diffusion coefficient (Df) of AAFS concretes was between 0.4-1.8×10-12 m2/s. The slag content and w/b were found as dominant parameters affecting the long-term chloride transport in AAFS concretes, while the sand-aggregate ratio presented a limited effect. MIP results indicated that capillary pores in AAFS reached percolation and became disconnected after 180 days. The long-term chloride diffusivity of AAFS concretes was closely related to the threshold pore diameter and volume of pores > 5 nm. The more larger pores, the higher chloride diffusion coefficient was. Keywords: Alkali-activated concrete, Natural chloride diffusion tests, Chloride diffusion coefficient, Pore structure

1. INTRODUCTIONAlkali-activated materials (AAMs), manufactured by the reaction between alkaline activator

and solid aluminosilicate powders, i.e. fly ash (FA), ground granulated blast furnace slag (GGBFS) and calcined clay, have attracted much interest in academic and industrial fields over the past decades [1, 2]. AAMs present comparable mechanical properties and considerably lower CO2 emission, thus are regarded as a promising alternative to ordinary Portland cement (OPC) [3-5]. To overcome the shortcomings of AAMs based on single raw material, alkali-activated fly ash/slag binary system (AAFS) is proposed to satisfy the performance criteria for concrete, and this binary system is also applied in real construction in Australia. Except for

4th International RILEM conference on Microstructure Related Durability of Cementitious Composites (Microdurability2020)

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early age properties, another technical issue that raises wide concern is the chloride resistance of AAFS and its efficient to protect steel bars in reinforced AAFS concrete.

This study presents a comprehensive investigation on the chloride diffusion of AAFS concretes with different mixing parameters by using natural chloride diffusion test at long term (till 180 days). Chloride transport was assessed according to the free chloride diffusion coefficient (Df). The effects of various parameters, including slag content, water–binder ratio, alkali content, and sand–aggregate ratio, were evaluated. The pore structures of AAFS mixtures were characterized by mercury intrusion porosimetry (MIP). Then, the relationship of pore structures and chloride diffusion properties was discussed.

2. MATERIALS AND METHODS

2.1 Materials Fly ash (Zhongshan Power Station, Guangdong, China) classified as Class F according to

ASTM C618, and ground granulated blast furnace slag (Shaoguan Steel Group Company Limited, Guangdong, China) were used as solid precursors to manufacture AAFS concrete. The chemical compositions of FA and GGBFS determined by XRF techniques are listed in Table 1.The alkaline activator used in this study was a mixture of NaOH and sodium silicate solution.Analytic grade NaOH pellets were dissolved in distilled water to prepare NaOH solution. NaOHsolution, sodium silicate solution (Na2O = 82.19 wt.%, SiO2 = 28.18 wt.%) and tap water werethen mixed in proportions to prepare alkaline activator solution with different Na2O and SiO2contents. The alkaline activator was prepared 24 hours before concrete casting. Natural riversand with a maximum size of 0.5 mm was used as fine aggregates. Coarse aggregates wereprepared by mixing crushed basalt with medium size (10–20 mm) and small size (5–10 mm) ina ratio of 60:40.

Table 1: Chemical composition of FA and GGBFS SiO2 Al2O3 CaO Fe2O3 MgO SO2 Na2O K2O LOI

FA 50.59 25.55 9.84 6.92 1.12 1.12 1.45 1.19 2.22

GGBFS 34.36 16.89 38.13 0.36 6.23 2.3 0.24 0.41 1.08

AAFS concretes with different slag content, w/b, Na2O content and sand-aggregate ratio were prepared in a single horizontal shaft concrete mixer. Table 2 presents the mixture proportions of AAFS concretes, which (Table 2) were denoted as: slag% (SL), w/b (W), Na2O% (N) and sand-aggregate ratio (S). Na2O content was designed as low as possible with theconsideration of concrete cost. The unit weight of AAFS concrete was designed in the range of2350-2450 kg/m3 and the binder content was 400 kg/m3. Firstly, sand, coarse aggregates andprecursor materials (FA and GGBFS) were dry-mixed for 2 mins. Subsequently, the alkalineactivator was added into the mixture and mixed for another 2 mins. Fresh concrete was thencast into cube moulds (100 mm × 100 mm × 100 mm). After 24 hours curing at 20 °C,specimens were cured in a curing chamber (relative humidity of 95% and temperature of 20 °C± 2°) before tests.


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Table 2: Mixture proportions of AAFS concrete Sample FA GGBFS Na2Ob SiO2

b Sand CA w/b s/a SL30 280 120

4% 6% 720.3 1080.4 0.45 0.40 SL50a 200 200 SL70 120 280

SL100 0 0 W40 200 200 4% 6% 720.3 1080.4 0.40 0.40 W45a 0.45 W50 0.50 N4 200 200 4% 4% 720.3 1080.4 0.45 0.40 N5 5% N6 6% S36

200 200 4% 6%

648.3 1152.5

0.45

0.36 S38 684.3 1116.5 0.38 S40a 720.3 1080.4 0.40 S42 756.3 1044.5 0.42 S44 792.3 1008.5 0.44

a SL50, W45 and S40 are the same mix proportion b Na2O% = Na2O/(FA + GGBFS) (%) and SiO2% = SiO2/(FA + GGBFS) (%)

2.2 Methods Natural diffusion test was performed according to a slightly modified form of ASTM C1543.

After curing for 28 days, surfaces of AAFS concrete specimens were sealed with water-resistant paraffin wax and only the bottom surface was exposed to the testing solution to ensure one-dimensional diffusion [6]. Subsequently, sealed specimens were immersed in 3.5% NaCl solution for another 90 and 180 days. The solution was renewed every 30 days, and the container was sealed with a lid to avoid water evaporation. At the end of each testing age, concrete specimens were taken out from NaCl solution and ground into powder layer by layer. After removing the surface paraffin wax, concrete specimens were fixed on the DRB-H1 concrete grinding machine. 10 layers were ground for each concrete specimen and each layer was kept in 2 mm. The grinded powder was then sieved with a 0.6 mm sieve-mesh, dried in an oven (55 °C ± 5°) for 2 hours, and put in a desiccator to cool to 20 °C [7]. For statistical reliability, 3 × 2 specimens were ground to powder for each concrete mixture with two testing ages (90 days and 180 days).

After that, the free chloride content of grinded powder was measured in accordance with JTJ270-98. 2 g concrete powder (G) was put in 50 ml distilled water (V1). After shaking for 20 min and standing for 24 hours, suspension liquid was filtered. Subsequently, 20 ml filtered solution (V2) was pipetted into an Erlenmeyer flask with 2 drops of phenolphthalein. Diluted H2SO4 solution was then used to neutralize until the solution became colorless. Afterwards, 10 drops of K2CrO4 solution were added and 0.02 mol/L AgNO3 solution was used to titrate until the solution become red. The volume of consumed AgNO3 solution (V3) was recorded. The free chloride content was calculated based on Equation 1 [7]:

C =𝐶𝐶𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴3𝑉𝑉3×0.03545

𝐺𝐺×𝑉𝑉2𝑉𝑉1

× 100% (1)

Where:


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C is free chloride content in concrete powder; CAgNO3 is content of AgNO3 solution, 0.02 mol/L; G is weight of concrete powder, 2 g; V1 is volume of water used to dissolve concrete powder, 50 ml; V2 is volume of filtrate, 20 ml; V3 is volume of AgNO3 solution for titration. For statistical reliability, each grinded powder was assessed three times; and the average result was calculated.

The Df was determined by fitting Fick’s second law as expressed in Equation 2 to the measured free chloride profile:

C(x, t) = 𝐶𝐶0 + (𝐶𝐶𝑆𝑆 − 𝐶𝐶0) �1 − erf � 𝑥𝑥2√𝐷𝐷𝐷𝐷

�� (2)

Where: C(x,t) is chloride content at depth x and time t; C0 is initial chloride content of the concrete;

Cs is chloride content at exposure surface; erf is error function. MIP was used to determine the total porosity and pore size distribution of AAFS mortar

specimens. Mortar with a size of 2 mm × 2 mm × 2 mm was obtained from concrete specimens (after immersion for 180 days). Coarse aggregates were eliminated and the mortars were then immersed in ethanol for 2 weeks to stop further reaction. Afterwards, mortar samples were put in freeze-dryer for another 2 weeks of drying process. Micrometritics Poresizer 9500 was applied for the MIP measurement (0.485 N/m, 130°).

3．RESULTS AND DISCUSSION

3.1 Chloride diffusion Figure 1 shows the Df of AAFS concrete with different FA/GGBFS, w/b, Na2O content and

sand-aggregate ratio after 90 and 180 days of immersion. The w/b exhibited the most prominent effect on the Df of AAFS concretes. As shown in Figure 1, the Df increased from 1.1×10-12 m2/s to 3.2×10-12 m2/s as the w/b increased from 0.4 to 0.5 at 90 days. Such results were different from the findings in AAS concrete [8], where they claimed that excess water was not affecting the pore structure and chloride penetration. However, in their study [8], a considerably higher w/b (from 0.55 to 0.7) and higher NaCl concentration (165 g/L) were applied, under which condition the real Df cannot be correctly reflected. In fact, the extra water in the alkaline activator acted as capillary pores in the concrete matrix, particularly around the ITZ [9, 10]. At longer age (180 days), the Df of W50 reduced to 1.3 ×10-12 m2/s, indicating a significantly pore refinement between 90 to 180 days.

For AAFS concrete with higher slag content, the Df decreased from 1.8×10-12 m2/s (SL30) to 0.4 ×10-12 m2/s (SL100), which was consistent with the findings from previous researches [11-13]. Compared with N-A-S-H gels, C-A-S-H gels could incorporate water into their crystalline structure during hydration process [14]. As a result, pores were filled and the chloride resistance of AAFS concrete was improved. However, the decrease of Df was more pronounced with age for SL30 and SL50, which indicated that after 90 days, the continuous reaction of AAFS concrete was dominated by fly ash.

The Df of AAFS concretes gradually decreased with increased Na2O content. Such findings were different from the results by Babaee and Castel [11], in which they noted increasing Na2O content increased the Df of AAFS. They ascribed it to the high alkaline environment that hinder the diffusion of calcium from slag. However, in their experiments, the SiO2 content increased


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as the Na2O content increased in order to keep a constant modulus (SiO2/Na2O). The SiO2 content is known to decrease the overall pH of activator, leading to a lower degree of reaction. With a constant SiO2 content, higher Na2O content was expected to promote a more rapid dissolution of raw materials and reduce chloride transport in AAFS. However, the effect of Na2O content was relatively small compared with FA/GGBFS and w/b.

Compared with other parameters, the sand-aggregate ratio had no significant effect. AAFS concrete with different sand-aggregate ratio presented similar Df both at 90 days and 180 days, which is consistent with the findings in prior work [15].

Figure 1: Free chloride diffusion coefficients of AAFS concrete with different FA/GGBFS, w/b, Na2O content and sand-aggregate ratio after 90 and 180 days of immersion

3.2 Pore structure Figure 2 presents the pore size distribution and differential pore size distribution of AAFS

mortar with different slag content, w/b and Na2O content at the age of 180 days. Generally, AAFS with higher slag content, lower w/b and higher Na2O content showed lower total porosity. However, SL50, SL70 and SL100 (Figure 2A), W40 and W45 (Figure 2B), N4 and N5 (Figure 2C) presented similar total porosity. Therefore, further study and discussion are required

The peak in differential curve was also corresponded to pore system in AAFS. In view of the ion diffusion characteristic, the pore diameter corresponding to the peak (also known as threshold pore diameter) was regarded as the minimum diameter of pores that form a continuous network throughout materials [16]. AAFS mixtures with lower threshold pore diameter had a smaller and denser pore system under the similar total porosity. For example, SL50 displayed a distinct peak at larger pore diameter (20 nm), while the peaks for SL70 and SL100 were at 4 nm (SL70 had bimodal curve) (Figure 2a). W40, with w/b of 0.4, presented two peaks at 5 nm and 15 nm, while W45 exhibited only one peak at around 20 nm (Figure 2b). N4 presented two peaks at 5 nm and 10 nm, while N5 exhibited only one peak at around 5 nm (Figure 2c).

From the above analyses, the pore structure of AAFS was closely related not only to total porosity, but also to threshold pore diameter. AAFS mixtures with higher slag content, lower

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w/b and higher Na2O content showed lower total porosity and/or lower threshold pore diameter, which means they had more excellent pore structure.

Figure 2: Porosity and pore size distribution with different FA/GGBFS (A), w/b (B) and Na2O content (C) at 180 days

3.3 Pore structure vs. chloride diffusion The Df of AAFS depended not only on total porosity, but also on pore size distribution. It

was observed that total porosity could not fit well with the Df (Figure 3a), while a good correlation was obtained between the Df and porosity of pores larger than 5 nm (Figure 3b). It was reported by Powers et al.[17] that capillary pores (pores > 10 nm) were regarded as disconnected when the capillary porosity reduced to 18%-20%. It implied that the transportation of ions in material was mainly depended not only on capillary pores, but also on a part of gel pores. Therefore, the threshold pore diameter of gel pores became vital to the ion transportation.

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In this study, all the AAFS mixtures had total porosity lower than 18% (Figure 3a), and the minimum threshold pore diameter was around 5 nm. It is likely that capillary pores and gel pores larger than 5 nm contributed to ion transportation in AAFS. Therefore, the Df exhibited a well correlation with the pores larger than 5 nm.

Figure 3: Relationship between free chloride diffusion coefficient (Df) and total porosity (a); free chloride diffusion coefficient (Df) and porosity of pores larger than 5 nm (b).

4．CONCLUSIONS

This study investigated the chloride diffusion and pore structure of AAFS concretes prepared with different parameters, including FA/GGBFS, w/b, Na2O content and sand-aggregate ratio. Then the relationship between pore structure and chloride diffusion was explored. The main conclusions from the experimental work can be summarized as follows:

1. FA/GGBFS and w/b had a more prominent effect on the chloride diffusivity resistanceof AAFS concretes than the effect of Na2O and sand-aggregate ratio both at 90 days and180 days. More C-A-S-H gels (by increasing slag content) and lower w/b (≤0.45) werepreferred for durable AAFS concretes. The Df gradually increased with increasing Na2Ocontent, while sand-aggregate ratio had no obvious effect.

2. AAFS mixtures with higher slag content, lower w/b and higher Na2O content showedlower total porosity and/or lower threshold pore diameter, which means achieving moreexcellent pore structure

3. At 180 days, the capillary pores in AAFS reached its percolation and becamedisconnected. The chloride transportation was mainly depended on capillary pores anda part of gel pores. Therefore, the Df of AAFS was closely related to the volume of pores >5 nm.

ACKNOWLEDGEMENTS The authors thank the Pearl River S&T Nova Program of Guangzhou (201806010004), Australian Research Council Discovery Project (1006016), National Natural Science Foundation of China (51561135012) and State Key Laboratory of Silicate Materials for Architectures Foundation (SYSJJ2017-05) for funding the project.

(a) (b)


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chloride diffusion of alkali-activated fly ash/slag concrete

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