Influence of MFPSA on mechanical and hydrophobic behaviour of fiber cement products

Construction and Building Materials 223 (2019) 1016–1029

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Influence of MFPSA on mechanical and hydrophobic behaviour of fiber cement products Chunguang Li a,b, Annan Zhou b,⇑, Jianfeng Zeng a, Zhenzhong Liu a, Zhijun Zhang a a b

School of Resources, Environment and Safety Engineering, University of South China, Hengyang, Hunan Province 421001, China Discipline of Civil and Infrastructure Engineering, School of Engineering, Royal Melbourne Institute of Technology (RMIT), Melbourne, Vic 3001, Australia

h i g h l i g h t s
The optimal calcination temperature of MFPSA is 800 °C and the dosage is 4%.
MFPSA can increase the 28d flexural strength of fiber cement by 11.5%.
MFPSA can increase the compressive of fiber cement strength by 5.7%.
MFPSA decreases the 28d capillary water absorption of fiber concrete by 31.9%.
MFPSA promotes hydration and introduces hydrophobic groups.

a r t i c l e

i n f o

Article history: Received 12 February 2019 Received in revised form 26 July 2019 Accepted 31 July 2019 Available online 7 August 2019 Keywords: Modified Fenton paper sludge ash (MFPSA) Cement products Calcination temperature Dosage Mechanical property Hydrophobicity

a b s t r a c t The modified Fenton paper sludge ash (MFPSA) was employed to replace cement in different proportions to prepare fiber cement products. The mechanical and hydrophobic properties of the MFPSA/fiber cement and concrete were studied. When the calcination temperature is 800 °C and the dosage is 4%, the MFPSA increases 28d flexural strength of fiber cement by 11.5%, compressive strength by 5.7%, and decreases water absorption by 2.6%. The MFPSA also decreases the 28d capillary water absorption of fiber concrete by 31.9%. The XRD, SEM/EDS, FTIR were employed to reveal how the MFPSA affects the hydration mechanism, and microstructure of MFPSA/fiber cement products. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Cement products such as concrete and mortar are inherently hydrophilic, porous and micro-cracked. Therefore, they usually cannot prevent the invasion and transportation of water [1]. Water invasion and transportation in cement products, especially when the water contains ions, can induce the corrosion, the degradation of freeze-thaw resistance, the reduction in durability and the strength loss of cement products [2–5]. Water invasion to cement products can also cause corrosion of steel bars inside, which seriously threaten the safety of hydraulic structures and underground structures [6]. Increasing the density of cement products by using a low watercement ratio is helpful to enhance the durability of watertight cement products [7,8]. However, it is still hard to eliminate water ⇑ Corresponding author. E-mail address: [email protected] (A. Zhou). https://doi.org/10.1016/j.conbuildmat.2019.07.338 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

ingress. To prevent water from entering the interior of cement products, two approaches have been commonly used according to the literature. One approach is adding fibers to reduce the micro-porosity and lower the possibility of cracking [9,10]. The other approach is changing the property of cement products from hydrophilicity to hydrophobicity by coating, impregnating or adding admixture that can form a natural waterproof barrier [11–14]. Fibers commonly-used in cement products can be classified into four categories according to materials. They are natural, steel, glass, and Polymer-based fibers [15]. Natural fibers are low-cost and environmental-friendly. However, a major disadvantage in using natural fibers is that the fibers will decay along with the time. Steel fibers can significantly enhance the strength of the cement products. But they are disadvantaged by a high volumetric density and corrosive characteristics [16]. Employment of glass fibers in cement products may not fulfill the requirement on strength. The cement products with glass fibers show lower strength than those with steel fibers in general. Their compressive

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strength even lower than blank cement products [17]. Polymerbased fibers are more popular than the others because of the long-term durability owing to good hydrophobicity and excellent chemical stability. One of the mostly used polymer-based fibers is the polypropylene fiber [18]. On one hand, the polypropylene fiber can decrease the quantity and size of micro-porosity in the cement products, and reduce the cracking at the interfacial transition zone around the aggregates [19]. On the other hand, the optimum dosage of the polypropylene fiber is low, which is about 0.9 kg/m3 [20] and thus ensures the economy of the products. Regarding the change of the hydrophilic property of cement products, adding admixture is superior to impregnation and coating to enhance the hydrophobicity, since it usually can provide a longer effective period than the other two methods [21]. Cement products hydrophobized through adding admixture also does not require regular maintenance. Moreover, adding admixture endows internal hydrophobicity for cement products rather than just working on a certain range of the surface, thus saving repair and renovation costs [22]. However, some of the hydrophobic admixtures may decrease the mechanical strength of cement products. Therefore, it is especially demanding to develop good admixtures with hydrophobic potential but without reducing mechanical strength. One of the options for the hydrophobic admixture is the paper sludge (PS). Using properly treated (PS) to be an admixture to enhance the hydrophobic properties for cement products has been proposed and validated [22–25]. Using the PS as an admixture can also be conducive to the disposal and reuse of the very large quantities of waste sludge from the paper recycling industry globally [26]. According to the previous study [27], the chemical and mineral composition of ordinary PS include: cellulose (C12H20O10), calcite (CaCO3), muscovite (Al2O3)3(SiO2)6K2O(H2O)2, talc (Mg3Si4O10(OH)2), kaolinite (Al2O32SiO22H2O), quartz (SiO2), and some refractory contaminants. In the paper recycling industry, to reduce the waste volume and producing energy, the PS is usually combusted at 500 °C to 1100 °C. After combustion, the PS turns to the primary sludge ash (PSA) which has high pozzolanic properties and metakaolin [28,29]. To reduce the refractory contaminants and meet environmental requirements, a Fenton oxidation catalyst has been widely using in the treatment process for PS to separate the organic matters from the sludge. After treatment, a new type of sludge named Fenton paper sludge (FPS) can be produced [30], which contains up to 40% of Fe cement. FPS is usually incinerated to form Fenton paper sludge ash (FPSA) [31]. Compared with the PSA, the FPSA contains relatively higher amorphous hydrated iron oxide. Similar to the PSA that can be modified by stearate [21], the FPSA can also be modified (e.g. with stearate) to produce the MFPSA which has a better hydrophobicity. When used as an admixture material, the FPSA/MFPSA is expected to change the hydration, setting and hardening mechanism of cement products and thus affect their hydrophobic and mechanical properties. Most of the existing research on waterproofing enhancement for cement products focus on either the first approach (such as usage of fiber) or the second approach (such as usage of PS or PSA, especially PSA modified by stearate) [21]. However, using the FPSA/MFPSA as the admixture to enhance the hydrophobicity of cement products is seldom discussed in the literature. Furthermore, the research on combined usage of fiber and admixture is extremely rare. This research aims to examine the joint impact of polypropylene fiber and MFPSA on the hydrophobic and mechanical properties of cement products. In this research, the MFPSA was replaced by ordinary Portland cement in different proportions to prepare fiber cement products, and its mechanical properties and hydrophobic properties were tested. The mechanism behind the observation on the mechanical/hydrophobic properties will also be probed in this study. XRD, SEM/EDS, FTIR were used to analyze

the composition, structure, functional group changes of the MFPSA and the mechanism of MFPSA affecting the hydration, interface and structure of cement products. 2. Materials In the research, Fenton paper sludge (FPS) was obtained from a sanitary landfill of Guangxi Guitang Paper Mill in Southern China. The FPS was oxidized by Fenton agent after PS desulfurization and dedusting, in liquid form with a water content of 87% and a PH value of 10.46 in distilled water. The main chemical element composition of FPS was determined by XRF (Spectro 2000 Analyser) and presented in Table 1. The FPS was naturally dried firstly, placed in an electric blower box at 105 °C to be completely dried, and then crushed to 2–5 mm by a jaw crusher. Then, the crushed FPS was continuously calcined at 800 °C and 1000 °C, respectively, for 2.5 h in a rapid heating furnace to produce the FPSA. After cooling to room temperature, the stearic acid modifier (4% of the FPSA mass) was added to the FPSA to improve the hydrophobic property. The mixture was grinded in a planetary ball mill at 350 r/min for 10 h, to prepare the modified FPSA (i.e., MFPSA-800 and MFPSA1000). Ordinary Portland Cement 42.5 and dry-cleaning natural aggregate from Xiangjiang river were used for the tests. The fine aggregate was a medium sand with the apparent density of 2620 kg/m3 and the fineness modulus is 2.8. The coarse aggregate was gravel with apparent density 2630 kg/m3 and the particle size is 10– 20 mm. In addition, polycarboxylic acid water-reducer, white shiny leaf crystal stearic acid and polypropylene fiber were adopted as well. Water reducing rate of the employed water reducing agent was 20%. The molecular weight of the stearic acid was 248.48. Technical indicators of the polypropylene are shown in Table 2.

3. Experiments 3.1. Preparation of the MFPSA/fiber cement paste Mix proportions of six blended fiber cement specimens are shown in Table 3, where 0%, 4%, 8%, 12%, 16% and 20% of cement was replaced with the MFPSA by mass. As shown in Table 3, M-0 represents the benchmark sample without MFPSA and M-‘x’-‘y’ stands for the samples mixed with MFPSA (x = calcination temperature and y = MFPSA/cement replacement ratio). First, cement, MFPSA and polypropylene fiber (0.9 kg/m3) were stirred until uniform. Then, 35% water by mass of cement was added and the mixture was stirred continuously until there was no solidarity. The fluidity was adjusted to the same with superplasticizer and measured by an expansion barrel (36  60  60 mm). Then, slurry specimens were cast in steel molds (40  40  160 mm) and compacted in two layers (60 times for each layer) by using a vibrating table. Finally, the specimens were put in a standard environment room for curing until the corresponding age. 3.2. Experiments on the performance of MFPSA/fiber cement blocks The flexural and compressive strength of the MFPSA fiber cement blocks were tested according to ASTM specification

Table 1 Chemical element composition of Fenton paper sludge (weight %). O

Ca

C

Fe

Si

Al

Others

47.2

27.7

10.5

10.1

2.9

1.1

0.5

Table 2 Technical indicators of polypropylene fiber. Specifications

Polypropylene fiber

Length (mm) Diameter (mm) Density (g/cm3) Compressive strength (MPa) Slenderness ratio MOE (GPa)

19 0.05 0.91 358 380 3.5

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Table 3 Mix proportions of MFPSA/fiber cement blocks. Unit (kg/m3)

Sample

M-0 M-800-4 M-800-8 M-800-12 M-800-16 M-800-20 M-1000-4 M-1000-8 M-1000-12 M-1000-16 M-1000-20

Cement

Water

Polyproplene fiber

MFPSA

Sand

Superplasticizer

1489 1430 1370 1311 1251 1191 1430 1370 1311 1251 1191

521 501 480 459 438 417 501 480 459 438 417

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

0 59 119 178 238 298 59 119 178 238 298

365 365 365 365 365 365 365 365 365 365 365

0 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5

[32,33] after standard curing for 7 days, 14 days, and 28 days, separately. Water absorption was tested by the following method. Cement blocks were first dried naturally at room temperature for 28 days, and then placed in an oven at 50 °C to dry to constant weight. The samples were quickly wrapped with polyethylene films to prevent external water absorption during cooling, and then they were completely submerged 10 cm underwater until saturated. The water absorption rates were determined by weighing. The hydrophobicity of cement blocks is also measured by the size of the area wetted by the ink, following the method presented in literature [21]. A burette filled with colored ink was employed to generate ink droplets. We adjusted the droplet size, drop height and dropping rate until the droplets are naturally spread out in the cement block. The photo was taken at the same position, and the inked area was calculated by CAD drawing software. Percentage of residual area (the area with ink) on a 40  40 mm2 section can be determined. More detailed measurement method here can be found in literature [21]. 3.3. Manufacture of MFPSA/fiber concrete According to ASTM standards [34,35], aggregate, cement, fiber and MFPSA (with 0%, 4%, 8% and 12% replacement rate of cement by mass) were mixed uniformly using a single horizontal forced mixer. After adding water (with water reducer) and stirring, the concrete strength test blocks (150 mm  300 mm) and the hydrophobic characteristic test blocks (100 mm  100 mm  100 mm) were produced. The detailed mix proportions are shown in Table 4. As shown in Table 4, C-0 represents the benchmark concrete sample without MFPSA and C-800-‘y’ stands for the samples mixed with MFPSA (800 is calcination temperature and y is MFPSA/cement replacement ratio). Based on the performance of MFPSA/fiber cement blocks, the MFPSA-800 performed better than the MFPSA-1000 in both flexural and compressive strength (see section 4.2.1). Therefore, only MFPSA-800 was employed for manufacturing the MFPSA/fiber concrete. The samples were demolded after molding, then placed in a standard condition (20 °C ± 2 °C, humid-

ity no less than 95%) for curing to 7 days, 14 days, and 28 days, respectively. 3.4. Experiments on the performance of MFPSA/fiber concrete blocks The compressive strength and the splitting tensile strength of the MFPSA/fiber concrete blocks with 7-, 14- and 28-day curing were tested using a hydraulic press apparatus according to ASTM standards [34,35]. The arithmetic mean of the measured values of the three test pieces was taken as the measured value. When the difference between any measured value and the median exceeds 15% of the median value, the median value was taken as the measured value. When the difference between the two measured values exceeds the above requirements, the test results of the group are invalid, and the test was repeated. To study the capillary water absorption characteristics, each group of concrete samples (three pieces) with different ratios of MFPSA-800 at the curing age of 7 days and 28 days were cut into six pieces (100 mm  100 mm  50 mm) by an automatic rock cutter. The testing method on the capillary water absorption of the concrete samples follows the method presented in literature [36– 39]. These pieces were placed in an oven at 50 °C for 24 h to ensure the same initial moisture content. After cooling, surrounding sides of the pieces were sealed with paraffin and only two opposite faces of 100 mm  100 mm were left to allow water to migrate in a onedimensional condition. Hereafter, the test pieces were placed on spacer bars with a diameter of 5 mm, and their bottom surfaces were immersed in water with a depth of 5 mm (see Fig. 1). After taken out every 0, 0.5, 1, 2, 4, 8, 12 and 24 h, the excess water on the surface of the test pieces were dried, then which were weighed quickly within 30 s. Finally, the average value of the water absorption increments of six test pieces with the same age was used as the final value. The cumulative capillary water absorption height per unit area can be calculated using the following equation.

i¼

DW A  q0

ð1Þ

Table 4 Mix proportions of MFPSA/fiber concrete blocks. Sample

C-0 C-800-4 C-800-8 C-800-12

Cement (kg/m3)

MFPSA (kg/ m3)

Gravel (kg/m3) 16–20 (mm)

10–16 (mm)

458 444 421 403

0 17.9 36.7 55.0

466 466 466 466

1088 1088 1088 1088

Sand (kg/ m3)

Polyproplene fiber(kg/m3)

Water (kg/ m3)

Superplasticizer (kg/m3)

732 732 732 732

0.9 0.9 0.9 0.9

208 213 221 225

2.5 2.5 2.5 2.5

C. Li et al. / Construction and Building Materials 223 (2019) 1016–1029

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Fig. 1. Schematic of capillary water absorption test.

where i is the cumulative capillary water absorption height per unit area (mm); DW is the mass of absorbed water (g); A is the crosssectional area of the test piece (mm2), and q0 is the density of water (g/cm3).

FPSA can increase the volcanic ash activity of sludge, but the formation of minerals such as calcium aluminum feldspar, Ca2Fe2O5 and wollastonite reduces the volcanic ash activity of sludge ash to some extent.

3.5. Microscopic experiments

4.1.2. Microstructure analysis of FPSA at different temperatures The microstructure and composition of FPSA at different calcination temperatures obtained by SEM/EDS and local point markers are presented Fig. 3. Before being calcined, as shown in Fig. 3(a), the mineral particles in the uncalcined FPS are obviously blocky, and the minerals are mostly connected in an irregular sheet form. It is clear that the bonding in microstructure of the uncalcined FPS is weak. The analysis of the partial scanning surface by EDS shows: (1) the inorganic matter is the main component of the uncalcined FPS, (2) the organic phase is mainly fine fibers, and it is attached to the periphery of the floc, and (3) the highest mass fraction is from Fe, which accounts for 38.4%. As shown in Fig. 3(b), the large-grained structure of the FPSA (after calcination at 800 °C) and the sheet form connection among grains sharply decrease. Meanwhile, the fine-grained structure and the specific surface area increase remarkably. As indicated by the EDS label of Spectrum 2, Calcium ferroalumnates exist in granular forms. Amorphous SiO2 and Al2O3 are adsorbed on flake-like calcite (as labelled in Spectrum 1) and in the vicinity of calcium aluminoferrite. Most of the particle aggregates are small granular and relatively smooth, which can be hydrated together with cement. When the calcination temperature is set to be 1000 °C, the particles in the FPSA are sintered in a large amount to form an expanded honeycomb structure (see Fig. 3(c)). It can be seen from the EDS mark of Spectrum 3 that the major elements of the honeycomb structure are Ca and Fe, indicating that the crystal phase mainly consists of Ca2Fe2O5. An increase in the degree of sintering may reduce the pozzolanic activity of the sludge.

XRD, SEM/EDS, and FTIR were used to reveal the changes in the mineral phase, microscopic morphology, and functional group during the reaction. The mineral phases of FPS, FPSA, and MFPSA-fibercement blocks were surveyed by an XRD (SIMENS D-500). Their microscopic morphologies were examined by a SEM/EDS (FIB 600i). The functional group change during the modification of MFPSA was analyzed using a FTIR (WQF-510A). The MFPSA fiber cement samples used for test were obtained by the following treatment methods: (1) the sample blocks were broken into pieces of 2–5 mm with a small hammer, (2) terminated their hydration by adding anhydrous ethanol, (3) dried in a 50 °C oven to a constant weight, and (4) milled through a 400-mesh square sieve. Before SEM tests in particular, the samples need to be plated gold in the vacuum. 4. Results and discussions 4.1. Phase, microstructure and functional groups 4.1.1. Phase analysis of Fenton sludge calcined at different temperatures The X-ray diffraction patterns of the FPSA powder at different calcination temperatures are shown in Fig. 2. It can be seen from Fig. 2(a) that the diffraction peaks of the minerals CaCO3, Fe(CO3) and Al2Si2O5(OH4) in the uncalcined FPS are strong. After calcination at 800 °C, as shown in Fig. 2(b), the diffraction peaks of CaCO3 and Fe(CO3) in the FPSA are weakened. Clinker minerals such as Ca2(Fe(Fe0.866Al0.134)O5) are formed, accompanying with a small amount of f-CaO. The kaolin phase disappeared completely, which decomposed into metakaolin whose main components are amorphous Al2O3 and SiO2. The X-ray diffraction peak of the FPSA at 800 °C is dispersive, and the mineral crystal structure is unstable. Therefore, the FPSA calcined at 800 °C is suitable to be employed as a mineral admixture to react with cement. After calcination at 1000 °C, the diffraction peak of the unstable mineral crystalline phase Ca2(Fe(Fe0.866Al0.134)O5) in the FPSA completely disappeared. It can be also observed that a large amount of active Al2O3, inert Ca2Fe2O5 and wollastonite were thermally decomposed. Calcite completely thermally decomposed and was transformed to anorthite. Further melting reaction occurred in the FPSA, during which clinker minerals such as calcium aluminum yellow feldspar, calcium iron phase solid solution and dicalcium silicate were formed. The thermal precipitation of Al2O3 in the

4.1.3. Comparison of functional groups before and after modification Fig. 4 shows the function group changes in FTIR spectra between FPSA (without modification) and MFPSA (with stearic acid ball milling modification) after FPS calcinated at 800 °C and 1000 °C. It can be seen that the functional group peak change of the MFPSA-800 is significantly higher than the MFPSA-1000. It can be seen from the FTIR spectrum (see Fig. 4(a)) that, the MFPSA exhibited stretching vibration absorption peaks of C–H at 2916 cm1 and 2848 cm1, and C@O at 1701 cm1. At 1440 cm1, the COO-based symmetric stretching vibration absorption band of the MFPSA was significantly enhanced, and the peak intensity was also increased compared with the FPSA. Besides, a more pronounced –OH peak appeared at 3461 cm1 in the FTIR spectrum of MFPSA than FPSA. It was proved that the stearic acid reacted with CaCO3 agglomerated on the sludge surface. On the other hand, an absorption peak of a new C-O group appears at 1298 cm1

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(b) FPSA calcined at 800 °C

(a) Uncalcined FPS

(c) FPSA calcined at 1000 °C Fig. 2. XRD patterns of FPSA at different calcination temperatures.

(see Fig. 4(a)). One of the possible reasons is that RCOOH forms a hydrogen bond with the active hydroxyl group on the surface of Ca2Fe2O5. It may also be attributed to the formation of COOH. . .O (Ca2Fe2O5) by chemisorption, or the C–O. . .Fe bond formed by dehydration. As shown in Fig. 4 (b), there is almost no peak of C-O appearing at 1298 cm1 in the FTIR spectrum of MFPSA. The peaks of –CH3 and –CH2 at 2910 cm1 and 2874 cm1, in addition the peak of –COOH at 1705 cm1 also can be seen, which are all peaks of stearic acid, indicating that they have not been grafted with FPSA. At the same time, the weak peak of –OH can be found at 3739 cm1 in the FTIR spectrum of MFPSA-1000, indicating few preface adsorptions of the –OH on the MFPSA-1000. That is owing to the crystal structure of the FPSA is more complete and chemically stable by calcinating at 1000 °C. Therefore, the FPSA is difficult to adsorb or graft by stearic acid, deducing strong hydrophobic characteristics because of a lot of free stearic acid. The hydrophobic phenomenon of MFPSA-800 is demonstrated in Fig. 5. Compared with FPSA, most of the MFPSA800 powder is suspended on the water surface, which display a strong single-sided non-wetting property to water. The observation proves that the modification effect of FPSA ball milling with stearic acid is excellent, that can enhance hydrophobic property to FPSA.

4.2. Strength and hydrophobicity of MFPSA/fiber cement 4.2.1. Flexural and compressive strength Fig. 6 shows the variation in the flexural and compressive strength of eleven sets of fiber cement blocks with different MFPSA powder ratios (0%–20%) and at different calcination temperatures (800 °C and 1000 °C). Regarding these eleven sets of testing blocks, each set contains 6 specimens in which 3 were used for the flexural and 3 for compressive tests. The final flexural and compressive strength for each set are averaged from the results of 3 repeated tests, respectively. As shown in Fig. 6, the solid bar (the 1st bar) represents the benchmark test (namely, M-0). The dot bars (the 2nd–6th bars) represent the flexural and compressive strength of the fiber cement blocks with the MFPSA calcined at 800 °C (namely, the M-800 series). The bars with short lines (the 7th– 11th bars) represent flexural and compressive strength of the fiber cement blocks with the MFPSA calcined at 1000 °C (namely, the M-1000 series). As shown in Fig. 6(a), with the same ratio of the MFPSA, the flexural strengths of the series M-800 are all higher than that of series M-1000. The peak flexural strength of the series M-800 is 9.92 MPa at the MFPSA ratio of 4%. The peak flexural strength of the series M-1000 is 7.44 MPa at the same MFPSA ratio. The average flexural strength of the series M-800 is 8.84 MPa, which is

C. Li et al. / Construction and Building Materials 223 (2019) 1016–1029

24.7% higher than the average flexural strength of the series M1000 (7.09 MPa). Moreover, the flexural strengths of both the M800 and M-1000 series decrease along with the increase of the content of the MFPSA. Different from the M-800 series, all flexural strengths for the M-1000 series are lower than the benchmark (M-0). It is worth noting that the specimens with the 4%–12% cement mass ratio replaced by the MFPSA at 800 °C demonstrate better flexural strengths, with 11.5%–3.7% higher than the benchmark (M-0). As shown in Fig. 6(b), the average compressive strength of the series M-800 is 72.08 MPa, which is higher than that of the series M-1000 (average = 65.48 MPa). The peak compressive strengths of the M-800 and M-1000 series are 76.62 MPa (4% MFPSA) and 73.08 MPa (4% MFPSA), respectively, which is 5.7% and 0.8% higher than the blank samples without MFPSA. It is worth noting that the compressive strengths for both series M-800 and M-1000 are decreasing with the increasing of the MFPSA content. The mechanical properties (the flexural and compressive strengths) of the MFPSA/fiber cement are affected by calcination temperature of the MFPSA powder and the mix ratio. When the calcination temperature is 800 °C and the ratio of 4%–12%, the flexural and compressive strengths are better than the other cases. The overall mechanical properties of the M-1000 series are worse than

1021

that of the M-800 series, the major reason can be attributed to that the MFPSA calcined at 1000 °C is more complete and therefore less active.

4.2.2. Hydrophobicity According to the mechanical test results, sample blocks with MFPSA content of 16% and 20% were ruled out for hydrophobicity due to insufficient strength. Since the mechanical behaviour of the M-1000 series is worse than that of the M-800 series, the M-1000 series will not be used for hydrophobicity tests as well. Therefore, sample blocks M-0, M-800-4, M-800-8 and M-800-12 were employed for the hydrophobicity tests. For hydrophobic tests, the result is averaged from the results of 3 repeated tests. Fig. 7 shows that the ink residual area and water absorption for sample blocks M-0, M-800-4, M-800-8 and M-800-12. As shown in Fig. 6, the residual area of the ink droplets gradually decreases along with the increase of MFPSA content. When the ratio of MFPSA-800 was 4%, 8% and 12%, the percentage of ink residual area in the cross section decreased by 8.9%, 41.0% and 62.1%, respectively. It is worth noting that the admixture of the MFPSA can improve the water repellency of the fiber cement block, and then reduce the area infiltrated by the ink droplets.

(a) Uncalcined FPS

(b) FPSA calcined at 800 °C Fig. 3. SEM/EDS images of FPSA at different calcination temperatures.

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(c) FPSA calcined at 1000 °C Fig. 3 (continued)

Fig 5. The hydrophobic phenomenon of MFPSA-800 powder.

(a) After calcined at 800 °C

It can be seen from the test results that the water absorption drops at first and then increases along with the increase of the MFPSA content. When the MFPSA-800 ratio is 4%, the water absorption of the specimen shows a bottom value of 1.25%, which is much lower than the value for the M-0 (3.81%). However, at a ratio of 12%, the water absorption is 5.31%, 1.5% higher than that of M-0. The right amount of MFPSA-800 instead of cement can significantly reduce the water absorption of fiber cement test pieces. However, when the MFPSA content is excessive, the water absorption of the fiber cement increases in turn. The above changes in water absorption are related to the hydrophobicity of the material and the internal pore structure of the MFPSA/fiber cement, which will be discussed in Section 4.4. 4.3. Strength and capillary water adsorption of MFPSA/fiber concrete

(b) After calcined at 1000 °C Fig. 4. FITR analysis of FPSA and MFPSA at different calcination temperatures.

According to the mechanical test results for cement blocks, only MFPSA-800 was employed for making the MFPSA/fiber concrete and the mix ratio of MFPSA is ranged from 0 to 12%. The mix pro-

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Residual area

Water absorption

Residual area / %

70

5

60 4

50 40

3

30

2

20 1

10 0

Water absorption / %

6

80

0 M-0

M-800-4

M-800-8

M-800-12

Sample ID (a) Flexural strength

(b) Compressive strength

Fig. 7. Water absorption and residual area of MFPSA/fiber cement blocks.

(a) Compressive strength

Fig. 6. Flexural strength and compressive strength of MFPSA/fiber cement blocks.

portions of MFPSA/fiber concrete blocks can be found in Table 4. Sample concretes C-0, C-800-4, C-800-8, C-800-12 were tested. 4.3.1. Compressive and splitting strength Fig. 8 shows the variation in the compressive and splitting strength of the MFPSA/fiber concrete with different curing periods (i.e., 7, 14 and 28 days) at different MFPSA-800 contents (i.e., 0%, 4%, 8% and 12%). For both compressive and splitting strength tests, the results are averaged from the results of 3 repeated tests. The solid bars represent the benchmark test (namely, C-0). The bars with short straight line represent the fiber cement blocks with the MFPSA-800 at the ratio of 4% (namely, the C-800-4 series). The bars with diagonals represent the fiber cement blocks with the MFPSA-800 at the ratio of 8% (namely, the C-800-8 series). The bars with dots represent the fiber cement blocks with the MFPSA-800 at the ratio of 12% (namely, the C-800-12 series). As shown in Fig. 8 (a), when the ratio of the MFPSA-800 is 4%, the compressive strengths of the concrete blocks after standard curing for 14 days and 28 days are higher than those using other admixture ratios. The 14-day and 28-day compressive strength for C-800-4 are 10.8% and 1.5% higher than the normal fiber concrete (i.e., C-0), respectively. But the 7-day compressive strength for C-800-4 is a bit lower than the benchmark (C-0). When the ratio of the MFPSA-800 is set to be either 8% or 12% (see C-800-8 and C-800-12), the 7-, 14-, and 28-day compressive strengths of the MFPSA/fiber concrete are all lower than that of the benchmark (C-0). The low content of MFPSA-800 can promote the rapid

(b) Splitting strength Fig. 8. Compressive and splitting strength of MFPSA/fiber concrete with different curing days.

growth of the compressive strength in 14 days, which can be related to the formation of the calcium aluminoferrite (Ca2(Fe1.866Al0.134O5)) and the active Al2O3, SiO2 from calcination of FPS. That will promote early hydration, leading to more AFt, then trigger moderate expansion, thus providing strength support for the previous period. The effect of MFPSA-800 ratio on the splitting strength of MFPSA/fiber concrete at different ages is shown in Fig. 8(b). As shown in Fig. 8(b), splitting strength of all the fiber concrete

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reduces after increase along with the curing periods. the 14-day splitting strengths of the MFPSA/fiber concretes (the MFPSA contents are 0%, 4%, 8% and 12%) are 8.6%, 3.9%, 4.3% and 16.0% higher than the corresponding 28-day splitting strengths, respectively. The reason for the slight decrease of the splitting strength in the later stage can be attributed to the microcracks formation due to the delayed release of the hydration heat coming from the chemical reaction of C3A. In addition, for all the MFPSA-800 fiber concretes, there are lower splitting strengths than the reference fiber concrete, especially at the ratio of 8%. When the MFPSA-800 ratio is 12%, a large splitting strength regression occurs in the later stage. Therefore, the splitting strength of the M-800-4 containing 4% MFPSA-800 is more stable in the early, middle and late stages than other ratios. It is also closest to the benchmark fiber concrete C-0. It is obvious that the excessive addition of MFPSA-800 is not conducive to the increase of splitting strength of fiber concrete, which may be related to the grafting of hydrophobic groups by FPSA. The XRD pattern in Fig. 2 (b) shows that FPSA at 800 °C contained Ca2Fe2(CO3) and some Unpyrolyzed CaCO3, which have similar crystal structures to the converter steel slag. In the system reacting with cement, their activity is slow, and some do not participate in the

reaction. That makes the concrete brittle and reduces its splitting strength.

4.3.2. Capillary water adsorption As shown in Fig. 9, at the ages of 7 days (short-term) and 28 days (long-term), the capillary water absorption heights (averaged from the results of 3 repeated tests) of all the concrete test blocks mixed with MFPSA-800 (C-800-4, C-800-8, and C-800-12) are lower than that of the benchmark (C-0). That was calculated by the formula (1) using the cumulative capillary water absorption height per unit area of the specimens. It is obvious that the minimum capillary water absorption height occurs when the blending amount of MFPSA-800 is 4%. As shown in Fig. 9(a), after 7-day curing, the average 24-h capillary water absorption heights of C-800-4, C-800-8 and C-800-12 are 0.625, 0.728 and 0.800 mm/day (37.1%, 26.7% and 19.5% lower than the benchmark, C-0), respectively. It implies that, with the MFPSA-800, the pore walls formed during the hardening process are accompanied by water repellency, which affects water migration and capillary water adsorption significantly. As shown in Fig. 9(b), after 28-day curing, the average 24-h capillary water absorption heights of C-0, C-800-4, C-800-8 and C-80012 are 0.612, 0.417, 0.498 and 0.523 mm/day respectively. Comparing with C-0, the decrease in the average 24-h capillary water absorption height for C-800-4, C-800-8 and C-800-12 is 31.9%, 18.5%, and 14.4%, respectively. The above shows that the MFPSA800 can reduce the long-term capillary water absorption height of the fiber concrete. The optimal effect can be achieved when the mix ratio exceeds 4%.

4.4. Mechanism analysis To study the mechanism of how the MFPSA affecting the performance of fiber cement products, different specimens were selected for microscopic testing after standard 28-day curing period: (1) the M-0 and M-800-4 samples were tested by XRD for hydration products analysis; (2) the M-0, M-800-4 and M-1000-4 samples were tested by SEM/EDS for weak interfaces and microstructures; (3) The M-0 and M-800-4 samples were tested for porosity and density. The pore structures of the M-0 and M-800-4 samples are imaged by a CT scanner; and (4) The interfacial transition zone

(a) After 7 days

(b) After 28 days Fig. 9. Water absorption height of C-0 and C-800-4/8/12 with different curing days.

Fig. 10. XRD results of M-800-4 and M-0 after hydration for 28 days.

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between cement and fiber for the M-0 and M-800-4 samples was tested by SEM/EDS. 4.4.1. Hydration products It can be seen from Fig. 10 that the hydration products of both M-0 and M-800-4 are mainly C2S, CH, C3S, AFt and C-S-H represented by a broad dispersion band. From the perspective of the intensity of the diffraction peak, the dispersion peak intensity of

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the M-800-4 is higher than that of the M-0, which can be proved that more C-S-H formed in M-800-4. The AFt peak of M-800-4 is also higher than M-0, but the CH peak is lower than M-0. These indicate that active substances such as Al2O3 and SiO2 in MFPSA800 can be reacted with hydration product Ca(OH)2, forming more C3ASH32 (AFt) that results in an increased early strength of fiber cement block. The calcium ferrite inert granules similar to steel slag crystals are encapsulated by C-S-H gel formed by cement

(a) Weak interface of M-0

(b) Weak interface of M-800-4 Fig. 11. SEM/EDS of weak interface of fiber cement with different MFPSA.

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(c) Weak interface of M-1000-4 Fig. 11 (continued)

hydration, which acts as nucleation to improve the hydration environment and promote the hydration rate and hydration degree of the cement. Calcium aluminoferrite (Ca2Fe(Fe0.866Al0.134)O5) in the MFPSA800 can be hydrated with gypsum in the Portland cement to form AFt and Fe(OH)3 gel containing a small amount of calcium, which is also the cause of high early strength. Its hydration reaction formula are:

4C 4 AF þ 15CaSO4  H2 O þ 140H2 O ¼ CaðOHÞ2 þ 6FeðOHÞ3 þ 5½C 3 ðA0:8 F 0:2 Þ  3CaSO4  32H2 O and 3C 6 AF 2 þ 12CaSO4  2H2 O þ 125H2 O ¼ 6CaðOHÞ2 þ 10FeðOHÞ3 þ 4½C 3 ðA0:75 F 0:25 Þ  3CaSO4  32H2 O

4.4.2. Weak interface The test results on weak interface of the M-0, M-800-4 and M1000-4 are shown in Fig. 11. The element Au is not part of the sample which was introduced owing to the gold spray treatment for SEM. The interface of M-800-4 is dense and has no obvious cracks and pores, but there are many cracks and large pores at the interface of M-0 and M-1000-4. As shown in Fig. 11(a), the element at the weak interface of M-0 is mainly Ca, indicating that Ca(OH)2 is the main mineral that restricts the strength of ordinary hardened fiber cement. As shown in Fig. 11 (b), the elements Al, Fe and Si at the weak interface of M-800-4 are significantly increased, and the Ca element is relatively reduced, indicating there are many MFPSA particles around the weak interface, which cause the local volume expansion clogging capillary channels. Ca2(Fe(Fe0.866Al0.134)O5) increases the mechanical strength of the fiber cement and reduces the water absorption. The hydrophobicity of M-800-4 is higher than M-0, mainly because the MFPSA can increase the surface energy of the cement products, reduce the defects on the surface of the pores, and transform the transition zone into hydrophobicity [40]. The zone creates a more stable meniscus shape at the interface between air and water, preventing free water from entering its interior. The strength of the cement products decreases with the increase of surface energy but increases

with the formation of the hydration products by MFPSA [41]. The strength of the cement products increases along with the increase of hydration products, decrease of surface energy and decrease of porosity [41]. The involvement of MFPSA can increase the formation of hydration products and meanwhile reduce the porosity, benefiting the strength increase. But the involvement of MFPSA can also increase the surface energy, which increases hydrophobicity but decreases the strength. Therefore, compared with M-0, the performance of M-800-4 can be complicated (compressive strength increases but splitting strength decreases) because the joint effect of change of hydration products, surface energy and porosity due to MFPSA. As shown in Fig. 11(c), the Fe content at the weak interface of M-1000-4 is significantly higher than that of fiber cement, mainly in the form of Ca2Fe2O5, which acts as micro-aggregate and filling. Ca2Fe2O5 hinders the hydration of the cement, resulting in a decrease in the formed C-S-H gel and therefore a reduction on strength. 4.4.3. Microstructure The SEM results of the M-0, M-800-4 and M-1000-4 are shown in Fig. 12, which indicates that the microstructure of M-800-4 seems to be denser than that of M-0 and M-1000-4. As shown in Fig. 12(a), the CH crystals in the M-0 hydrated product show a bridge shape, and the C-S-H gel seems to be aligned. While the CH crystals in the M-800-4 hydration product change from bridge-shape to block-shape, as shown in Fig. 12(b). They are entangled by the C-S-H gel, AFt and fiber, in a directionless manner, which is beneficial to improve mechanical strength and hydrophobicity of the interface. As shown in Fig. 11(c) that the CH crystals in the M-1000-4 are not tightly connected. At the crack, no obvious rod-shaped AFt was observed, and most of the surface of the granules was covered by a flocculent C-S-H gel. Therefore, in the hydration process, MFPSA-1000 mainly acts as a nucleation, which is the main reason for the decrease in strength of cement-based materials with it. 4.4.4. Porosity and macropores Following ASTM method [42], the porosity of M-0 and M-800-4 are measured as 17.8% and 17.6% respectively. Macroscopically, the

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(a) Microstructure of M-0

(b) Microstructure of M-800-4

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4.4.5. Interfacial transition zone The interfacial transition zone (ITZ) between cement and fiber was tested by SEM/EDS, following the method presented in literature [45–48]. As shown in Fig. 14(a) and (b), the ITZ between cement and fiber in M-800-4 is denser than M-0 by producing more hydrated calcium aluminate and hydrated calcium iron aluminate. It can be seen that, by replacing the cement with MFPSA800 by 4% mass fraction, more C-S-H and AFt can be formed in the fiber cement, which improves the overall density and decreases the porosity (see section 4.4.4). Furthermore, the influence of fiber on concrete is greatly affected by its content, the amount of 9 kg/ m3 can significantly increase the porosity of concrete [46]. However, the literature [45,47] indicates that the porosity of concrete is almost unchanged if the dosage is lower than 0.9 kg/m3 [45]. Therefore, following the literature [45,47], a constant fiber content of 0.9 kg/m3 was selected to study the effect of MFPSA on fiber concrete performance in this paper. 4.4.6. Summary At a suitable calcination temperature (800 °C) and dosage (4%), the MFPSA/fiber cement perform can achieve the optimal performance in both mechanical and hydrophobic behaviour. On one hand, the MFPSA contains particles that have volcanic ash active material and calcium aluminoferrite, which are polar, promote the hydration process of cement, improve the density of cement stone structure. and enhance the interface strength. On the other hand, the MFPSA also contains a non-polar hydrophobic group, which can be oriented on the surface between air and hydration products and plays a hydrophobic role. When the dosage of MFPSA is too much (>4%), or the calcination temperature is too high (=1000 °C), the MFPSA may hinder the hydration process, reduce the structural gel content, degrade the fiber cement structure, reduce the structural density, because of easy agglomeration and excessive inert iron particles. Finally, either over-dosage or overcalcination of the MFPSA reduces the overall strength and the hydrophobic ability of the MFPSA/fiber cement products.

5. Conclusions This research aims to examine the joint impact of polypropylene fiber and MFPSA on the hydrophobic and mechanical properties of cement products. In this research, the MFPSA was replaced by ordinary Portland cement in different proportions to prepare fiber cement products, and its mechanical properties and hydrophobic properties were tested. The following concluding remarks can be drawn.

(c) Microstructure of M-1000-4 Fig. 12. SEM of fiber cement with different MFPSA ratio.

inclusion of MFPSA can reduce the porosity of concrete and therefore increase the overall density. Meanwhile, the CT scanned images for M-0 and M-800-4 are presented in Fig. 13(a) and (b), respectively, where the light gray parts represent the mortar while the black parts represent the air voids. The detailed testing method for using CT scanner to investigate the pore structure of cement products can be found in literature [43,44]. By comparing Fig. 13 (a) and 13 (b), it can be seen that the macropores in M-800-4 sample has been reduced compared with that in M-0, which is in accord with the macroscopic measurement on porosity.

After calcination at 800 °C, FPS can be converted into FPSA containing a certain pozzolan active material, a small amount of calcium aluminosilicate and inert particles (Ca2Fe2O5). By incorporating stearic acid into FPSA and performing ball milling modification, MFPSA-800 can be produced, which has the dual properties of pozzolanic activity and hydrophobicity. By replacing the cement with MFPSA-800 by 4% mass fraction, more C-S-H and AFt can be formed in the fiber cement, which improves the overall density, pore structure and interface characteristics, thus improving its mechanical strength and hydrophobic properties. When the calcination temperature is 800 °C and the dosage is 4%, MFPSA can increase the 28d flexural strength of fiber cement by 11.5%, the compressive strength by 5.7%, decrease the water absorption by 2.6%. MFPSA also dropped the 28d capillary water absorption of fiber concrete by 31.9%.

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(a) Pore structure observed in M-0

(b) Pore structure observed in M-800-4

Fig. 13. CT images for pore structure change due to MFPSA replacement.

(a) ITZ between cement and fiber of M-0

(b) ITZ between cement and fiber of M-800-4 Fig. 14. Change of interfacial transition zone between cement and fiber due to MFPSA replacement.

If the dosage of MFPSA-800 is higher than 4%, the MFPSA-800 promotes the development of weak interface due to excessive expansion or agglomeration, reduce the strength and increase the water absorption. If the calcination temperature is set to

be 1000 °C, more inert particles are formed in the FPSA powder, playing the roles of micro-aggregate and filling in the fiber cement, which negatively affects the strength and hydrophobicity.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements The financial supports from the National Natural Science Foundation of China (51679004, 51774187) and China Scholarship Council are highly appreciated. In addition, the authors would like to acknowledge Key Laboratory of Nonferrous Metal Materials Science and Engineering in Central South University, College of Urban construction in University of South China and Key Discipline Laboratory for National Defense for Biotechnology in Uranium Mining and Hydrometallurgy for their support for the experiments.

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