Effect of sepiolite addition on fibre-cement based on MgO-SiO₂ systems

Effect of sepiolite addition on fibre-cement based on MgO-SiO₂ systems

Cement and Concrete Research 124 (2019) 105816 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier...

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Cement and Concrete Research 124 (2019) 105816

Contents lists available at ScienceDirect

Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres

Effect of sepiolite addition on fibre-cement based on MgO-SiO₂ systems a,⁎

a

b

b

Gonzalo Mármol , Holmer Savastano Jr , Elena de la Fuente , Rubén Miranda , Ángeles Blancob, Carlos Negrob

T

a

University of São Paulo, Faculty of Animal Science and Food Engineering, Department of Biosystems Engineering, Duque de Caxias Norte Street, 225, 13630-000 Pirassununga, SP, Brazil Department of Chemical Engineering, Complutense University of Madrid, Avda. Complutense s/n, 28040 Madrid, Spain

b

ARTICLE INFO

ABSTRACT

Keywords: Sepiolite MgO-SiO2-H2O cement Fibre-cement Flocculation Drainage

The addition of sepiolite in fibre-cement is analysed as a possible additive constituent of MgO-SiO2 binders. In order to optimise the Mg-SiO2 cementitious system in fibre-cement sheets, small cement replacement (1 and 2 wt %) by sepiolite is introduced and its effects are studied, firstly, in hardened cement pastes and, later, in fibrecement systems. When used only in cement pastes, non-destructive studies are employed to assess flexural performance over time as well as the possible reactivity between additive and cement, proving that Elastic Modulus is increased. When sepiolite is introduced in fibre-cement, the properties of the slurry suspensions and its interference in the Hatschek process are studied. It is shown that sepiolite addition increases floc size of MgOSiO2 systems. Finally, flexural tests are performed to understand the outcome of this additive of the mechanical performance of the samples, proving that sepiolite improves the product homogeneity, which is essential for industrial applications.

1. Introduction Sepiolite is a fibrously structured clay with an empirical formula of Si6Mg4O15(OH)2·6H2O. This mineral has a wide range of uses due to its sorptive, optical, rheological and molecular sieve properties. Sepiolite in cementitious materials behaves as a rheological additive to enhance fresh and hardened state properties. Sepiolite has a thixotropic effect that improves workability, adherence to the substrate, pumpability and surface finish [1]. In the case of fibre-cement (FC) products produced by a dewatering-pressing technique (Hatschek or similar), this additive has an important effect in the rheology of the slurry. The modification of the rheological properties of the FC suspensions entails better fibre distribution and better fine particles retention [2]. Besides, in Portland cement (PC) products reinforced with cellulosic fibres, it enhances interlaminar bonding of the sheets and promotes an excellent surface finishing [2,3]. All this features are strongly reliant on sepiolite dispersion, which likewise depends on sepiolite surface charge. The dispersion of sepiolite in FC is favoured by reducing the anionic charge of this mineral [4]. In addition, higher anionic charge affects cement hydration by reducing the extent of oxygen atoms in the sepiolite surface with free pairs of electrons to interact with water through hydrogen bonding. When a moderate-to-low anionic charge surface sepiolite is added in FC



suspensions, mechanical properties (modulus of rupture, limit of proportionality and specific energy) are improved. However this enhancement is not reflected in an improvement of the physical properties (density, porosity and water absorption) [4]. Sepiolite presents good compatibility with anionic polyacrylamide (A-PAM) flocculants for different ionic charges and molecular weights in FC suspensions. When sepiolite is dispersed in water, either with cement or silica, also interacts with the flocculant particles [3]. Sepiolite in FC competes with the rest of the components (cellulose mainly) to absorb flocculant, increasing the flocculation efficiency in terms of chord size and stability of the flocs [3]. Besides, the higher small particles flocculation, the easier water flows through floc spaces [2], which favours FC drainage. Apart from its rheological and flocculation properties, this clay also reinforces cementitious materials due to its morphology. Because of its needle-like shape, with an average length of 0.2–2.0 μm and crossed section of 100–300 × 50–100 Å for single fibres and 0.2 × 0.2 μm for fibre bundles [5], sepiolite fibres fill the pores in cement paste and mortars. When it is added in PC mortars, it is found that a sepiolite addition of 10 wt% enhances the mechanical properties (compressive and bending strengths) compared to plain PC samples [6]. It is also proved that an optimal fibre distribution is necessary for a proper linkage of the fibres in the cement matrix [6]. Also, because of the short

Corresponding author. E-mail address: [email protected] (G. Mármol).

https://doi.org/10.1016/j.cemconres.2019.105816 Received 17 August 2018; Received in revised form 11 June 2019; Accepted 9 July 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.

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length and the random distribution of its microfibres, it reduces cracking and shrinkage [7]. Thus, it contributes to volume stability [6]. MgO-SiO2 (MS) cement hydrated phases mainly comprise Mg(OH)2 and poorly crystallised magnesium silicate hydrates (M-S-H gels) [8–10] that has similar chemical composition to sepiolite. In FC products, this type of cement has successfully functioned, allowing the substitution of expensive synthetic fibres by globally available cellulose elements with excellent durability results [11]. Due to the low solubility of MgO, MS binders in FC have higher water demand and slower hydration reaction compared to PCFC elements. This factor spoils the industrial implementation of this type of cement for FC production. Considering the sepiolite absorption properties in low humidity conditions [12], sepiolite addition may act as a water reservoir and contribute to hydrate MgO-SiO2 over time. This effect was found for sepiolite addition in mortars with pozzolan addition, particularly metakaolin, since it provided a wet environment and favoured pozzolanic activity [13]. Thus, a reactivity enhancement may increase the Mg2+ concentration in the liquid phase of MgO-SiO2 systems and, therefore, the formation of more Mg-based hydrates. FC industry, in a further attempt to cut down production costs, might both replace the type of reinforcing fibre by cheaper elements and increase its content. A material that meets these criteria is cellulose, which has been widely applied at lab-scale. This technique showed a satisfactory long-term performance using MS cement. In order to improve the water demand of MS cement for its industrial application, it is essential to study the effects of sepiolite addition in FC products only reinforced cellulose-based fibres. In this study, the influence of sepiolite addition on the rheological and mechanical properties of fibre-cement based on MgO-SiO₂ systems was assessed.

particle size of 20.1 μm and 90.37% purity to recreate analogous conditions as used during industrial production. Due to its wide availability, low-cost and high content in cellulose, unbleached eucalyptus pulp (characterised in Table 1) was used as cellulose fibre source. In order to reduce in 64% the cellulosic material water absorption during the fibre-cement production, 4 hornification cycles were used following the procedure described by Ballesteros et al. [15]. The rest of the solid mass of the samples corresponds to MS cement. 0.1 wt% of anionic polyacrylamide (APAM) was dosed as a flocculant (Praestrol 1225) with high molecular weight and low-anionic charge, with values of 7.4 kg/mol and 13.4% respectively. 2.2. Sample production 2.2.1. Pastes Cement pastes were prepared in a mixer, conforming to EN 196-1, and using deionized water for making the specimens. Water demand was established by standard consistence test and later pastes were mixed according to EN 196-3. After mixing, they were transferred to a 80 × 30 × 8 mm casting moulds and compacted by vibration table at 1 kHz for 20 s. Delamination consisting in high-energy Ultraturrax stirring at 10,000 rpm for 10 min in order to assure a colloidal system and the proper dispersion of sepiolite needles was applied before its addition to MS cement paste. For Dynamic Elastic Modulus (DEM) Pastes, determination pastes were placed in plastic sealed bags at 25 °C. For Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Thermogravimetry analysis (TGA), paste samples were steam cured at 55 °C for 5 days until their characterization (Fig. 1). 2.2.2. Fibre-cement Fibre-cement thin sheets of approximately 6 mm thickness were produced to carry out 4-point bending tests according to [11]. For FC flexural testing, 300 g of solids, meeting the same proportions as noted in Table 2, were used following a dewatering and pressing technique described in [15]. With this technique, a fibre and cement suspension with excess of water (1.4 L) was stirred and immediately transferred to a 25 × 20 cm casting box for vacuum-drainage at 650 Hg mm. While the excess of water was vacuum-pumped, flat tamping was used until 6 mm thick pads were obtained. Later, 5 MPa hydraulic pressure was applied for final compaction for 5 min. After casting, FC samples were steam cured at 55 °C for 5 days until bending tests. Flocculation experiments were carried out following the protocol published by Fuente et al. [2], summarised in Fig. 1. Suspensions in 400 mL of water are produced and analysed after mixing 18.4 g of different powder and powder-fibre solids in accordance with Table 2. For water retention analysis, 250 mL of FC suspensions in water were prepared according to Table 2 and the method of Fuente et al. [2] was also adopted except water saturation in Ca(OH)2, with the aim of obtaining a calcium free system that avoids cellulose degradation [11,12,16].

2. Materials and methods Sepiolite addition is separately analysed in both cement pastes and fibre-cement samples. Thus, it is possible to infer the partial effects of this mineral on the cementitious matrix and on the overall product. 2.1. Raw materials 2.1.1. Pastes 60%MgO-40%SiO2 cement composition employed in [14] is applied in this study. MgO with a purity of 97.4%, specific gravity of 3.5 g/cm3, average particle size of 17.41 μm and surface area of 22.7 m2/g was employed. The silica source was undensified silica fume with SiO2 wt % > 95.1, specific gravity of 2.3 g/cm3, average particle size of 19.02 μm and surface area of 16.1 m2/g. In order to assess the effect of sepiolite surface charge, 2 types of sepiolite with different anionic charge, higher and lower, HAC and LAC respectively, both provided by TOLSA GROUP, were used in this study. The main sepiolite properties are summarised in Table 1. 1 and 2% by mass replacement of sepiolite was applied for MS cement. Ordinary Portland cement (PC), CEM I 52.5 meeting EN 197−1:2000 standard was used as control reference.

2.3. Testing methods 2.3.1. Pastes Different techniques were used to characterize cement pastes: DEM, XRD, SEM and TGA. Flexural DEM analysis was adapted from the ASTM C1548-02 standard, using a SONELASTIC® equipment, ATCP brand, Brazil. DEM was recorded over time after 24 h of curing for every

2.1.2. Fibre-cement Fibre-cement is made of a 92 wt% of cement matrix and 8 wt% fibres. Dry cement matrix is 25 wt% limestone filler (CaCO3) with an average Table 1 Properties of the different sepiolite and lignocellulosic fibres used in this study. Sepiolite properties −3

Bulk density (g·cm ) pH Brookfield viscosity at 5 ppm (cP) % of particles smaller than 5 mm Specific surface (BET m2·g−1) Zeta Potential (mV)

HAC

LAC

Fibres properties

2.2 7.7 45,000 89.9 319 45.52

4.45 8.7 56,000 96.4 309 36.25

Number (106/g) Length weighted in length (mm) Average Width (mm) Coarseness (mg/m) Rate in length of Macrofibrills (%) Break end (%)

2

21.38 841 16.7 0.073 0.725 16.81

Cellulose (%) Hemicellulose (%) Lignin (%) Water retention value (g/g) Index of crystallinity (%) Fines number

84.5 10.0 1.6 1.09 76.5 9242

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Fig. 1. Schematic summary of every test used in this work. Tests performed on cement past samples (DEM, XRD, TGA and SEM) are encircled in red. Tests performed on fibre-cement products (flocculation, drainage, flexural and physical tests) are encircle in black. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

formulation. All samples were mixed using a w/c ratio of 0.5 and MELFLUX 2651F superplasticizer for MS samples. For XRD tests, pastes were finely crushed and sieved, using the particle fraction passing through 63 μm mesh. X'Pert Pro MPD PANalytical's X-ray diffractometer was used (Cu Kα radiation of 1.54 Å) to collect data between 2θ = 5° and 65° at a scanning speed of 1°/min TGA of the samples was carried in nitrogen atmosphere (50 mL/min) from 35 to 1000 °C using a Seiko Exstar 6000 TGA/DTA thermobalance. SEM analysis was performed on a JSM 6335F scanning electron microscope, JEOL brand with an intensity of 10 kV.

flexural testing, fibre-cement samples were cut into 1 × 1 × 1 cm cubes and embedded in epoxy resin. Samples were firstly polished with abrasive silicon carbide and then with alumina until smooth surface was achieved. A Quanta 600 FEG microscope (FEI, Eindhoven, Netherlands) was used. In order to study the effect of sepiolite on FC flocculation, the evolution of the aggregate size is carried out using a M500L focused beam reflectance measurement probe (FBRM) (Mettler Toledo, Seatle, USA). FBRM technique detailed in Fig. 1, according to Fuente el al. [2], allows recording floc chord length distribution over time for all the compositions. A chord length of the intercepting particle is determined as the product of the time duration of the pulse by the linear speed of the focal point movement. Drainage tests were performed on different suspensions after their vacuum filtration (650Hg mm) and drained water values were recorded until constant weight (Fig. 1).

2.3.2. Fibre-cement Different techniques were used to characterize cement pastes: flexural and physical tests, SEM, flocculation and drainage tests. FC pads were cut into 160 × 40 mm specimens for mechanical tests. After 5 days of curing were immersed for 24 h to assure saturated testing conditions as indicated in [14]. Flexural properties were tested according to Fig. 1, using a 1kN cell load at a deflection rate of 5 mm/min. A deflectometer at half-spam was placed to register sample deformation. Physical tests were carried out in accordance to ASTM C1185-08. For SEM tests, after

3. Results and discussions 3.1. Cement paste In order to assess the influence of sepiolite addition on the hydrated

Table 2 Mix proportions of the different elements of the tested fibre-cement products. Powder used

Cement + filler Cement + filler + delaminated sepiolite (CFS)

Cement

CaCO3

Sepiolite

(%)

(%)

Type and

69 68.3 67.6 68.3 67.6

23 23 23 23 23

3

1%HAC 2%HAC 1%LAC 2%LAC

Cellulose (%) ***

0.7 1.4 0.7 1.4

(%) 8 8 8 8 8

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Fig. 2. XRD of the different analysed MgO-SiO2 cement pastes. (a) MS paste with no sepiolite addition diffractogram, in which grey circle stands for any possible trace of unreacted SiO2 (b) diffractograms of the more representative peaks of the hydrated phases using different anionic charge sepiolite, lower (LAC) and higher (HAC), and different addition contents, 1% and 2% and (c) diffractogram of 2% HAC samples.

phases of MgO-SiO2 systems (MS), XRD test were performed. Fig. 2(a) displays the MS cement XRD pattern, where MgO and Mg(OH)2 are the main crystalline phases, while M-S-H gel is the only amorphous compound [8–10,17]. The most representative diffractogram peaks and halos for the hydrated phases in MS cement were highlighted in the central region of the diffractogram in Fig. 2(a), between 33 and 43.5°. For a better comparison of the different hydrated, this central region of MS pattern was zoomed in Fig. 2(b). It is clearly seen that the presence of MgO (at around 43°) is reduced with sepiolite addition, with further decrease for higher sepiolite content and surface charge. Since MgO peaks are related to unreacted particles, it is inferred that sepiolite enhances MS hydration. SiO2 in water is dissolved as a hydroxocomplex, silicic acid (Si(OH)4), which at basic pH deprotonates to form H3SiO3− to further deprotonate to H2SiO42− at higher pH. Thus, silicic acid reacts with Mg(OH)2 to form M-S-H that is confirmed by the reduction of the Mg(OH)2 peak (at around 38°) and the increase of the M-S-H hump (between 33 and 36°) as result of the sepiolite addition. It was especially noticeable the diffractogram for 2% HAC samples, Fig. 2(c), that resembles the synthetic M-S-H gel diffraction pattern [18] with insignificant amounts of Mg(OH)2. Thus, the addition of 2% by mass of sepiolite to MS cement significantly boosts the formation of poorly crystallised M-S-H. Similar effects are obtained by TG analysis (Fig. 3), where higher mass losses mainly associated to M-S-H dehydration, from 75 to 200 °C, and dehydroxylation, between 300 and 600 °C, [10] were observed for higher sepiolite content and anionic charge. Similarly to XRD results, there was a reduction in the mass loss due to Mg(OH)2 dehydroxylation (between 340 and 420 °C) for the sepiolite containing samples, proving that these samples present a great reactivity between Mg(OH)2 and SiO2 to form more M-S-H gel (Table 3). Once more, 2% HAC samples

Fig. 3. TG and DTG curves of the different analysed MgO-SiO2 (MS) cement pastes, using different anionic charge sepiolite, lower (LAC) and higher (HAC), and different addition contents, 1% and 2%.

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Table 3 Mg(OH)2 content in the samples obtained from the mass loss due to water release during Mg(OH)2 dehydroxylation. Mg(OH)2 content (%) MS 1% 2% 1% 2%

LAC LAC HAC HAC

22.34 20.31 19.78 13.02 2.01

showed a differentiated performance, exhibiting DTG curves with an insignificant Mg(OH)2 dehydroxylation peak (Table 3) and greater M-SH gel dehydration area. Thus, XRD and TG analysis show that higher anionic charge affects MS hydration since the reduction of oxygen in the sepiolite surface leaves free pairs of electrons to interact with water through hydrogen bonding [4]. In addition, the high surface area (approximately 300 m2/g) and consequently more active centres of sepiolite along with its high density of silanol groups (-SiOH) yields higher ability to adsorb Mg compounds from cement as analogously reported for Portland cement elements [6]. According to the techniques employed in this research, from TG analysis is possible to quantify the amount of Mg(OH)2 present in the samples by integrating the first derivative of the peak relative to Mg (OH)2. After deconvolution fitting, the area of this peak stands for the amount of water released during thermal decomposition of Mg(OH)2, so it is possible to define the degree of formation of MSH based on the Mg (OH)2 consumption. SEM depicted the addition of sepiolite with different surface charges and different contents to MS pastes (Fig. 4). Fig. 4(a) displays an MgOSiO2 sample with no sepiolite addition, where a crust of M-S-H gel is surrounding bigger Mg(OH)2 particles. These compounds are also displayed in the rest of the images in Fig. 4. In Fig. 4(b), MS cement dosed with 1% of LAC sepiolite presented plenty of needle shaped particles interconnecting M-S-H gel particles and Mg(OH)2 crystals. These

Fig. 5. Dynamic Elastic Modulus evolution over time determined by ultrasound tests of the different analysed MgO-SiO2 (MS) cement pastes, using different anionic charge sepiolite, lower (LAC) and higher (HAC), and different addition contents, 1% and 2%.

acerose particles perfectly matched the size of sepiolite bundles, with 1–2 μm length and 0.2 μm width. It was also shown that hydrated products covered acicular particles. Fig. 4(c), where 2% HAC sepiolite is used, showed different sides of a fractured surface packed with hydrated particles and sepiolite particles springing out in between. In contrast to 1% LAC samples, 1% HAC present a more compact structure with smaller voids between hydrated compounds. Fig. 4(d) exhibits MS samples with 1% of HAC sepiolite, where sepiolite particles interconnected M-S-H gel, creating a 3D net. Part of these sepiolite needles were covered by hydrated products while some other are not. This area with high concentration of sepiolite is more porous in contrast with

Fig. 4. SEM photomicrographs of hydrated MgO-SiO2 systems without sepiolite addition (a), with 1% of LAC sepiolite (b), with 2% HAC sepiolite (c) and 1% HAC sepiolite (d). 5

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areas with more M-S-H particles. Since Dynamic Elastic Modulus (DEM) is the most significant mechanical parameter in order to apply MS in fibre-cement products, its evolution over time was assessed by non-destructive tests (ultrasound). Regarding DEM results displayed in Fig. 5, the effect of sepiolite on MS pastes is well-defined. As it was expected, the addition of sepiolite to MS cement increases the paste mechanical performance for every type and content of sepiolite [6,13]. The results obtained in this study showed up to 27% improvement at 14 days for 2% HAC samples. This increment in DEM is related to the higher content in M-S-H gels, the filling of the cavities in hardened cement and the better linkage between the hydrated compounds. It is also noticed that samples with HAC and higher sepiolite addition have higher DEM values from early stages. Considering that all samples were mixed with the same w/c, adjusting the amount of superplasticizer, both the higher sepiolite surface area and content led to a densification of the pastes. However, because of the adsorption and absorption properties of this clay mineral, the addition of HAC may affect the FC properties when produced by the mixing with excess of water and later removal. Therefore, it is necessary to evaluate the performance of the FC products when sepiolite is added. 3.2. Fibre-cement The study of the addition of sepiolite in FC suspensions is necessary to verify unsought side effects. In this sense, flocculation study is essential to assure satisfactory production conditions. In Fig. 6(a), the chord size evolution of MS powder suspension in water is exhibited. Mean chord size values of 38 μm were obtained before flocculant addition (point A in Fig. 1), which are higher compared to FC suspensions with Portland cement [2–4,19,20]. After flocculant addition, the maximum mean chord size (point B in Fig. 1) was of 109 μm. Adding CaCO3 to MS powder slightly decreases the chord size before flocculant addition (point A), while does not affect the chord size after flocculant addition (point B). Floc stability and chord size at deflocculation stage (point C) were also reduced. The addition of delaminated sepiolite to MS powder, shown in Fig. 6(b), increases the chord size at the flocculation stage (from B to C) for 1% sepiolite gel content samples, while it was reduced for 2% sepiolite gel content samples. Shorter chord size was exhibited when HAC sepiolite was used. This is in accordance to what has been already reported in the literature [3,20], where lower anionic charge sepiolites presented larger and more stable flocs. The floc size evolution over time when eucalyptus fibres are introduced in the FC suspension is displayed in Fig. 6(c). The addition of delaminated sepiolite to MS powder dispersed in a eucalyptus pulp suspension rises chord size at every stage, showing similar performance for every formulation regardless sepiolite type and content. After maximum chord size (point B), at approximately 940 s a faster floc size reduction takes place with a quick a reflocculation event 20 s later during flocculation stage. Another remarkable fact takes place with the deflocculation process after 1140 s (point C), when stirring intensity was increased up to 800 rpm. With increased rotation speed, floc size immediately decreased for every FC formulation. Nonetheless, FC suspensions presented higher floc size and stability compared to MS samples with no sepiolite addition. Since good particle dispersion is desired during FC suspension, high rotation speed is essential before dewatering. Thus, MS cements presented a more stable floc under deflocculation stage which is more effective for industrial FC production compared to FC produced with PC [3,20]. The flocculation results reported in this study are consistent with results reported with PC suspensions [3], where maximum floc size of FC suspensions ranged between approximately 45 and 60 μm. Water drainage curves for every FC suspension (standard curve displayed in Fig. 1) showed similar trends, with an initial rapid drainage where most of the water was drained, a transition zone from fast to low drainage and residual drainage, where horizontal asymptote is

Fig. 6. Evolution of the mean chord size of 18.4 g of FC suspensions in 400 mL of water according to Table 2.

shown, keeping stable the drained water values over time. The higher flocculation efficiency of MS cement compared to PC cement is not enough to assure good water drainage since MgO surface area of MgO is approximately 10 times higher compared to cement Portland particles [9]. Even flocculant addition to FC suspensions may not be sufficient drain the water required for an optimal w/c due to the increased fine solid particles retention efficiency [2], which partially clogs drainage routes through. This was confirmed attending to Vacuum Drainage Tests when delaminated sepiolite was added to MS powder dispersed with eucalyptus pulp in water suspension (Table 4). It was observed that sepiolite addition decreases the amount of total drained water (C values) from the samples except for samples with 1% LAC. The amount of drained water was mainly reduced in samples containing HAC sepiolite, where the greatest reduction was shown by 2% HAC sepiolite content. HAC sepiolite presents more viscosity and surface area (Table 1) and this justifies the higher water retention of the samples with this type of sepiolite. This is in accordance with flocculation 6

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Table 4 Water drainage test values of Fibre-Cement suspensions. (A) drained water after 10 s of vacuum, (B) time until constant weight is achieved and (C) total drained water. A (g) MS 1%LAC 2%LAC 1%HAC 2%HAC

145.24 176.35 169.25 153.49 132.21

B (sec) 32 23 29 26 42

C (g)

w/c

197.13 197.93 188.14 184.32 174.52

0,51 0,51 0,62 0,66 0,75

results that showed lower flocculation efficiency of HAC compared to LAC. However, it increases the drainage speed, rising the amount of drained water after 10s for every sample (A values) and reducing the speed until constant weight (B values) except for 2% HAC samples. Thus, w/c is given for the different composites produced after drainage test in Table 4. Even though a compaction pressing is required after water draining so further water removal from samples is achieved, it is seen that the higher sepiolite content and the higher anionic charge samples present the higher water to cement ratios. It exists a neat relation between w/c ratio and mechanical properties [21] so flexural tests have to be carried out in order to assess the influence of this differential water demand on the performance of FC products. According to flexural tests (Table 5), all the mechanical parameters were increased except MOE when 1% by mass of MgO-SiO2 cement was replaced by LAC sepiolite. The rest of the replacements did not improve the mechanical performance and statistical significant reduction was observed for HAC samples. This is in contrast with DEM tests (Fig. 5) performed on cement paste, where every sepiolite addition increased cement rigidity. This difference in rigidity between paste and FC samples is related to the increased water demand of the FC samples when sepiolite was added, contrary to paste samples that all were mixed with the same w/c. Based on water retention after drainage (Table 4), samples with 1% of LAC sepiolite does not affect the drainage capacity of the suspensions compared to MS samples. Since hydraulic compaction was alike for every sample, 1% LAC samples w/c ratio stays at the same level and simultaneously present more fibre-shaped elements within the matrix. Therefore, the improvement of the FC flexural properties with 1% LAC addition may be explained by the better fibre distribution and linkage of the fibres in the cement matrix [6]. Furthermore, LAC samples present bigger flocs that are related to better-connected fibre nets so samples with larger flocs also offer better properties related to the binder rigidity. LAC when added 2% by mass also increases the limit proportionality, this is the maximum flexural strength before the first

Fig. 7. Stress-deflection curves from bending tests of the most representative sample of each samples composition.

cracking episode, probably due to the short length and the random distribution of its microfibres that contributes to reduce cracking [7]. In addition, sepiolite as additive reduces the coefficient of variation for every studied parameter in most of the studied compositions, which is closely related to more consistent elements [2]. It is noteworthy the importance of achieving more homogeneous materials since many of the FC production issues are related to a lack of materials uniformity. The only composition that was not in accordance with this trend was the one with 2% of HAC sepiolite, which showed more variable properties related to the cement matrix rigidity (LOP and MOE) and significant lower flexural performance. From Fig. 7 it is evident that properties related to the efficiency of the reinforcing elements (SE and SD) are lower for samples including sepiolite, except 1% LAC samples. At the initial stages of the flexural test, samples with sepiolite show similar trends to samples without sepiolite. In the beginning, when only cement matrix withstands bending loads, all samples behaved in an identical manner, yet when loads exceeded LOP and were transferred to the fibres, samples with sepiolite reached MOR earlier and were quickly deformed at lower flexural stresses. Given that the foremost difference between the tested samples was the retained water, the lack of toughness shown by sepiolite-formulated samples in the plastic zone of the curve may be attributed to a worse fibre-matrix

Table 5 Flexural results after 5 days curing at 55 °C. Modulus of rupture, limit of proportionality, specific energy (at MOR and 50% MOR), specific deflection (at MOR and 50% MOR) and modulus of elasticity values for the different studied formulations. Sample

MS 1% LAC 2% LAC 1% HAC 2% HAC

Mean Std dev Var coef (%) Mean Std dev Var coef (%) Mean Std dev Var coef (%) Mean Std dev Var coef (%) Mean Std dev Var coef (%)

MOR

LOP

SE at MOR

SE at 50% MOR

Total SE

MOE

SD

SD at 50% MOR

(MPa)

(MPa)

(J/m2)

(J/m2)

(J/m2)

(MPa)

(mm/mm)

(mm/mm)

9.36 1.13 12.11 9.78 0.75 7.67 9.22 1.08 11.69 9.00 0.91 10.14 8.56 1.19 13.96

4.29 0.91 21.21 4.27 0.48 11.30 4.57 0.89 19.43 3.84 0.71 18.57 3.52 1.02 29.04

4613 1391 30 5544 1182 21 4027 692 17 3856 926 24 3881 821 21

6105 1029 17 5835 743 13 5513 948 17 5715 632 11 4942 844 17

0.060 0.018 30.20 0.073 0.013 18.30 0.053 0.011 21.68 0.047 0.012 25.13 0.053 0.007 13.45

2741 1143 42 3471 713 21 2279 538 24 2022 584 29 2176 494 23

4316 1402 32 5088 1221 24 3595 767 21 3288 786 24 3396 810 24

7

0.094 0.021 22.01 0.106 0.023 21.78 0.082 0.015 18.57 0.076 0.015 20.28 0.082 0.008 10.10

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Fig. 8. SEM micrographs of the surroundings of reinforcing fibres for each composition with sepiolite: (a) 1% LAC, (b) 2% LAC, (c) 1% HAC and (d) 2% HAC.

interface. As water retention was higher for samples containing sepiolite, areas that are more porous at the interface transition zone (ITZ) between cement and matrix were promoted [22]. A clear example of porous ITZ was exhibited with the addition of sepiolite to the MS binding systems, excluding 1% LAC samples. Tests after longer curing are required to assess a possible ongoing hydration with time and thus achieve a denser ITZ. Since longer ageing promotes further hydration in MgO-SiO2 systems in fibre-cement products [11,14], it is likely to improve the fibrematrix adherence for these samples. Another interesting possibility to explore is the carbonation curing, since the greater amount of water would be beneficial to generate additional cementitious material and close the voids between matrix and fibre [23]. In Fig. 8, where eucalyptus fibres were centred in the middle of SEM images for every sample with sepiolite addition, it was observed that the fibre in 1% LAC samples, in Fig. 8(a), presents microcracks in contrast to the rest of the samples (b, c and d), that show an undamaged fibre. The rest of the samples also show a large gap between matrix and fibre because of the larger porosity of these samples. Imperfections in the fibre observed in 1% LAC sample (a) comes from necking episodes in the fibre after larger strains. For this reason, effects on the fibre related to necking events generate higher SE performance of the specimens, since the fibre requires more energy to be pulled out from the matrix. Physical tests performed on samples after flexural tests (Table 6) confirmed the reported trends with mechanical results, where MS samples were denser and less porous due to the lower water retention values compared to samples containing sepiolite. Greater porosity and lower density was observed in samples with lower mechanical performance, which is consistent with a more porous ITZ. Similarly to bending test results, the addition of sepiolite reduces the variation coefficient of the different parameters. Therefore, it is possible to conclude that the incorporation of sepiolite in fibre-cement products with MS also increases the homogeneity of the samples.

water molecules. These water molecules are surrounded by Mg and Si atoms on the sides and both at the top and bottom of the channels respectively. This singular structure makes sepiolite to exhibit a rodshaped particle instead of a laminar structure as happens for the rest of clay minerals. These channels confers an enormous specific surface to sepiolite particles, 300 m2/g, which promotes the hydration of MgO to form Mg(OH)2 initially and M-S-H later, because of the interaction between Mg(OH)2 and H2SiO42−. In addition, the presence of (OH)− on the surface provides to the particles a greater anionic charge density and, therefore, the capacity of adsorbing dissolved cations, which also leads to a faster dissolution of MgO and a hydration of Mg compounds. Sepiolite addition in MgO-SiO2 pastes enhances the hydration of this cement regardless the sepiolite type and replacement amount. The higher sepiolite replacement and the higher sepiolite anionic surface, the more hydrated phases were found in these pastes. The improvement in the MgO-SiO2-H2O system reactivity is translated into an improvement in Dynamic Elastic Modulus, which is related to a densification of the matrix and better interlinking between hydrated particles created by sepiolite particles. Regarding flocculation, the addition of sepiolite to MgO-SiO2 systems increases the floc size of the fibre-cement suspensions in an aqueous liquid. This improves the dewatering process in terms of fines retention. The higher fines retention would be a very positive effect in an upscaled production process due to the reduced amount of tiny particles susceptible of being stack and blocking industrial machinery. The introduction of sepiolite in suspensions where both cement and fibres are dispersed hampered water removal in MgO-SiO2 systems. However, the addition of sepiolite increased drainage speed so most of the water is removed in a faster manner. Once again, this factor supposes an advantage in a larger production scale since it makes more efficient the Hatschek process with regard to energy saving during the dewatering stage. Samples with higher specific surface sepiolite retain more water than those with lower specific surface. Since sepiolite induces a higher water retention, it decreases flexural and physical properties of the fibre-cement products. Nevertheless, this increase in retained water may be counterbalanced with the interlocking effect of the sepiolite particles that can improve flexural

4. Conclusions Sepiolite has a similar chemical composition to talc and clays, but its crystalline structure presents parallel channels that may include 8

Cement and Concrete Research 124 (2019) 105816

G. Mármol, et al.

Table 6 Physical characterization of the samples for every formulation after 5 days curing at 55 °C. Bulk density (g/cm3)

Sample MS 1% LAC 2% LAC 1% HAC 2% HAC

Mean (std dev) Var coef (%) Mean (std dev) Var coef (%) Mean (std dev) Var coef (%) Mean (std dev) Var coef (%) Mean (std dev) Var coef (%)

1.42 1.42 1.38 1.37 1.36

4.00 2.86 2.59 3.59 3.05

Water absorption (%) (0.06)

29.61

(0.44)

29.37

(0.04)

30.91

(0.05)

31.32

(0.04)

31.52

performance and physical properties when dosed in reduced amounts (1%). This is very important since it allows the combination with carbonation curing techniques since the greater water in the samples enhances the carbonation process in Mg-based cements. According the presented results, the mechanical properties of MS pastes, in which water/binder ratio is constant for all the samples HAC sepiolite increases the dynamic modulus of elasticity. However, for fibre-cement samples, mechanical properties are decreased since HAC displayed higher water retention values that are related to a higher porosity and reduced density after setting. The higher porosity of the samples is translated into lower cement strength as well as into a lower adherence between fibre and matrix. Altogether affects negatively to the flexural strength. The most remarkable effect of adding sepiolite into fibre-cement products with MgO-SiO2 systems is the augment of the fibre-cement homogeneity. Uniformity is especially significant in the case of physical properties, which is desired as it makes possible the obtainment of a more reliable material. It also would be of importance to optimise this type of specimens in the future, since the use of sepiolite would reduce the amount of material to obtain reliable test results.

10.25 6.53 6.03 8.51 6.41

Permeable void volume (%) (3.03)

41.82

(1.92)

41.50

(1.86)

42.66

(2.67)

42.90

(2.02)

43.29

6.10 3.62 3.57 4.93 3.55

(2.55) (1.50) (1.52) (2.12) (1.54)

[6] T. Kavas, E. Sabah, M.S. Çelik, Structural properties of sepiolite-reinforced cement composite, Cem. Concr. Res. 34 (2004) 2135–2139, https://doi.org/10.1016/j. cemconres.2004.03.015. [7] H.-J. Kang, M.-S. Song, Y.-S. Kim, The effects of sepiolite on the properties of Portland cement mortar, J. Korean Ceram. Soc. 45 (2008) 443–452, https://doi. org/10.4191/KCERS.2008.45.8.443. [8] T. Zhang, L.J. Vandeperre, C.R. Cheeseman, Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate, Cem. Concr. Res. 65 (2014) 8–14, https://doi.org/10.1016/j.cemconres.2014.07.001. [9] S.A. Walling, H. Kinoshita, S.A. Bernal, N.C. Collier, J.L. Provis, Structure and properties of binder gels formed in the system Mg(OH)2–SiO2–H2O for immobilisation of Magnox sludge, Dalton Trans. 44 (2015) 8126–8137, https://doi. org/10.1039/C5DT00877H. [10] D. Nied, K. Enemark-Rasmussen, E. L'Hopital, J. Skibsted, B. Lothenbach, Properties of magnesium silicate hydrates (M-S-H), Cem. Concr. Res. 79 (2016) 323–332, https://doi.org/10.1016/j.cemconres.2015.10.003. [11] G. Mármol, H. Savastano, Study of the degradation of non-conventional MgO-SiO2 cement reinforced with lignocellulosic fibers, Cem. Concr. Compos. 80 (2017) 258–267, https://doi.org/10.1016/j.cemconcomp.2017.03.015. [12] R.D. Toledo Filho, F. de A. Silva, E.M.R. Fairbairn, J. de A.M. Filho, Durability of compression molded sisal fiber reinforced mortar laminates, Constr. Build. Mater. 23 (2009) 2409–2420, https://doi.org/10.1016/j.conbuildmat.2008.10.012. [13] S. Andrejkovicôvá, Fine sepiolite addition to air lime-metakaolin mortars, Clay Miner. 46 (2011) 621–635, https://doi.org/10.1180/claymin.2011.046.4.621. [14] G. Mármol, H. Savastano, M.M. Tashima, J.L. Provis, Optimization of the MgOSiO2 binding system for fiber-cement production with cellulosic reinforcing elements, Mater. Des. 105 (2016) 251–261, https://doi.org/10.1016/j.matdes.2016.05.064. [15] J.E.M. Ballesteros, S.F. Santos, G. Mármol, H. Savastano, J. Fiorelli, Evaluation of cellulosic pulps treated by hornification as reinforcement of cementitious composites, Constr. Build. Mater. 100 (2015) 83–90, https://doi.org/10.1016/j. conbuildmat.2015.09.044. [16] J. de A. Melo Filho, F. de A. Silva, R.D. Toledo Filho, Degradation kinetics and aging mechanisms on sisal fiber cement composite systems, Cem. Concr. Compos. 40 (2013) 30–39, https://doi.org/10.1016/j.cemconcomp.2013.04.003. [17] B. Lothenbach, D. Nied, E. L’Hôpital, G. Achiedo, A. Dauzères, Magnesium and calcium silicate hydrates, Cem. Concr. Res. 77 (2015) 60–68, https://doi.org/10. 1016/j.cemconres.2015.06.007. [18] D.R.M. Brew, F.P. Glasser, Synthesis and characterisation of magnesium silicate hydrate gels, Cem. Concr. Res. 35 (2005) 85–98, https://doi.org/10.1016/j. cemconres.2004.06.022. [19] R. Jarabo, E. Fuente, M.C. Monte, H. Savastano Jr., P. Mutjé, C. Negro, Use of cellulose fibers from hemp core in fiber-cement production. Effect on flocculation, retention, drainage and product properties, Ind. Crop. Prod. 39 (2012) 89–96, https://doi.org/10.1016/j.indcrop.2012.02.017. [20] G.H.D. Tonoli, E. Fuente, C. Monte, H. Savastano, F.A.R. Lahr, A. Blanco, Effect of fibre morphology on flocculation of fibre–cement suspensions, Cem. Concr. Res. 39 (2009) 1017–1022, https://doi.org/10.1016/j.cemconres.2009.07.010. [21] F.M. Lea, P.C. Hewlett, Lea's Chemistry of Cement and Concrete, (2004). [22] S.F. Santos, R.S. Teixeira, H. Savastano Junior, Interfacial Transition Zone Between Lignocellulosic Fiber and Matrix in Cement-based Composites, Sustain. Nonconv. Constr. Mater. Using Inorg. Bond. Fiber Compos, Elsevier, 2017, pp. 27–68 http:// linkinghub.elsevier.com/retrieve/pii/B9780081020012000036 , Accessed date: 17 August 2018. [23] S.F. Santos, R. Schmidt, A.E.F.S. Almeida, G.H.D. Tonoli, H. Savastano, Supercritical carbonation treatment on extruded fibre–cement reinforced with vegetable fibres, Cem. Concr. Compos. 56 (2015) 84–94, https://doi.org/10.1016/j. cemconcomp.2014.11.007.

Acknowledgements The authors acknowledge the Brazilian financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP Thematic Project, Grant n: 2012/51467-3). We also thank Magnesita Refratários S.A. and Elkem Materials South America for providing the materials for the cement production, Tolsa S.A. for providing the sepiolites and Universidad Complutense de Madrid for facilitating the development of part of the research in their facilities. References [1] A. Álvarez, J. Santarén, A. Esteban-Cubillo, P. Aparicio, Current Industrial Applications of Palygorskite and Sepiolite, Dev. Clay Sci, Elsevier, 2011, pp. 281–298. [2] E. Fuente, R. Jarabo, A. Moral, Á. Blanco, L. Izquierdo, C. Negro, Effect of sepiolite on retention and drainage of suspensions of fiber–reinforced cement, Constr. Build. Mater. 24 (2010) 2117–2123, https://doi.org/10.1016/j.conbuildmat.2010.04. 048. [3] R. Jarabo, E. Fuente, A. Moral, Á. Blanco, L. Izquierdo, C. Negro, Effect of sepiolite on the flocculation of suspensions of fibre-reinforced cement, Cem. Concr. Res. 40 (2010) 1524–1530, https://doi.org/10.1016/j.cemconres.2010.06.006. [4] R. Jarabo, E. Fuente, H. Savastano, C. Negro, Effect of sepiolite on mechanical and physical properties of fiber cement, ACI Mater. J. 111 (2014), https://doi.org/10. 14359/51686891. [5] M. Suárez, E. García-Romero, Variability of the surface properties of sepiolite, Appl. Clay Sci. 67–68 (2012) 72–82, https://doi.org/10.1016/j.clay.2012.06.003.

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