Evaluation of chloride resistance of silica fume and glass waste MWCNT- geopolymer composite

The target of the present paper is to study the enhancement effect of MWCNT on geopolymer resistivity to chloride attack. Geopolymer composites were made from metakaolin and ground granulated blast furnace slag and to either 10 % silica fume or glass waste. The added MWCNT were in the ratio of 0.01: 0.09 % by weight. Results revealed the high resistivity and stability against magnesium chloride solution; in addition glass waste mixes acquired high mechanical strength as compared with silica fume mixes as related to the high amorphous structure as well as high finesty of glass waste as compared with silica fume. The results revealed an increased enhancement in characteristics of geopolymer composites with MWCNT up to 0.07 % in both matrices.


Introduction
Interest in the development of alternative building materials such as alkali-activated binders has been promoted by growing development in building industry and the need for materials with high superior properties with high sustainable properties. Alkali-activated binders represent an attractive alternative for the partial or almost complete substitution of cement in construction industry as they offer durable, high performance in addition to low production cost [1], low greenhouse gas emissions [2,3]. There is a general agreement that alkali-activated cements can offer cradle-to-gate Greenhouse emissions savings approaching 40-80% compared to Portland cement for a performanceequivalent material.
Geopolymers are inorganic polymer binder that processed at room temperature upon activation of alumino-silicate sources using alkali metal oxides. Metakaolin-based geopolymer is one of the most abundant and applicable alumino-silicate polymers worldwide [4]. The synthesis of geopolymer proceeds via polycondensation reactions of metakaolinite (kaolinite calcined at 600-800°C) or other natural and industrial aluminosilicate reach materials as water cooled slag, silica fume fly ash with alkaline solution [5]. In spite metakaolin-based geopolymers possess interesting properties; they have not enough high strength and show brittle behavior. These make the products unsuitable for structural and infra-structural applications; thus, it is necessary to improve the fracture properties of metakaolin-based geopolymers [6]. Many previous studies investigated their reinforcing with various macrofiber as steel, polypropylene (PP), poly vinyl chloride (PVC), and basalt fibers [7][8]. The addition of these fibers increased the flexural strength, fracture energy, and controlled the crack propagation.
Multiwall carbon nanotubes (MWCNTs) are being considered as a potential reinforcement in composites because they have mechanical properties superior to those of traditional fibers and possess a Young's modulus of 1TPa, a yield stress of100-300 GPa, and a tensile strength of 63 GPa [9]. However, the higher mechanical properties of MWCNTs alone do not ensure a mechanically superior nanocomposites geopolymer. It is clear that the dispersion of the MWCNTs within the metakaolin-based geopolymers and the bonding properties between the surface of the MWCNTs and the metakaolin-based geopolymers around them are the main challenges in the fabrication of metakaolin/MWCNTs composites [10,11].
Maeva et al., [12] showed that using nanodispersed carbon additives as a modifier in anhydrite compositions provided a dense, low-defect structure of crystalline hydrates, leading to the increased mechanical strength and water resistance of the material. However, the effective application of multiwalled carbon nanotube dispersions required the solution of the problem connected with partial separation of carbon nanotubes into individual particles in the aqueous dispersion medium as well as the problem of stabilizing nanostructures in dispersion while being stored. The solution of the problem of stabilizing suspensions is adsorbing and solvent layer on the surface of nanotubes which prevents their approximation. The best variant to limit coagulation is to use surfactants [13].
Previous studies have shown that sodium hydroxide (NaOH) acts as a surfactant and removes oxidation debris from the surface of the MWCNTs, consequently allowing them to de-bundle and form well-dispersed nanotubes within geopolymer matrix [11]. Recently, Khater and Abdel Gawwad [14,15]  Morsy et al. [17] have investigated also the effect of different concentrations of MWCNTs with nanoclay particles on the compressive strength of cement mortar and they noticed that the nanoclay particles improved the dispersion of CNTs within the mortar. Lio et al. [18] studied also the influence of MWCNTs mixed with silica fume (SF) on the mechanical property of cement-based composites.
Hunashyal et al. [19] on the other hand fabricated hybrid composites of plain cement by integrating both carbon micro-fibers (CMFs) and MWCNTs.
As known that the immersion in chloride solution resulted in rapid deterioration and softening to the concrete structure, this can be related to the corrosive action depends on the concentration of chloride solution and the kind of cation linked to the chloride ion as reported by Barnes and Bensted [20]. The chloride ions penetration is dependent not only on the porosity of the matrix, but also on the paste composition and the ion-exchange capacity of the system. The damage influence of chloride is either chemical by interaction with liberated lime from the hydration reaction forming what is known as Friedel's salt or hydrocalumite (C 3 A·CaCl 2 ·10H 2 O) causing softening to the composition.
Jun et al., [21] investigated the chloride-binding capacity of alkali-activated fly ash and slag samples and their synthesized Cl-bearing phases, which are capable of binding and immobilizing chloride when seawater is used as the mixing water.
In the current work, we aim to examine the stability of the geopolymer composites enhanced with MWCNT against immersion in magnesium chloride solution up to 6 months and trace the formed structures using XRD, FTIR, and DTG as well as study the mechanical stability up on immersion in the solution.

Materials
The materials, which used in this investigation, are ground granulated blast furnace slag (GGBFS), sourced from Iron and Steel Factory-Helwan, Egypt. Kaolinite collected from El-Dehesa, South Sinai, Egypt, which was thermally treated at 800 o C for 2 hrs with a heating rate of 5 o C/min to produce metakaolin (Mk) [22,23]. Silica fume sourced from Suez Cement Company (Helwan plant) also known as micro-silica is a by-product of the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys; it was in the powder form with 95% SiO 2 , where about 90 % of particles are of 200 μm diameter, while glass waste powder prepared by grinding glass up to fine structure with an average particle size of 38 μm as measured by laser particle size distribution.
Chemical compositions of the starting raw materials are given in Table (1 Multi-walled carbon nanotubes used in this study were synthesized using a Co/MgO based catalyst using CVD instrument (housing and building national research center, Egypt), the physical properties of the MWCNTs used in the study are given in Table (2).

Dispersion of MWCNTs
MWCNTs were first mixed with Gelenium Ace 30-polycarboxylate-based superplasticizer and 50 % of the added water. This Polycarboxylate-based superplasticizer has been proven to be effective for CNTs dispersion [24,25]. The solution was sonicated using a Fritish 450 Sonifier Analog Cell distributor for 15 min [26]. Solutions with concentration of 0.01, 0.03, 0.05, 0.07 and 0.09-wt % of the total weight of the matrix were used to identify the MWCNT optimum concentrations and their resistivity to aggressive medium.

Geopolymerization, curing and casting
Geopolymerization and curing are processed according to the following steps:

1.
First activators prepared 24 hrs prior to casting, whereas binder reinforced with 2.
MWCNT particles sonicated for 15 min using half of the used water as well as used superplasticizer for better dispersion under a temperature of 40oC.

3.
The geopolymer binder passing a sieve of 90 µm as represented in Table (3) were hand mixed with the alkaline activator solution dissolved in the remaining used water for 10 min followed by a further 5 min using rotary mixer and mixed at medium speed (80 rpm) for another 30 seconds.

4.
MWCNT and superplasticizer were then added and stirred with the mixture at high speed for additional 30 seconds.

5.
Paste mixtures were cast into cubic-shaped moulds with 2.5 cm length, vibrated for compaction, sealed with a lid to minimize any loss of evaporable water, left to cure undisturbed under ambient temperature for 24 hrs, demolded and then subjected to curing at 40°C with 100% relative humidity (R. H.) for 28 days [27],

6.
All mixes were then immersed in 5 % MgCl2 solution [28], whereas the solution was replaced monthly, while their strength values were recorded, finally the crushed cubes were stopped for further hydration using acetone/methyl alcohol method (1:1) [29][30], finally tightly conserved in container until examination time.

Exploration techniques
Chemical analysis was carried out using Axios (PW4400) WD-XRF Sequential Spectrometer (Panalytical, Netherland). Compressive strength tests were carried out using five tones German Brüf pressing machine with a loading rate of 100 kg/min determined according to [31]. X-ray diffraction are characteristic of Friedel's salt [37], where all bands except one at 532 cm −1 were overlapped by other bands, so only this peak shows the formation of Friedel's salt after 3 months of immersion in chloride solution. However the increased content of MWCNT leads to agglomeration, so more sodium cations are available for carbonation as clearly will be seen in Fig. (6).
On the other hand, silica fume based mixes immersed in chloride solution at 1 and 6 months [ FTIR spectra of geopolymer mixes incorporated glass waste and silica fume immersed in 5% magnesium chloride solution up to 6 months, and enhanced by 0.07 % MWCNT (Fig. 4A, B). The pattern of both mixes exhibit an increased intensity in the amorphous geopolymer constituents as represented by asymmetric stretching vibration of T-O-Si at about 1000 cm -1 with slight shifting to lower wave number with time. Also, there is an increased dissolution of unreacted slag materials with curing time up to 6 months as illustrated from the decreased asymmetric band at about 1100 cm -1 for non-solubilized particles as well as symmetric vibration of α-quartz at about 797 cm -1 , confirming the activation and nucleating efficiency increase by MWCNT [38]. The band at 690 cm −1 is attributed to the symmetric vibration mode of the Si-O-Si or Al-O-Si bonds. This band corresponds to C-A-S-H type phases, and the zeolites forming in these systems, as identified by XRD (Figs. 5, 6).. This band has also been observed in unreacted slag [39], and has been attributed to gehlenite. All infrared band assignments follow references [41,42].
There is a decrease in the carbonation bands observed with MWCNT and time, where the carbonate constituents in slag materials lead to the growth of the carbonate band as discussed above, showing that the carbonates identified in this raw material do not react significantly under alkaline activation conditions [43]. The CO 3 2vibration band decreases with time as attributed to the fact that the increased nucleation of MWCNT increase the formation of geopolymer structure and decrease the availability of free Na + species which will be subjected to carbonation. It can be seen an increased broadness of this hump up on increasing MWCNT to 0.07 % with the increase of CSH as well as reversedite phases (C 3 S 2 H 3 ) which is one of the calcium hydrate phases that add additional strengthening as well as nucleating sites for geopolymer formation and accumulation [44], the increased intensity of this hump favors the activation done by MWCNT resulting in enhancement of geopolymer reaction. Further increase in nano results in an agglomeration, forming weak points within the matrix which hinder geopolymer propagation leading to formation of short zeolite chains as confirmed by the increased intensity of zeolite phases (faujasite, zeolite A, sodalite) on the expense of the amorphous geopolymer structure. This decrease in the amorphous geopolymer phases made the matrix more prone to chloride attack as indicated by the appearance of Friedel's salt, but its intensity still much lower than control mix which suggests the ability of MWCNT even in high dose to suppress the chloride attack.

X-ray diffraction (XRD
The behavior of silica fume based mixes almost similar to those of glass waste mixes (Fig.5b) The main remark in silica fume mixes also is the increased zeolite content as related to its lower reactivity which favor the zeolite formation than amorphous geopolymer constituents specially up on using high MWCNT dose while accompanied by the increased intensity of Friedel's salt.
On examining the effect of immersion time up to 6 months on the optimum mixes incorporating 0.07 % MWCNT in glass waste and silica fume mixes as illustrated in Figure (6), there is continuous growth in amorphous geopolymer hump with time in addition to the increased CSH-phases as attributed to the effect of the NaOH alkaline solution during processing as discussed in a previous investigation [45]. The carboxylate based superplasticizer was effective in dispersing MWCNTs at 0.0.07 wt., % and lower, where most of MWCNT interact and activate the geopolymerization reaction forming three dimensional networks. Another main concluded remark was the increased faujasite phase in silica fumes mixes up to 3 months whereas in glass waste mixes there is continuous growth and crystallization of CSH. The increased zeolite phases in silica fume mixes made the matrix more prone to chloride attack especially at 3 months as represented by peaks due to Friedel's salt, while beyond this age the progress of geopolymer formation results in increasing the medium alkalinity which will destabilize the formed Friedel's salt leading to lowering in its intensity [46][47].
00It is likely that a significant proportion of Na + species is consumed in the activation of binding constituents to form a sodium aluminosilicate-type gel, reducing the availability of Na + species in the pore solution [48] and, therefore, reducing the alkalinity from the high levels which would favor zeolite growth, even with the higher alkali dosage added to these samples. It can be noticed also the absence of calcite phases mostly in all mixes as well as with immersion time which reveals the absence of free alkalis as they are mostly consumed in geopolymer formation. This observation come in accordance with FTIR pattern where the intensity of carbonate bands are very small while in XRD there are no carbonates as the XRD tool concerned about crystalline content of the minerals which can detect only more than 3%, which means that calcite phases are too small in the matrix. present in large pores of the aluminosilicate type product (geopolymer gel) for Mk -geopolymer [26,49], while the second associated with the dehydration of a C-S-H gel [50], as this is the main reaction product identified in GBFS-rich blended binders [40,51]. In addition to another endothermic peak at In case of silica fume geopolymer mixes (Fig.8), the same behavior with respect to MWCNT addition except that the endothermic peak for freely evaporable water in geopolymer shifted to lower temperature (52 o C) indicating the lower matrix density of those mixes as compared with glass waste mixes and the free water of geopolymer is not tightly bound as the previous mixes. Also, the CSH shifted to lower temperature to 131 o C as the formed CSH binding materials are not of dense structure as compared with previous mixes, whereas the carbonate peaks are almost equal in intensity.
The total weight loss reflected the increased the amorphous aluminosilicate content with nano materials up to 0.07 %, while its decrease as compared with glass waste related to the absence of zeolite peak at about 310 o C in silica fume mixes. On the other hand, prolonged immersion of glass waste geopolymer and silica fume mixes incorporating 0.07 % MWCNT in magnesium chloride solution up to six months (Fig.9, 10 is known by its non-cementitious properties and leads to softening on prolong exposure time leading to destabilization of the hydration materials [53,54]. In our case, optimization of the strength values for geopolymer composites containing 0.07 % MWCNT ascribed to its spreading throughout the geopolymer matrix with a uniform density. Where, The good dispersion of MWCNTs was also attributed to the effect of the NaOH alkaline solution during processing as discussed in an earlier publication [45] and hydrophilic groups (-COOH) in carboxylate based superplasticizer forming covalence-modified CNTs to improve interfacial interactions within composites, in which the -COOH groups form strong coordinate bonds with Ca 2+ ions in geopolymer matrix, thus enhancing the formation of three dimensional geopolymer in addition to CSH [39,55].
However, The carboxylate based superplasticizer was ineffective in dispersing MWCNTs in ratios more than 0.07 %, where most of MWCNT were agglomerated in the alumino-silicate gel and hinder the propagation of the three dimensional network.
Incorporated MWCNTs increase the geopolymerization rate after addition to the same system; therefore, the MWCNTs inhibit the retardation effect caused by aggressive chloride solution as reflected on the decreased intensity of the formed zeolite as well as Faujasite phases at the optimum their efficiency in resisting aggressive media can be explained by their ability to work as nucleationsites in the system, where the functionalized MWCNTs provide different reaction sites for the crystal growth of geopolymer products [22,23,39,56].
The negative effect of MWCNT overdose than 0.07 % was confirmed by the increased zeolite as well as Faujasite with the increased MWCNT, as the geopolymer network was terminated forming the crystalline zeolitic structures with lower reactive characteristics which facilitate the ingress of aggressive chloride solution as confirmed by slight peak at 0.09 % in XRD pattern ( Figure 5) for Friedel's salt. Also, the intensity of the formed Friedel's salt in glass waste mixes is lower than that of silica fume mixes due to variation in reactivity of both as can be seen from their compressive strength values, where strength values for optimum MWCNT (0.07%) after six months of immersion reaches about 88MPa while for silica fume mix at the same ratio reaches about 48 MPa. Zeolite formation is also well known to take place in KOH-activated geopolymer, similar to their NaOH-containing counterparts. However, crystallization is less rapid in KOH/metakaolin geopolymer compared to the NaOH/metakaolin system [58] which confirmed the lower intensity in DTG for zeolite phases and lower intensity detected in XRD.

Conclusion
Hybridization of various aluminosilicate precursors including either glass wastes or silica fume produce sustainable building materials with superior resistivity to aggressive magnesium chloride solution.
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