Physico-mechanical and Thermo-gravimetric Properties of a Silica Fume-based activator for use in Geopolymer Binders

Ndigui Billonga*, John Kinuthiab, Jonathan Otib.

a Laboratory of Materials Analyses, Local Materials Promotion Authority - Mission de Promotion des Materiaux Locaux (MIPROMALO), P.O. Box 2396, Yaoundé, Cameroon.

b Faculty of Computing, Engineering and Science, University of South Wales, Pontypridd, UK, CF 37 1DL.

Abstract

The effectiveness of using silica fume-based sodium silicate in the production of metakaolin (MK) and ground granulated blast furnace slag (GGBS) based geopolymer was investigated. Un-densified silica fume used as the source amorphous silica was appropriately dissolved in a 10M sodium hydroxide solution to obtain an alternate sodium silicate (SSA) solution having SiO2/Na2O molar ratio of 2. The SSA solution was used together with the 10M sodium hydroxide solution in 1:1 volume ratio as alkaline solution to elaborate MK and GGBS based respectively 1.5 and 0.7 liquid/solid ratios geopolymers pastes at ambient temperature of 23±2°C. Setting time, density, compressive strength, tensile strength, water absorption were the physical-mechanical tests performed on fresh and hardened pastes at 3, 7 or 28 days of curing. Powder of hardened paste at 28 days underwent thermogravimetric (TG/DTG) analysis at up to 1000°C. For comparison, similar MK and GGBS based geopolymer pastes were prepared with a standard commercial sodium silicate (SSC) having the SiO2/Na2O molar ratio of 2 and submitted to similar tests. The silica fume-based sodium silicate was found to effective in geopolymer production as evidenced by the similar or slightly better performance in physical-mechanical and thermo-gravimetric properties when SSA or SSC was used in MK or GGBS geopolymers. Un-densified silica fume constitutes a suitable raw material for sodium silicate production with additional sustainability potential in the reduction of embodied energy in the production process, since the by product is not tumbled, heated or grinded and the dilution process with the sodium hydroxide solution happens at ambient temperature.

Key words: Sodium silicate; Alternate silica sources; Silica fume; Geopolymer; Physical-mechanical properties; Thermo-gravimetry.

1.0Introduction

Geopolymer binders are inorganic polymers which comprise of amorphous and three-dimensional structures formed from polycondensation of alumino-silicate monomers in the presence of alkaline activator solutions [1, 2]. The binders have potential and scope for use in a wide range of applications beyond and besides traditional civil engineering applications in paste, mortar and concrete. Specialist applications may include use in aerospace, automobile, non-ferrous foundry and metallurgy, pollutants immobilization, plastic industry, and in energy storage and possibly other niche uses [3-5]. Because of the environmental concerns due to the high-energy consumption and carbon dioxide emissions associated with the manufacture of Portland cement (PC) [6], viable geopolymer binders that are likely to present attractive and/or substitutes to Portland cement have been reported by in numerous researches [7, 8]. The novel new binders have attracted considerable interest in the past few decades due to their interesting physical, thermal and structural properties [9]. Hassan et al. [10] in their review on mechanical properties and microstructure of geopolymer concrete for cleaner and sustainable environment concluded that the geopolymer binders may be attractive in terms of fire resistance compared to Portland Cement-based concrete.

Rapid geopolymerization reactions take place when natural and/or artificial alumino-silicate precursors such as Ground Granulated Blast-furnace Slag (GGBS), red mud, volcanic ash and calcined clays react with alkaline solutions such as sodium hydroxide (Na-based activator) or potassium hydroxide (K-based activator) or the mixture of these hydroxide solutions and their silicate solutions [11-13]. Depending on the nature of the precursor, Na-based activators have been observed to promote higher compressive strength, associated with more refined pore structure compared to K-based activators [13], making sodium silicate (also commonly known as water glass,) the more common and effective activator. Because of the fast setting times, coupled with rapid and early strength development, these geopolymer formulations are gaining new interest in specialist applications such for use as binders in 3D printing techniques [11].

Studies have shown that while industrial waste ashes are re-utilized to produce geopolymer at ambient temperatures, the amount of greenhouse gas emitted to the environment may be lowered by 26-80 % compared to Portland cement [10, 14-17]. Turner and Collins [18] indicated that CO2 emission of geopolymer concrete was 9 % less than PC-based concrete. Differences between studies arise due to proximity, availability and composition of raw materials, energy/fuel types, concrete mixture compositions and manufacturing process for the alkaline activators used. Although sodium silicate has been extensively used in alkaline activation, it was however reported that its use significantly increases the embodied energy and the CO2 emissions associated with the geopolymer manufacturing process. This typically happens during the calcination of a mixture of sodium carbonate (Na2CO3) and of quartz (SiO2) at temperatures as high as 1400-1500°C [19] to produce commercial sodium silicate. This energy intensive and consequently costly manufacture of sodium silicate is one of the factors that can hinder the sustainability of geopolymer binder production and reduces its promotion compared to Portland cement [13, 20].

Vinai et al. [21] recently estimated that the cost of Alkali-Activated Concrete (AAC) varies from 41.2 £/m3 to 66.92 £/m3, 19% lower to 48% higher compared to PC-based concrete. Therefore, the uptake of geopolymer binders as a sellable commodity on a global basis will take time, if appropriate solutions are not found to optimize the sustainability in their production process. The development of alternative activators of reduced CO2 footprint and less energy demand can increase the sustainability credentials of geopolymer binders in concrete [5, 19]. Past studies aiming to reduce or eliminate the use of energy intensive sodium silicate in the production process of geopolymer have showed potential viability. Kamseu et al., [22] researched on the substitution of sodium silicate with rice husk ash solution in a metakaolin-based geopolymer cement. They concluded that sodium silicate based on an RHA-NaOH solution cured at room temperature in open air had characteristics like those of standard/commercial sodium silicate at the equivalent bulk composition and SiO2/Na2O molar ratio of 3.1. In a similar alternative approach, Tchakounte et al., [23], obtained satisfactory results on metakaolin-based geopolymer production when they used alternative alkaline activators with SiO2/Na2O molar ratio of 1.5, by using waste glass and rice husk ash. Further, Moraes al., [24] prepared sodium silicate by reacting sugar cane straw ash with sodium hydroxide and obtained similar results with Ground Granulated Blast-furnace Slag (GGBS) based as when rice husk ash was used.

There have not been many studies targeting/focussing on the use of silica fume as the source of amorphous and reactive silica, even though this material has shown numerous applications in concrete as a Supplementary Cementing Material [25, 26]. For this reason, the currently reported study aimed at investigating the effectiveness of silica fume-based sodium silicate in geopolymer production, using silica fume dissolved in a concentrated 10 M sodium hydroxide solution to obtain a Sodium Silicate Alternative (SSA). A SiO2/Na2O molar ratio of 2 was adopted, and the SSA produced was trialled using two alternative precursor alumino-silicate materials – Metakaolin (MK) and Ground Granulated Blast Furnace Slag (GGBS). The geopolymers were produced adopted a 1:1 volume ratio for the SSA and the 10 M sodium hydroxide solution to form Alkaline Activator (AA), and 1:2 ratio for the AA and the Precursor Material (PM).

Results were compared to similar geopolymer binder formulations made with commercial sodium silicate (SSC) solution at a similar SiO2/Na2O molar ratio of 2. The subsequent analyses comprised of comparisons of engineering performance suggested by results from tests for setting times of the fresh pastes, density, water absorption, compressive strength of hardened pastes after 3, 7 and 28 days, and of tensile strength after 28 days of moist curing. In addition, analytical tests to enable formulation of hypotheses for the reaction mechanisms were carried out by comparing chemical and mineralogical compositions of the raw materials, and thermo-gravimetric and derivative thermo-gravimetric (TG/DTG) behaviour of hardened pastes at 28 days of moist curing, to temperatures of up to 1000°C.

2.0Materials and methods

2.1. Materials

Silica Fume (SF): The Silica Fume used in the study to produce silicate-based activator was an undensified variety (USF) designated a commercial code - 971U of 97.1% purity and was manufactured by Elkem Silicon Materials of Norway. SF is a by-product of the silicon and ferro-silicon industries. The reduction of high purity quartz to silicon at temperatures of up to 2000°C produces SiO2 vapour, which oxidizes and condenses in the low temperature zone to ultra-fine particles consisting of 85 to 99% non-crystalline silica, also known as undensified silica fume (USF) [27, 28]. SF is a highly reactive pozzolanic material having particles 100 times smaller than the average cement particles [28]. The surface area of the particles varies from 13,000 to 30,000 kg/m2. When the undensified silica fume is collected, it is very light with bulk density ranging from 120 to 220 kg/m3. For storage, reduction of transport cost and dust pollution problems, the USF is treated (densified) to increase its bulk density to the range of 400 to 720 kg/m3 [29]. This treatment is usually accomplished by tumbling silica fume particles in a silo, which causes surface charges to build up. These charges draw the particles together to form agglomerates with consequences of increasing the density and the embodied energy of the final material [30].

Precursor materials (PM): The alumino-silicate precursor materials used in the study comprised of commercial Ground Granulated Blastfurnace Slag (GGBS) and industrial metakaolin (also referred to as metakaolinite) (MK). GGBS was manufactured by Civil & Marine Ltd., in accordance to BS 15167-1. For the industrial MK, Metastar 501 was used, and was manufactured by IMERYS company in the UK.

Chemicals: Commercial laboratory grade sodium silicate pellets of 97% purity were obtained from Fisher Scientific, UK. The manufacturer indicated the molar ratio SiO2/Na2O of the pellets as 2. From these pellets, a 10 M sodium hydroxide (NaOH) solution was obtained by dissolving the appropriate mass (400 grams) in one litre of de-ionized water. The concentration of NaOH was chosen based on previous optimization works [31, 32]. The solution was prepared 24 hours before its subsequent use.

The chemical composition of raw materials was determined by X-Ray Fluorescence (XRF) spectrometry, using a Bruker S4 Pioner wavelengths dispersive (WD-XRF) spectrometer. X-ray diffraction (XRD) analysis was carried out on the MK, USF and GGBS materials at room temperature using a STOE Powder Diffraction System with operating conditions of CuKα sealed tube, operated at a radiation of 1.54060nm.

The characteristics of the input materials are shown in Table 1. SiO2 was observed as the major oxide in USF. MK consisted of SiO2 and Al2O3 while GGBS had CaO, SiO2, Al2O3 and MgO as major constituents. The mineralogical analyses suggested that all the materials used were essentially amorphous in nature, as shown in Figure1.

2.2. Methodology

Preparation of sodium silicate solutions (SSA and SSC): The chemical equation 1 was used as the basis for the calculations for the preparation of the Sodium Silicate Alternative (SSA) with a SiO2/Na2O ratio = 2 using USF as raw material.

(1)

According to equation 1 and considering i) the 97.1 % SiO2 content in the USF and ii) the molarity of the sodium hydroxide solution desired (10M), 123.58 grams of USF were mixed per 200 mL of the 10M NaOH solution. Proportions of the constituents in the mixture were multiplied by 20 to obtain the desired quantity of SSA. This mixture was left to react for 48 hours in a closed container before subsequent use. The viscosity the solution was adjusted by a controlled addition of de-ionized water to obtain SSA having a density of 1.34 g/mL in accordance to previous study [22].

The control commercial sodium silicate solution (SSC) was obtained by dissolving 1kg of sodium silicate pellets per 100 mL of de-ionized water. After 24 hours of dissolution, controlled addition of de-ionized water was used to dilute the solution to a similar density as SSA.

Preparation Alkaline Activators (AA): The alkaline activator was prepared by mixing SSA or SSC with the 10M NaOH solution in a volume proportion of 1:1. This is a common sodium silicate-sodium hydroxide mixing ratio in geopolymer formulations [20].

Preparation geopolymer pastes and test specimens: The alkaline activator was mixed with the precursor materials (PM - MK or GGBS) for 3 minutes using Liquid/Solid ratios of 1.5 for MK and 0.7 for GGBS (Table 2). These were the mixing ratios observed to provide workable paste mixes. After homogenization, the fresh pastes were tested for initial and final setting times. In addition, 50 × 50 mm paste cubes and 8-shaped briquette gangs were prepared according to standard ASTM C 307-03 (2012). These were for compressive and tensile strength tests respectively. The steel molds were vibrated slightly to eliminate air bubbles, and the pastes then left to cure for 24 hours in the molds at ambient temperature. After demolding, the hardened paste samples were kept in a container and allowed to moist cure at 23±2°C and at least 80% humidity.

In summary,

Production of Sodium Silicate Alternative (SSA)- SiO2/Na2O = 2 (w)

Production of Alkaline Activator (AA)- SSA (or SSC) /NaOH = 1 (w)

Production of Geopolymer - AA/PM = 1 (MK) or 0.7 (GGBS) (w)

w - by weight

Characterization of fresh and hardened geopolymer pastes: The initial and final setting time tests of pastes was carried out using an automatic penetrometer (Vicatronic) equipped with a penetration needle, in accordance to EN 196-2005 standard at the ambient temperature of 23±2°C. The solid PM material and the alkaline activator (SSA (or SSC) + NaOH) were mixed thoroughly during 3 minutes before performing the test. Hardened 50 × 50 mm paste cubes underwent compressive strength measurement after 3, 7 and 28 days of moist curing, while others were tested for water absorption and dry density after 28 days. The compressive strength test was performed using a Denison 89084 hydraulic press equipped with a Matest cyber-plus data recorder at a load rate of 1.5 kN/s. The briquette gangs were tested for tensile strength testing according to ASTM C 307-03 (2012) after 28 days of curing. The test was carried out using a Zwick Roell Z010 equipment at a rate of 1 mm/minute. The stress (in MPa) were recorded as a function of their elongation (in mm). The stress-elongation curves of samples having the highest ultimate stress were compared. The water absorption (WA) of test specimens was carried out based on ASTM C642-13. Calculations were performed using Equation 2.

(2)

Where Mh was the weight in grams of the specimen after immersion in water for 48 hours and Ms its weight after drying at 105°C for 48 hours.

Thermogravimetric analyses of test powders of hardened samples at 28 days up to 1000°C were performed using a TA Instruments TGA 55 Discovery series apparatus.

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3.0Results and discussion

3.1. Setting times of fresh geopolymer pastes

The setting time test results for the various geopolymer pastes are indicated in Figure 2. It is seen that setting time of the pastes was influenced by the type of precursor material (MK or GGBS) and nature silicate solution (SSA or SSC) used. The rapid initial setting was observed with GGBS based pastes compared to MK-based pastes. With GGBS-SSC pastes, the initial setting time was observed to be achieved at as about 5 minutes. Thereafter, the penetration decreased significantly. With the GGBS-SSA paste a slight increase of 2 minutes is observed on the initial setting time. The initial setting times of the GGBS-based pastes containing either SSC or SSA were very similar and happened about 4 minutes after the mixing period. In contrast, the initial setting times of the MK-based pastes containing either SSC or SSA were slightly different. The paste containing MK and SSA exhibited an initial setting at about 24 minutes after the end of the mixing period, while that made with SSC showed a setting time that was about 4 minutes longer. The setting times exhibited by the pastes containing the alternative sodium silicate (SSA) and the commercial sodium silicate (SSC) appear to suggest that the reactivities of the two silicate forms are quite comparable.

3.2. Characteristics of hardened geopolymer pastes

Density

According to EN 1602:2013 standard, the apparent density refers to the ratio of the mass and the volume occupied by a tested solid including hollow spaces. It depends on the form of the constituent particles, the composition of the solid components, and the storage method. Figure 3 shows that higher densities were observed with GGBS-based geopolymer pastes compared to MK-based pastes. The difference can partly be explained by the difference in specific gravity between GGBS and MK which are 2.85 and 2.50 respectively. It is also due to the high-water demand of pastes with metakaolin compared to those made using GGBS in order to achieve workable pastes. The high-water content is associated with high porosity after the drying process. There was no significant difference in apparent density between SSA- and SSC-based geopolymer pastes.

Compressive strength

Compressive strength results of geopolymer pastes are shown in Figures 4(a) and 4(b). Figure 4(a) shows the preliminary trial on geopolymer pastes using SSA and GGBS. The least workable mixtures were achieved at an AA/PM ratio of 0.6. The subsequent compressive strength test results obtained appeared to suggest that self-desiccation of the mixtures took place upon curing for more than 7 days, such that at 28 days the strength development was severely compromised. With this information, some adjustments were made and subsequent mixes using GGBS were made at an AA/PM ratio of 0.7. For MK, similar workability was achieved at AA/PM ratio of 1.5.

Using these second generation geopolymer paste mixtures, Figure 4(b) shows that a great difference was observed between the strength of hardened pastes containing GGBS or MK. GGBS-based mixtures demonstrated higher compressive strength results compared to MK-based pastes. This difference has been attributed in previous studies to the differences in the binding system. Since SiO2 and Al2O3 are the major compounds in MK and CaO, SiO2, Al2O3 and MgO in GGBS, the MK- and GGB-based systems may be considered as (Si+Al) and (Si+Ca) respectively [33]. Differences in mechanical performance can also due to the internal micro-texture induced by the quantity of water used to achieve both workability and prolonged strength development. However, any excess water can also affect the porosity of the paste and influence mechanical performance. For both GGBS- and MK-based geopolymer pastes, the compressive strength increased with the curing duration. Pastes containing SSA showed marginally higher 28-day strength compared to SSC for both precursor materials used. However, the commercial sodium silicate (SSC) induced more rapid strength gain compared to the silica fume-based sodium silicate (SSA). For GGBS-based pastes, about 72 % of the 28-days strength was achieved at 7 days of curing using SSA while the equivalent was 88 % using SSC. For MK-based pastes, strength development was additionally rapid, and the equivalent values are 79% and 92% respectively. Overall, compressive strength test results appear to suggest that silica fume can be used to make a sodium silicate alternative that can be utilised in viable geopolymer binders, without having to result to expensive commercial sodium silicates, and hence leading to improvement of the sustainability of geopolymer binders.

Tensile strength

In addition to compressive strength, the tensile strength provides additional information of the nature of the cross-linkages developed in a material matrix. Tensile strength may also suggest the level of densification of the matrix and its consequence on the microstructure features within the matrix [22]. In the present study, it was evaluated using stress-elongation curves shown in Figure 5. Within experimental variations, both GGBS- and MK-based geopolymer pastes exhibited identical stress-stain paths, with only minor differences between SSA- and SSC-based pastes. Results in Table 3 show the maximum stresses and elongations achieved. The GGBS-based pastes failed at higher tensile stresses compared with the MK-based pastes, corroborating the trend observed with both density and compressive strength. The mode of failure in all the pastes were deemed to be typically of brittle materials with no plastic deformation. Within the minor variation that were observed, the GGBS-based pastes made with SSA presented marginally lower ultimate stress and elongation magnitudes compared to those made using SSC. This order was reversed with the MK-based pastes, with SSA pastes demonstrating marginally higher stress and elongation magnitudes.

Water absorption

Water absorption is a capacity of a material to absorb moisture from its environment. It is related to the internal pore structure of the material and hence plays an important role in the permeability of the material, and subsequent mechanical performance and durability [1]. The water absorption of the geopolymer paste in this study are shown in Figure 6. The results show that for the material compositions studied, GGBS-based pastes indicated lower water absorption compared to MK-based pastes. The results corroborate those of the density and of the compressive and tensile strength measurements. The high-water absorption of the MK system agrees with the less dense and compact nature of the system, associated with high internal porosity and high permeability compared to the GGBS system. There were no major differences in the water absorption of pastes containing SSA compared to pastes containing SSC, hence demonstrating applicability of the alternative method of producing sodium silicate for geopolymer cementation route.

Thermogravimetric analysis

Davidovits et al. [34] has reported the presence of three different types of water in hardened geopolymer pastes. These are:

i) water that is physically held in the microstructure. This is relatively easily lost from ambient temperature to 100°C due to dehydration during heating of the geopolymer. From 60 to 70 % of reaction water in geopolymers is reportedly physically bonded, and there is consequent significant weight loss depending on the composition of the mixture.

ii) water that is chemically bound by H-bonds, and retained at the surface of hydroxylated silica structures [35]. It is associated with weight losses taking place above 100oC and may continue to about 300 °C.

iii) water from hydroxyl groups (Si-OH, Al-OH, and Na-OH) [34]. It is lost as a result of the de-hydroxylation of the silicate structures and takes place above 300oC.

With the above weight losses in consideration, Figures 7 and 8 show the thermogravimetric results obtained from the reported study. For the GGB-based systesm all the three major weight loss peaks above were identified, and took place below 600oC. The MK-paste system also demonstrated the three major peak identified with the GGBS-system, but were rather different in nature and took place at different temperature zones. For the MK-system, the fist peak was far bigger and sharper than that for the GGBS-system. It is hypothesised that for the MK-system, the presence of high amounts of alumina resulted in a sytem that had more crystalline alimuno-silicate compounds.

For both GGBS- and MK-based systems, the second peaks (100-200oC) were very similar. Changes between 100-200°C can be attributed to the loss of interlayer water of C-S-H gel and, when present, the decomposition of ettringite [39]. Ettringite is not anticipated in the system studied, due to absence of sulphate. It is therefore hypothesized that the similarity in the peaks between GGBS- and MK-based systems is due to similarities in the C-S-H gels in the GGBS-system, and the decomposition of the gels associated with the polycondensation sodium alumino-silicate hydrate gels (N-A-S-H) in the MK-system [34,36,37,38,39].

The peaks above 300oC are different for the GGBS- and MK-based systems. For the GGBS-system, the third pead centred at about 400oC. Is is associated with the high Ca-based compounds due to the high CaO content of the GGBS that may be associted with some Ca(OH)2 content which is known to decompose at about this temperature range. In contrast, the third peak for the MK-based system is a very diffuse peak starting to take place at about 600oC. In the absence of Ca-based compounds in this system, the diffuse peak is more likely to be due to the de-hydroxyalation of the remnants of the clay-like structure of calcined kaolin that consitute MK. It is rather weak because the structure is severely broken in the dehydroxylation process during the manufacture of MK by calcination process.

There no weight loses above 650oC. Typically, in addition to the above weight losses, the decomposition of carbonate compounds can also contribute to weight losses, and takes place at temperatures from about 650oC [40]. This did not appear to be the case for both systems. Also for both systems, there were no major differences between SSA- SSC-based geopolymer pastes.

…….consequence of the dissolution and reorganization of aluminosilicates during the geoplymerization process which involves three phases comprising the dissolution and reorganization, condensation and polymerization. The dissolution and reorganization create several types of oligomers known as of silanol or alumino groups that connect and form large polymers or gel as indicated in equation 3 [36]. When oligomers connects, the OH groups at their ends meet and release water by sharing an oxygen atom with NaOH being the primary source of OH group [34,37, 38].

(3)

Where T is Si or Al.

From figure 6 and 7, it is clearly noticed that the decomposition trend for both pastes containing SSA or SSC is almost similar with a slight difference in the total weight loss which was 11.95% for paste containing SSA and 11.68 for the paste containing SSC. The difference in the nature of DTG peaks at temperature lower or equal to 100 °C can be attributed to the differences in pore structure in the two materials which retained differently evaporable water [38]. The similarity in the release of chemically bounded water and water from hydroxyl groups from the geopolymerization process is evidenced by the overlapping of the TG thermograms of the paste with SSA and SSC at temperatures above 100°C. Since the difference in weight loss of metakaolin geopolymer is mainly attributed to the decomposition of their different polycondensation sodium aluminosilicate hydrate (NASH) type gels, the major binding gels, which are responsible of mechanical performance, the slightly higher total weight loss of MK based geopolymer paste containing SSA is in accordance with compressive strength results in which this type of paste presented better performances after 28 days of curing.

The trends of thermograms of GGBS based geopolymer containing SSA and SSC showed similarities. The thermograms consisted of four main weight loss changes. The change at temperature below 100°C are attributed to the loss of the remaining evaporable water in the materials. Differences in DTG peaks can be explained by the difference in the pore structure induced by the presence of SSA or SSC which can liberate differently water during heating. Between 200 and 600°C a weight loss change observed is due to the dehydroxylation process of gels such as calcium silicate hydrate (C-S-H) and calcium alumino-silicate hydrate (C-A-S-H). Binder consisting of these two phases are found to have better compressive strength, giving additional explanation on why GGBS-bases geopolymer presented better mechanical performances compared to MK-based geopolymer [39]. The slight decomposition peak at about 900°C can be attributed to viscous sintering process of the alkali-activated matrix and the decomposition of anhydrites like carbonates [40]. The total weight loss of pastes with SSA is 11.67% while that of the paste with SSC is 12.88%. the discordance of the result with that of compressive strength in which GGBS based pastes with SSA presented higher performances compared to that with SSC can be explained by the fact that pastes with SSC presented higher amount of C-S-H interlayer water molecules as evidenced by a higher intensity of the DTG peak between 100 and 200°C which is not contributing to mechanical performance.

4.0Conclusions

This research focussed on the effectiveness of a silica fume-based sodium silicate in the development of mechanical performances in GGBS- and MK-based geopolymer pastes compared to a standard commercial sodium silicate. Undensified silica fume (USF) was used as raw material. The sodium silicate solution investigated had SiO2/Na2O = 2. From the findings, it was concluded that,

1. The setting performance of GGBS- and MK-based geopolymer pastes did not differ significantly when silica fume-based sodium silicate (SSA) was used compared to the standard commercial version (SSC).

2. Geopolymer pastes containing SSA showed slightly higher densities and higher compressive strength compared to those using SSC. Similarities in density and compressive of hardened GGBS and MK pastes have been observed no matter when SSA or SSC were used.

3. In relation to the stress-strains properties of the geopolymer pastes investigated, GGBS-based pastes made with SSA were observed to show lower ultimate strength and lower elongation compared to those containing SSC. In contrast, MK-based geopolymer pastes showed a reversal of this trend, with pastes containing SSA showing slightly higher ultimate tensile strength and higher elongation before rupture.

4. The results of water absorption test results in relation to mix composition and curing periods showed similar trends to results from density and compressive strength tests.

5. GGBS- and MK-based geopolymer pastes showed similarities as well as differences in their thermal characteristics. Peaks below 300oC were broadly similar, with the MK-based pastes demonstrating sharper peaks suggesting higher crystallinity. Crystalline phases are bound to decompose at more precise temperature ranges, resulting in sharper peaks. The peaks beyond 300oC were different, with the GGBS-based pastes showing dehydroxyalation peaks at regions where Portlandite (a form of Ca(OH)2) would be expected take place (350-450oC). In contrast, the MK-based pastes showed peaks at temperature regions where clay-like materials would dehydroxylate – temperatures within 500-600oC. In both materials, there was no evidence of the presence of carbonate.

6. The successful utilization of undensified silica fume (USF) in producing an alternative sodium silicate that compares favourably with the more expensive commercial sodium silicate in the formulation of geopolymer pastes was demonstrated by the test results reported. Since USF is a by-product material that did not require to be tumbled, heated or ground, and the dilution process with the sodium hydroxide solution took place at ambient temperature, the results suggest potential for enhanced sustainability in the reduction of embodied energy in the production of future geopolymer binders.

5.0Acknowledgements

The authors would like to thank the University of South Wales, UK, for their internal Faculty financing through the Research Investment Strategy (RIS) project on the development of capacity for Inter-disciplinary Engineering Research into Advanced Materials.

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Tables

Table 1 - Chemical composition and specific gravity of materials used in the study

Oxide (%)

MK

USF

GGBS

CaO

SiO2

Al2O3

MgO

Fe2O3

MnO

S2−

SO3

K2O

N2O

L.O.I.

Specific gravity

0.71

58.5

36.8

0.11

0.61

/

/

/

2.0

0.31

0.11

2.50

−

97.1

0.1

0.15

0.2

/

/

0.06

/

/

/

2.20

41.99

35.35

11.59

8.04

0.35

0.45

1.18

0.23

/

/

/

2.85

Table 2 - Details of the geopolymer pastes investigated

Paste code

Alumino-silicate

Silicate type

Liquid/Solid wt. ratio

MK-SSA(1.5)

MK-SSC(1.5)

GGBS-SSA(0.7)

GGBS-SSC(0.7)

MK

MK

GGBS

GGBS

SSA

SSC

SSA

SSC

1.5

1.5

0.7

0.7

Table 3 - Ultimate tensile stresses and elongations of the geopolymer pastes

SSA-GGBS(0.7)

SSC-GGBS(0.7)

SSA-MK(1.5)

SSC-MK(1.5)

Ultimate

stress (MPa)

1.32

1.61

0.94

0.88

Elongation

(mm)

2.44

2.90

2.36

2.11

Figures

Fig. 1 - XRD patterns of MK, USF and GGBS showing the amorphous nature of all the materials.

Figure 2 - Setting times of fresh geopolymer pastes

Fig. 3 - Densities of hardened of geopolymer pastes after 28 days of curing

Fig. 4(a) - Compressive strength of trial hardened geopolymer pastes at 3, 7 and 28 days of curing

Fig. 4(b) - Compressive strength of hardened geopolymer pastes at 3, 7 and 28 days of curing

Figure 5 - Stress-elongation curves of hardened geopolymer pastes at 28 days

Fig. 6 - Water absorption of hardened geopolymer pastes at 28 days

Fig. 7 - Thermograms of GGBS-based geopolymer pastes at 28 days

Fig. 8 - Thermograms of MK-based geopolymer pastes at 28 days

2