Rice straw and husk are the major by-products of cultivation of rice, which is the second leading agricultural crop after corn. The open field burning of rice straw and husk is gradually being replaced by the recovery and utilization of these resources. For this purpose, silica recovery from rice residues is one of the main options. Different technologies are being developed for silica production from rice straw and husk. In this chapter, various techniques are categorized and reviewed within a platform to explain most existing methods. In addition, the most important characteristics of the recovered silica and their relation to the operational conditions as well as the applications of the produced silica are presented.
The electronics industry is one of the worlds fastest growing manufacturing industries. However, e-waste has become a serious pollution problem. This study reports the recovery of e-waste for preparing valuable MCM-48 and ordered mesoporous carbon for the first time. Specifically, this study adopts an alkali-extracted method to obtain sodium silicate precursors from electronic packaging resin ash. The influence of synthesis variables such as gelation pH, neutral/cationic surfactant ratio, hydrothermal treatment temperature, and calcination temperature on the mesophase of MCM-48 materials is investigated. Experimental results confirm that well-ordered cubic MCM-48 materials were synthesized in strongly acidic and strongly basic media. The resulting mesoporous silica had a high surface area of 1,317m2/g, mean pore size of about 3.0nm, and a high purity of 99.87wt%. Ordered mesoporous carbon with high surface area (1,715m2/g) and uniform pore size of CMK-1 type was successfully prepared by impregnating MCM-48 template using the resin waste. The carbon structure was sensitive to the sulfuric acid concentration and carbonization temperature. Converting e-waste into MCM-48 materials not only eliminates the disposal problem of e-waste, but also transforms industrial waste into a useful nanomaterial.
Bhagiyalakshmi M, Yun LJ, Anuradha R, Jang HT (2010a) Utilization of rice husk ash as silica source for the synthesis of mesoporous silicas and their application to CO2 adsorption through TREN/TEPA grafting. J Hazard Mater 175:928938
Chen D, Bi X, Liu M, Huang B, Sheng G, Fu J (2011) Phase partitioning, concentration variation and risk assessment of polybrominated diphenyl ethers (PBDEs) in the atmosphere of an e-waste recycling site. Chemosphere 82:12461252
Kaneda M, Tsubakiyama T, Carlsson A, Sakamoto Y, Ohsuna T, Terasaki O, Joo SH, Ryoo R (2002) Structural study of mesoporous MCM-48 and carbon networks synthesized in the spaces of MCM-48 by electron crystallography. J Phys Chem B 106:12561266
Kim H, Karkamkar A, Autrey T, Chupas P, Proffen T (2009) Determination of structure and phase transition of light element nanocomposites in mesoporous silica: case study of NH3BH3 in MCM-41. J Am Chem Soc 131:1374913755
Liu X, Du Y, Guo Z, Gunasekaran S, Ching CB, Chen Y, Leong SSJ, Yang Y (2009) Monodispersed MCM-41 large particles by modified pseudomorphic transformation: direct diamine functionalization and application in protein bioseparation. Microporous Mesoporous Mater 122:114120
Peng X, Cao D, Wang W (2008) Heterogeneity characterization of ordered mesoporous carbon adsorbent CMK-1 for methane and hydrogen storage: GCMC simulation and comparison with experiment. J Phys Chem C 112:1302413036
Samadi-Maybodi A, Teymouri M, Vahid A, Miranbeigi A (2011) In situ incorporation of nickel nanoparticles into the mesopores of MCM-41 by manipulation of solvent-solute interaction and its activity toward adsorptive desulfurization of gas oil. J Hazard Mater 192:16671674
Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, Siemieniewska T (1985) Reporting physisorption data for gas/solid systems with specific reference to the determination of surface area and porosity. Pure Appl Chem 57:603619
Zhao D, Budhi S, Rodriguez A, Koodali RT (2010a) Rapid and facile synthesis of Ti-MCM-48 mesoporous material and the photocatalytic performance for hydrogen evolution. Int J Hydrogen Energy 35:52765283
Liou, TH. Recovery of silica from electronic waste for the synthesis of cubic MCM-48 and its application in preparing ordered mesoporous carbon molecular sieves using a green approach. J Nanopart Res 14, 869 (2012). https://doi.org/10.1007/s11051-012-0869-6
This study was conducted to recover silica (desilication) as a valuable metalloid from waste foundry sand (WFS) by a leaching process and to find application for desilicated foundry sand (DFS). The leaching time applied was 5h; 3M of potassium hydroxide (KOH) was used as a leaching reagent. The agitation speed of 200rpm and the liquid/solid ratio of 25 were found to be the best conditions for optimum leaching results. A geopolymer from DFS was developed by using NaOH as an alkaline activator. The results obtained showed that the optimum conditions for the synthesis of a geopolymer were 15M NaOH, 150m DFS particle size, and a curing temperature of 80 for 72h. The geopolymer strength development was due to the formation of Phillipsite and Kalsilite as new hydration products. At the optimum alkaline solution concentration, the highest unconfined compressive strength (UCS) of 4.8MPa was achieved. The developed geopolymer met the minimum strength requirements for load bearing material. This study provides an innovative and novel solution for the beneficiation of spent foundry sand and the recovery of a valuable metalloid, resulting to zero waste generation.
The high amount of waste foundry sand (WFS) produced in South Africa and the high disposal costs have necessitated the development of a solution to deal with the waste generated. WFS has been the focus of extensive research in recent years. South Africa has been reported to have more than 200 casting facilities . The foundry industry generates approximately 350,000 tons of silica sand . Due to the land issue the country is facing, the environmental pollution as a result of the wastes, and the high penalties the foundry industry pay for the disposal of spent foundry sand, the dumping of wastes on the landfill has become a problem for most of these foundry industries. When compared to other countries, the country is facing an over prize for waste disposal, whereby approximately R 63.91 is applicable for disposal in United Kingdom and R505.43 in South Africa .
About 3,000 foundries in the United States of America utilize 100 million tons of sand, which result in generating 610 million metric tons of foundry waste that is discarded to the landfills annually [3, 4]. Between 2012 and 2013, 9.3 million metric tons of WFS was reported .
From a sustainability perspective, the reuse of WFS to better the infrastructure development is favorable in all viewpoints: (1) ecologically: natural resources are preserved and space in landfills is spared, (2) economically: WFS is less expensive since it is found from other industries and there will be no need for the industries to pay large amounts of money to dump the waste, (3) socially: it has a minor effect on humans and the environment , and (4) it provides a solution to environmental pollution, whereby the sand pollutes the air during windy weather conditions and contaminates ground water when it is raining. The foundry sand used in this study is predominated with constituents such as silica and aluminum, which present an opportunity for the recovery of silicon metalloid, changing a perspective to view the sand as a resource instead of waste.
Metal recovery/removal from numerous waste materials is widely studied and investigated. Xiao et al.  investigated the recovery of silica from vanadium-bearing steel slag using a leaching process and sodium hydroxide solution as a leaching reagent . The results obtained showed that most of the silica was successfully removed from the slag and a residue produced contained low content of SiO2 content and a high V2O5. Several researchers have shown that biomass and silica fumes  can be used to supplement cementitious materials in building and construction materials because of the silica content in most of the materials. These processes have been reported to reduce greenhouse gas emissions and energy consumption. The biomass used is from waste such as sugar cane bagasse ash [9,10,11,12] and rice husk ash (RHA) [13, 14]. Akbar et al.  developed sugarcane bagasse ash (SBA) geopolymer, which incorporated propylene fibers. The prevalent element of sugar cane bagasse ash was SiO2 with a relative proportion of 66.7%. 12M sodium hydroxide (NaOH) and 2.5M sodium silicate (Na2SiO3) were combined and used as alkaline activators. The geopolymers were cured for 7, 14, and 28days. The highest unconfined compressive strength (UCS) was about 18MPa, 22.5MPa, and 25MPa for the curing ages investigated, respectively. The study showed that well-structured geopolymer composites were provided by the reaction of amorphous silicates in SBA with alkaline activators at elevated temperature . Nguyen et al.  used rice husk ash and coal bottom with sodium silicate solutions to develop geopolymers. Low energy consumption and reduced carbon dioxide emission were reported as the advantages for the geopolymerization process. The developed geopolymer specimens were cured at room temperature for 28days and rice husk showed to have silica content of over 80 wt%. Geopolymers with optimum UCS of 32.8MPa were developed from RHA and waste glass . RHA proved to be the main source of silica with a content of 83.05 wt%.
Geopolymerization is a chemical process between alkali metal silicate solutions and solid aluminosilicate oxides at mild temperatures and alkaline conditions. The process yields SiOSi and SiOAl amorphous to semi-crystalline structures . Silica and aluminum play a significant role in geopolymerization process, with silica making the structure and bonding of the geopolymer . Under alkaline conditions of pH>11, an alkali activator interacts with metals-metalloids to generate three-dimensional polymers (SiOSi, AlOSi) [18,19,20,21,22,23].
Desilicated foundry sand (DFS) is a by-product of WFS leaching. Studies show that the silica extraction on WFS can be conducted using potassium hydroxide as a leaching reagent. DFS contains minimal amount of silica, which can be a solution to the dry shrinkage, difficulties in construction, and temperature cracks during construction .
Geopolymers are alumina-silicate materials, which are synthesized in alkali solutions. The alumina-silicate compound causes the material to have a good fire-resistant performance and influences the thermal performance of a geopolymer. A geopolymer has a three-dimensional structure, which consists of AlO4 and SiO4 tetrahedra, which share all the oxygen atoms. Geopolymers are made by the alkaline activation of material and curing at medium temperatures; this promotes the process of polymerizations to produce pastes, which settles and hardens to produce solid materials . Jamieson et al.  studied the development of Bayer geopolymers for aggregates formation . In the study, Bayer solution and fly ash were used to produce geopolymers with compressive strength more than 30MPa .
Although research is being conducted for the beneficiation of WFS, there are limited studies reported on the use of DSF for building and construction applications. This therefore presents an opportunity for research in this area as it will allow for the use recovery of metalloid and use of WFS, which will result in reducing the waste disposal to the landfills. This study presents a novel and a holistic zero waste solution for the recovery of silica as a valuable metalloid from WFS and the development of geopolymers from DFS. Resource recovery and waste beneficiation were successfully achieved in this study.
The foundry sand was collected from a local South African foundry. Desilicated foundry sand (DFS) was generated from the leaching process. Potassium hydroxide (KOH) was used as a leaching reagent and sodium hydroxide (NaOH) as the alkaline activator.
The leaching process was carried out in a thermostatic shaker by varying and studying the following parameters: (1) leaching time; (2) liquid-to-solid ratio (L/S) from 20 to 30%; (3) KOH concentration from 1 to 4M; (4) particle size distribution; (5) agitation speed from 100 to 200rpm. For all the tests conducted under different leaching conditions, samples were taken at 1h interval, filtered, and characterized. The effect of leaching time was studied by allowing leaching process under the following conditions: 5h, liquid-to-solid ratio (L/S) of 25%, 2M NaOH solution, agitation speed of 150rpm, leaching temperature of 90C and foundry sand particle size as received. Using the optimum conditions obtained in step 1, to study the effect of S/L, the following conditions were applied: 2M KOH, 5h leaching time, temperature of 90 , foundry sand as received. The L/S ratio was varied from 20 to 30%. The next study conducted was to investigate the effect of KOH concentration on silica recovery, and the following conditions were applied, which were optimum in steps 1 and 2: 5h leaching time, L/S ratio of 25, agitation speed of 150rpm, leaching temperature of 90C, and foundry sand particle size as received. The concentration of KOH solution was varied from 1M, 2M, 3M, and 4M. To study the effect of particle size on the leaching of silica, the foundry sand was sieved into different sizes 75, 150, 300m, and as received. The following optimum leaching conditions were applied: 5h leaching time, L/S ratio of 25, agitation speed of 150rpm, leaching temperature of 90C, and 3M KOH concentration. The effect of agitation speed on the recovery of silica was studied by varying the speed from 100rpm, 150rpm, and 200rpm under the optimum leaching conditions obtained in step 14.
Silica extraction was conducted by using a thermostatic shaker. The analysis of silica leached was achieved by analyzing the leachate using an Atomic Absorption Spectrometer (Thermo scientific ICE 3000 Series). The yield was obtained by incorporating the volume change as presented in Eq.1 .
Here V0 is the initial volume (mL) of the solution, Vi is the volume (mL) of the ith sample, CSiO2 is the content of silica in the Camden Power Station FA (wt%), CSiO2i is the concentration of silica in sample i (mg/L), and m is the initial mass of FA (g) added into the reflux reactor [24, 27].
The leaching conditions that yielded the optimum recovery of silica were then used to leach foundry sand to generate DFS for the development of geopolymers. It is reported that sodium is the better reagent for the preparation of geopolymers, as compared to potassium . This is due to Na+ cation being better in promoting alumino-silicates than K+ cations . Geopolymers were developed by mixing DFS with NaOH solution as the alkaline activation reagent. 300g of DFS was mixed with a 60ml NaOH solution. The optimum concentration of NaOH was investigated by varying the solution from 5M, 10M, and 15M. After the development of a workable paste using different concentrations, the different paste was cast into a 505050 mm3 molds. The cast specimens were allowed to set and then removed from the molds. The effect of curing temperature on the UCS of the geopolymers was investigated by curing the specimens at different temperatures of 40C, 80C, and 100C for 24h, 48h, and 72h. The effect of PSD of DFS on the UCS was also studied by varying the particle size between 75m, 150m, and 300m. The specimens were produced in triplicate to ensure consistency in the results. After curing, the specimens were measured for unconfined compressive strength (UCS) in compliance with ASTM C 109 , using a Universal Testing Machine (UTM) with a capacity of 300 kN on the specimens. The porosity of the developed geopolymer was determined. Porosity is defined as the ratio of volume of voids to the total volume and open porosity is the fraction of volume that is occupied by the fluid in the interconnected porous network . The porosity of the composites was determined by following ASTM C373-14 a . The dry weight was determined, and the composites soaked in water for 24h. After the soaking period, the wet geopolymer was weighed. Open porosity was then calculated using Eq.2 .
The raw foundry and DFS were characterized for elemental composition by X-ray fluorescence (XRF; model Magix Pro Phillips). 10 g of the dried sample and 3g of Sasol wax were weighed. The Sasol wax and dried material was mixed using a pastel and mortar. The mixed sample was stored in the aluminum cup and pressed to 25 tons. The pressure was released after 2min. The chemical analysis was carried out on the 13g pressed sample using wavelength dispersive. The mineralogy of DFS and geopolymer that yielded the highest UCS was studied by X-ray diffraction (XRD, model Rigaku Ultima IV). A representative sample containing 10g material was milled using a mortar and pestle. A sample holder was used to mount a sample, mounted in a manner to minimize preferred orientation for XRD analysis. Scanning Electron Microscope (SEM; model Jeol JSM 5600) was used to study morphology of the geopolymers at different curing periods. The materials were carbon coated and mounted onto SEM particle morphology determination. The analysis of variance (ANOVA) was applied to investigate the significance of difference in the investigated variables. A statistical significance confidence level limit of 95% was used for the tests . F value and critical F value were utilized to evaluate the significance in varying the variables investigated. F value can be defined as a ratio of two variances, measuring how far the data are scattered from the mean or the dispersion . It is calculated as shown in Eq.3. This is conducted from computer generated data comparing the value of variables investigated .
The chemical composition of WFS is influenced and depends on the type of binders used for casting, metal molded, and combustible used . The WFS used in this study was predominated with SiO2 and Al2O3. The sand is rich in silica with a relative proportion of 95.18 wt%. Minor constituents of Cl, CaO, MgO, and ZrO2 were also detected. In the study by , SiO2 and Al2O3 were reported to have the highest relative proportion of 87.91 wt% and 4.71 wt%, respectively. Basar and Aksoy  investigated the effect of WFS as partial replacement of sand on the ready-mixed concrete. The XRF results of the WFS also showed SiO2 and Al2O3 as predominant constituents with relative proportion of 97.38 wt% and 1.89 wt%, respectively.
To measure the pH of WFS, 50g of WFS was added into 100mL of deionized water under continuous stirring and the pH was measured after 30min. According to Johnson , the pH of WFS is dependent on the type of metal cast, the binder used, and it normally ranges between 4 and 8. In the study conducted by , on the evaluation of physical and chemical properties of South Africa waste found sand, 39 WFS from steel, iron, and aluminum were studied and the pH value ranging from 6.7 to 10.2 was reported . The pH was found to be 9.10, indicating that the sand is basic and within the range of reported WFS pH.
The leaching of silica increased by 44g SiO2/kg sand from the first hour to the second hour. From the third to fourth hour, silica yield increased from 74 and 81g SiO2/kg, respectively. At 5h leaching, silica yield was 151g SiO2/kg. A 70% difference in yield was achieved between the fourth and fifth hour of leaching. 5h was therefore taken as the leaching time with maximum silica yield. 5h leaching period was based on the study conducted by Falayi et al. , whereby 6h was the optimum leaching period for the desilication of fly ash. Xu et al.  investigated the desilication and recycling of alkali silicate solution with red mud and the time employed for leaching was 3h.
Figure2 shows that the increase of L/S ratio from 20 to 30 resulted in the higher silica yield. This is due to an increase in solid loading, which results in reduction of efficient mass transfer . The maximum yield of 337g SiO2/kg of WFS was achieved for L/S of 25. The L/S of 30 resulted in a decrease by 122g SiO2/kg from L/S ratio of 25. This is due to the decrease in the amount of solid per amount of reagent in the reaction mixture. Table 2 shows the ANOVA computation of SiO2 removal with variation in liquid-solid ratio. The L/S of 25 was chosen as the optimum leaching L/S load.
A critical value of 5.3177 was obtained, a value less than the F value of 17.8398. This shows a significant difference in the recovery of SiO2 with different L/S ratios of 25 and 30. The L/S of 25 was taken as the optimum and then utilized to conduct further tests.
Figure 3 shows silica yield with variation of KOH concentration. There was an increase in the yield of silica as the KOH concentration was increased. This is due to the increase in the OHanion available for leaching. Silica yield at 2h and 3h for 3M and 4M was almost similar. However, at 4h and 5h leaching period, a slight decrease on the yield of silica from the 3M to the 4M was observed. A high concentration of KOH has been reported to result in the codissolution of alumina, which reacts with silica and precipitate cancrinite or sodalite ; this explains the sudden decrease in the yield of silica in 4M medium. Table 3 shows the ANOVA computation of SiO2 removal with variation of KOH concentration. 3M had the maximum yield of silica and was chosen as the leaching concentration.
A critical value of 5.3177 was obtained, a value greater than the F value of 0.1033. This shows that there was no significant difference in the recovery of SiO2 when 3M and 4M KOH was used as a leaching reagent. The concentration of 3M was then utilized to conduct further tests.
There was a very slight difference in silica yield with the variation of particle size. At 5h, the yield of silica for 75m particles was 130g SiO2/kg WFS, 139g SiO2/kg FS for 150m, 148g SiO2/kg WFS for 300m, and 152g SiO2/kg WFS particle size as received. This is because the particle size of WFS entailed over 85% of particle size below 400 microns. Over 50% of WFS particle size was less than 75m. Therefore, the particle size did not have a major effect on the SiO2 yield. The WFS particle size as received was chosen as the optimum.
As the agitation speed was increased from 100 to 150rpm, there was an increase in silica yield. At 5h, the yield of silica was 69g SiO2/kg WFS for 100rpm and 96g SiO2/kg WFS for 150rpm. This is due to the increase in contact between the KOH and the foundry sand. The silica yield increased further from 150 to 200rpm by 14g SiO2/kg. The 200rpm was taken as the leaching agitation speed.
The DFS was characterized by a low silica content due to leaching of silica. This also shows that silica was successfully leached from the foundry sand, as the relative proportion of silica decreased significantly from 95 to 37 wt%. There was also a decrease in the relative proportion of alumina in DFS from 2.0924 to 1.8826 wt%. The pH of DFS was found to be 11.15 and higher than that of WFS, as DFS has more basic oxides than WFS; thus, the significant increase in the relative proportion of K2O, which increased from 0.4 to 57.41 wt%, contributed to the higher alkalinity.
From Fig.6, it is evident that leaching WFS with KOH completely transformed the color of the sand. The sand changed from a dark brown color to cream like color, as silica was removed from the sand. The extracted silica can be used as a precursor for production of zeolites and xerogels .
This section presents the development of the geopolymers from DFS. The geopolymers were developed by studying the effect of NaOH concentration, particle size, temperature, and curing regime. The geopolymers were tested for the unconfined compressive strength (UCS).
Figure7 shows the UCS results obtained for the geopolymerization of DFS with variation in NaOH concentration. The specimens were cured at 80C for 72h. In the comparison of the best alkaline activator for the synthesis of geopolymers, it was reported that geopolymers developed with NaOH attained strength higher than that of KOH solution . NaOH was therefore selected as an activator due to the Na+ cations, which are more active than K+ and tend to favor better dissolution process of raw material [41, 42]
As NaOH concentration was increased from 5 to 15M, there was an increase in UCS. Higher concentrations of alkali enhance the geopolymerization, whereas the type of alkali also affects the process. The use of sodium hydroxide as an activator regulates hydration. The Na+ ions have a greater charge, which makes the sodium hydroxide to dissolve Si and Al fast. The use of NaOH produces an extra silicate in the system and allows the geopolymerization process to accelerate, which leads to the development of UCS . Geopolymers are said to gain strength when alkaline solutions and aluminum silica-rich source materials react . The polymer chain of inorganic aluminum silicate polymer is structured around tetrahedral coordinated Si4+ and Al3+ . The XRD results presented in Fig.11 also show high peak associated to SiO2, showing that silica might have not reacted in the process. It is also possible that the silica that was not recovered and remained in DFS was in crystal phases, which is inert, unreacted, and did not take part in the fabrication of a geopolymer, which contributed to the lower strength attained [45, 46]. In the strength development in either NaOH/KOH-based geopolymer, Na2O is the major alkali oxide required . However, in this study, Na2O constituent was 1.32 wt% for DFS, which also explains the lower strengths attained.
In the study to develop a geopolymer from waste foundry sand for building material, Doan-Salamtimur  reported 2.5MPa as the minimum compressive strength achieved, which was acceptable for building wall materials. It can therefore be concluded that the geopolymers developed in this study with the UCS between 3.9 MPa to 4.8 MPa can be used for different applications. Taking into consideration that the geopolymer did not entail the required constituents to contribute to the strength development, which is mainly the aluminate silica constituent, the developed DFS geopolymers can be categorized as innovative sustainable building and construction material.
The activator that yielded the highest UCS was then used to study the effect of temperature and regime. Figure8 shows the variation in UCS with curing regimes. There was an increase in UCS with increasing time up to 48h within the same curing region. Thereafter, there was a slight increase in UCS at 72h. An increase in UCS with temperature increase from 40 to 80 was also observed. The increase in UCS is attributed to the high rate of the geopolymerization process, which increased the rate of hardening of the DFS paste. An increase in curing temperature from 80 to 100 resulted in a decrease in UCS. The condensation-polymerization process has been reported to be complete at 80 , hence the decrease in compressive strength beyond 80 . The condensation of geopolymer is endothermic and heat plays an important role in the reaction and curing of the DFS paste. There was a rapid loss of moisture at 100 . This led to the reduction in time for equilibrium, reorganization, and hardening, which would result in a stronger geopolymer . Although the temperature of 80C yielded the highest UCS, the strength is low as compared to the strength of geopolymers reported in the literature. This is attributed to the low SiO2 and Al2O3 ratio, which was recovered and plays a significant role in the development of a geopolymer.
The 150m was found to be the optimum particle size, which yielded the highest UCS from the geopolymers developed. The particle size reacted faster and resulted in higher unconfined compressive strength. The 75m resulted in an increase in particle number, which would impede mass transfer . The particles larger than 150m had less surface area to allow for effective dissolution of aluminosilicate species. Kim and Lee  investigated the effect of PSD on the compressive strength for geopolymers developed from different fly ash materials, namely, fine ground bottom ash (50.5m), medium-sized bottom ash (102.4m), and coarse ground bottom ash (196.7m) . Fine ground bottom ash, the material with the smaller particle size, yielded the highest strength; this was because of PSD. The authors found that with larger particles size, the reaction between alumina and silica required for dissolution is reduced . However, in this study, the smallest DFS particle size investigated resulted in the lowest strength of 2.9MPa.
Figure10 shows the microstructure of WFS and DFS geopolymers cured at 80C for 72h. WFS is made up of angular, sub-angular, and round particle shapes . The mechanism of geopolymerization is seen from time-based SEM analysis. The geopolymer shows a heterogeneous morphology on the surface. The geopolymer is more compact as compared to WFS particles, indicating the particles bonded together. The surface of geopolymer is porous; this is due to the addition of sodium hydroxide, where the matrix became loose, with low space filling properties through the DSF activation formation of a gel ; this was supported by the open porosity of the geopolymer after 24h in water, which is 0.17, as shown in Table 5. After 24h of water soaking, the UCS of the geopolymer reduced by 11%. The geopolymer showed only 2% water absorption.
Figure11 shows the XRD diffractograms for DFS and the Geopolymer cured at 80 for 72h. The XRD shows the patterns of the geopolymer and DFS. The XRD reveals that there are crystalline phases present in the geopolymer, as the sharp intensity peaks are seen. The quartz are the remnant peaks in the geopolymer due to the unreacted DFS particles. In the study to synthesize sodium silicate from WFS, the micrograph of the WFS showed a rough surface after acid leaching Aleem et al. . Boussaa et al.  reported a thinner surface for solid WFS sample after leaching. This explains the lower UCS obtained for DFS geopolymers as compared to the high UCS of geopolymers developed from WFS [6, 44, 53, 54].
The XRD analysis of WFS was predominated with quartz (SiO2); this agrees with the elemental analysis results, which showed a relative proportion of 95.8 wt% for SiO2. The mineralogy results reported by Iloh et al.  also showed high peaks associated with silica in the WFS studied. Geopolymerization resulted in an increase in the intensity of Zeolite phillipsite K. Geopolymerization also resulted in the formation of kalsilite, which is seen in the XRD diffractogram at around 37 and resulted in the formation of mullite, which is seen in the XRD diffractogram at around 21. Kalsilite and the Phillipsite are responsible for the strength development as their presence on zeolitic material increases the strength of the geopolymer. The peak for quartz was high in the developed geopolymer, indicating that the geopolymerization process was not complete and not all the silica in DFS reacted . This, therefore, supports the lower strength that was obtained.
The geopolymer developed at optimum conditions, with the highest UCS presented in Fig.12. The product was intact and the highest strength of 3.52MPa is applicable for load bearing applications. In terms of UCS and durability, the developed geopolymer has equivalent strength values as C1C4 materials, which is applicable for subbase . The product also meets the requirements of South African Roads specification. According to SANS 1215, it is also applicable for masonry brick production .
The US Environmental Protection Authority (US EPA, 2015) conducted a risk assessment study to examine the environmental effects of WFS to the environment . The results obtained showed WFS may produce some environmental benefits and with application that does not harm human health. Studies by Dungan et al.  showed that under the conditions they have investigated, WFS has low metal leaching potential and when the sand was subjected to Toxicity characteristic leaching procedure (TCLP), it passed the toxicity characteristics. In the evaluation of silica-based WFS generated from iron, aluminum, and steel foundries, it was reported that the sand does not pose any hazardous concerns when used in manufactured soils . Deng  evaluated contaminants in waste foundry sand and its leachate. He tested 594 WFS samples from 123 foundries in US. The results obtained showed that WFS is non-hazardous, except for the sand generated from copper-based foundries . With this information, it is therefore concluded that WFS used in this study has minimal potential to affect the environment negatively.
This study investigated the desilication of WFS and geopolymerization of DFS. The results obtained show that KOH can successfully recover silica from WFS and NaOH is an effective activator for the development of geopolymers from desilicated foundry sand. The best leaching parameters were 3M KOH, L/S ratio of 25, particle size as received, agitation speed of 200rpm, and leaching time of 5h. Foundry sand can be used as a source of silica. The desilicated foundry sand can be used as a precursor for geopolymer reactions. The particle size, concentration, and quantity of NaOH and curing regime have a direct effect on the geopolymer brick strength. NaOH can be used to alkali activate DFS for the development of load bearing geopolymers. SiO2 and Al2O3 ratio plays a significant role in the development of a geopolymer. To improve the strength of the geopolymers developed in this study, alkali sodium silicate can be used, to make up for the silica recovered from the WFS. Curing temperature played a significant role in the strength of DFS geopolymer. The XRD analysis showed a possible unreacted SiO2, and it has been proven that longer curing periods results in strength development. Longer curing periods up to 28days, at ambient temperature, should be investigated and is highly recommended for DFS-based geopolymer. The study showed that it was possible to use WFS as a source of silica and alkali activate the residual solids from the leaching process to produce geopolymers. The study was a success as the objectives were successfully achieved. The study provides an opportunity for the beneficiation of WFS and recovery of silica as a valuable metalloid. The recovered silica metalloid can be recovered in pure metal form by using reductive crystallization and be utilized in different applications. The geopolymers attained the UCS of. 4.8MPa and thus show a great potential for the geopolymers developed to be used for load bearing application in building or construction.
Lyra GP, dos Santos V, De Santis BC, Rivaben RR, Fischer C, Pallone EMDJA, Rossignolo JA (2019) Reuse of sugarcane bagasse ash to produce a lightweight aggregate using microwave oven sintering. Constr Build Mater 222:222228
Ambedkar B, Alex J, Dhanalakshmi J (2017) Enhancement of mechanical properties and durability of the cement concrete by RHA as cement replacement: experiments and modeling. Constr Build Mater 148:167175
Nguyen HT, Pham TK, Promentilla MA (2017) Development of geopolymer-based materials from coal bottom ash and rice husk ash with sodium silicate solutions. In: Tran-Nguyen HH, Wong H, Ragueneau F, Ha-Minh C (eds) Congrs International de GotechniqueOuvragesStructures. Springer, Singapore, pp 402410
Tchakout HK, Rscher CH, Kong S, Kamseu E, Leonelli C (2016) Geopolymer binders from metakaolin using sodium waterglass from waste glass and rice husk ash as alternative activators: a comparative study. Constr Build Mater 114:276289
Koohestani B, Darban AK, Mokhtari P, Darezereshki E, Yilmaz E (2020) Geopolymerization of soil by sodium silicate as an approach to control wind erosion. Int J Environ Sci Technol. https://doi.org/10.1007/s13762-020-02943-2
ASTM C373-14 (2014) Standard test method for water absorption, bulk density, apparent porosity, and apparent specific gravity of fired whiteware products, ceramic tiles, and glass tiles. ASTM International, West Conshohocken, PA
Alonso-Santurde R, Andrs A, Viguri JR, Raimondo M, Guarini G, Zanelli C, Dondi M (2011) Technological behaviour and recycling potential of spent foundry sands in clay bricks. J Environ Manage 92(3):9941002
Abdul Rahim RH, Rahmiati T, Azizli KA, Man Z, Nuruddin MF, Ismail L (2015) Comparison of using NaOH and KOH activated fly ash-based geopolymer on the mechanical properties. In: Liyana J, Muhammad FMT (eds) Materials science forum, vol 803. Trans Tech Publications Ltd., Stafa-Zurich, pp 179184
Huseien GF, Mirza J, Ismail M, Hussin MW (2016) Influence of different curing temperatures and alkali activators on properties of GBFS geopolymer mortars containing fly ash and palm-oil fuel ash. Constr Build Mater 125:12291240
Davidovits J (1994) Properties of geopolymer cements. In: First international conference on alkaline cements and concretes, vol 1. Kiev State Technical University, Scientific Research Institute on Binders and Materials, Kiev, pp 131149
Davidovits J (2002) Years of successes and failures in geopolymer applications. Market trends and potential breakthroughs. In: Geopolymer 2002 Conference, vol 28. Geopolymer Institute, SaintQuentin, p 29
Kim H, Ji-Young L (2017) Effect of ash particle sizes on the compressive strength and thermal conductivity of geopolymer synthesized with alkali activated low-calcium ground coal bottom ash. In: World of Coal Ash, Conferences Proceedings
Aleem MA, Kavikumaran K, Prabunathan P, Hariharan A, Alagar M (2020) Synthesis and characterisation of sodium silicate from spent foundry sand: Effective route for waste utilisation. J Clean Prod 264:121689
Venkatesan M, Zaib Q, Shah IH, Park HS (2019) Optimum utilization of waste foundry sand and fly ash for geopolymer concrete synthesis using D-optimal mixture design of experiments. Resour Conserv Recycl 148:114123
Timakul P, Thanaphatwetphisit K, Aungkavattana P (2015) Effect of silica to alumina ratio on the compressive strength of class C fly ash-based geopolymers. In: Peerapong P (ed) Key engineering materials, vol 659. Trans Tech Publications Ltd., Stafa-Zurich, pp 8084
U.S. Environmental Protection Agency (2014) Risk assessment of spent foundry sands in soil-related applications -evaluating silica-based spent foundry sand from iron, steel and aluminium foundries; U.S. EPA Office of Resource Conservation and Recovery Economics and Risk Assessment Staff; U.S. Department of Agriculture-Agriculture Research Service; The Ohio State University and RTI International: Washington, DC, USA
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Mashifana, T., Sithole, T. Recovery of Silicon Dioxide from Waste Foundry Sand and Alkaline Activation of Desilicated Foundry Sand. J. Sustain. Metall. 6, 700714 (2020). https://doi.org/10.1007/s40831-020-00303-5
Feasibility of producing high purity silicon from amorphous silica fume, using a low temperature magnesiothermic reduction was demonstrated. Commercial silica fume containing 97.5% amorphous silica was first purified by acid leaching and roasting to remove large quantities of transition metals and carbon. The product was then reduced using magnesium as the reductant. The effect of the amount of reductant, initial temperature, and dwell time were investigated on the quantity and type of the reaction products. The optimum reduction conditions were decided based on the maximum yield of the Si metal. These corresponded to Mg/SiO2 molar ratio of 2.0, preheating temperature of 750 C, and holding time of 2h. High purity silicon (>99wt.%) containing <3 ppmw B and 12 ppmw P was obtained after leaching purification of the reduction products, showing that the material is superior to metallurgical grade silicon, for use as solar-grade silicon feedstock.