Unbakedmaterials frommixtures of waste sludge of a water purification plant , fly ash , and water glass

Use your smartphone to scan this QR code and download this article ABSTRACT In water purification plants, a large area of urban land is using to store waste sludge (WS). Thewaste sludge from water filtration plants is aluminosilicate, which can form a geopolymer. However, the waste sludge has low alkaline activity, so it must be used in combination with fly ash (FA) to create geopolymer products. Fly ash is a solid waste containing amorphous silica and it has high alkaline activity, so that it is suitable for treatment by the geopolymer method. The geopolymerization of waste sludge from water purification plants is a relatively new method. The geopolymer is a binder formed by the chemical reaction between aluminosilicate materials and alkaline activated solutions. The alkaline activated solution used in these experiments was water glass (WG). The water glass is the solution of sodium silicate (Na2O.nSiO2) dissolved in water. The research results of geopolymer materials from the mixture of fly ash, the waste sludge of Thu Duc water purification plant in Ho Chi Minh City (Vietnam), and water glass (WG) were introduced in this study. The activated Al2O3 and SiO2 oxides in the fly ash and the waste sludge can be dissolved in the water glass and polymerized into a geopolymer material. The test samples had pressed at a high pressure of 225 MPa to form cylindrical ones weighing approximately 3 grams, height about 18 mm, and 10 mm in diameter. These samples were then cured at 110 ◦C for 24 hours and at room temperature (30 ± 5 ◦C). The methods of Fourier infrared spectroscopy (FTIR) and scanning electron microscope (SEM) had used to detect the microscopic structure and geopolymer bond formation of the samples. The compressive strength of the tested samples at 28 days old was higher than 3.5 MPa, the pH was less than 12.5, meeting the Vietnamese National Standards for unbaked materials (TCVN 6477:2016) and National Technical Regulation on environmental impact (QCVN 50:2013 / BTNMT), respectively. The results show a new approach of solidifying the waste sludge for further applications such as the manufacture of geopolymer concretes or landfill materials.


INTRODUCTION
Waste sludge (WS) from water purification plants is composed mainly of aluminosilicate. The WS is usually treated by methods as landfills, making ceramic bricks 1-3 … However, due to the areas of landfill sites, WS management is a growing global problem 2 . It is necessary to study the treatment of WS by new methods. In recent years, the treatment of the WS by polymerization is also being interested in many studies [4][5][6] . Geopolymer is a binder formed by the chemical reaction between aluminosilicate materials and alkaline activated solutions. The alkaline activated solutions are composed of strongly alkaline solutions such as sodium hydroxide (NaOH), potassium hydroxide (KOH), soda ash (Na 2 CO 3 ), calcium hydroxide (Ca(OH) 2 ), and water glass (WG: Na 2 O.nSiO 2 .mH 2 O) 2,3 . Fly ash (FA), silica fume, kiln slag, and metakaolin are the industrial wastes used as raw materials to produce the geopolymers [4][5][6][7] . Unbaked materials made from WS of the Thu Duc water purification plant, Vinh Tan fly ash, and the WG are introduced in this research. Sodium silicate is a generic name for chemical compounds with the formula Na 2 O:nSiO 2 (molar ratio n~1 ÷ 3.75 named module of sodium silicate) and soluble in water with various amounts. The solution of the sodium silicate in water is called water glass or liquid glass. Activated Al 2 O 3 and SiO 2 oxides in aluminosilicate can be dissolved in an alkaline solution to form a similar bonding circuit of a WG. The bonding circuit is M{-(SiO 2 ) z -AlO 2 } n .wH 2 O, wherein M is a cation such as K + , Na + , Ca 2+ , and "n" is a degree of polycondensation, z is 1,2,3 [8]. When dehydrated, the WG condenses to form a gel-like polymer circuit with the features of poly (sialate) M n -(-Si -O -Al -O -) n and poly (sialate -siloxo)M n -(-Si -O -Al -O -Si -O -) n 7-9 . For increasing the rate of condensation, the unbaked materials can be applied to the treatments in the atmosphere by drying at 65 -110 o C 8,10 or using microwave energy 11 . When using WS to fabricate unburned materials by the geopolymer method, due to the weak alkaline activity of the WS, the substances with better alkaline activity such as FA, silica fume often need to be mixed 1 .
The use of strongly alkaline substances influences the surroundings, first of all, is pH. Using WG binders can reduce the impact of environmental pH less than using NaOH, KOH. Furthermore, WS is relatively inexpensive, easy to use, and non-toxic. This research presented some results on the physicomechanical properties and environmental pH effects of unburned materials made from WS, FA, and WG binder. The bonding and microstructure of the materials had to be studied by Fourier infrared spectroscopy (FTIR) and scanning electron microscope (SEM).

Raw materials
The raw materials are WS from Thu Duc water purification plant, FA from Vinh Tan thermal power plant, alkaline activator solution is water glass WG. The chemical composition of the raw materials was determined using the X-ray Fluorescence method. The alkaline activity of WS depends on the solubility of the oxides in alkaline solution with different concentrations.

Forming
The weight of each test sample was 3 g and pressed in a steel mold to form a cylindrical specimen with height h = (18 ± 1) mm, diameter d = (10 ± 0.2) mm ( Figure 1). The samples had pressed at 225 MPa pressure, and the moisture content of the mixture was about 7-8%. This pressure value was the parameter for the cohesion and high density of shaped samples. As the pressure increases further, the sample density hardly increases anymore 11 . After forming, the samples had cured under two different conditions: 1. At room temperature (average about (30 ± 5) • C) for 24 hours, and 2. In a Venticell laboratory dryer (MMM Medcenter Einrichtungen GmbH) at 110 • C for 24 hours. Then, both sample groups were cured at room temperature (30 ± 5) • C at the same time until the determining properties. The properties identified were volumetric density and compressive strength.

Determination of the properties
The physico-mechanical properties of the specimens were determined after 28 days of curing. This is the time that the specimen weight has been not changed, the properties were considered stable 11 . Volumetric density was determined by equation d = m/V (therein m is the weight of the sample and V is its volume, calculated by V = πd 2 h/4. Compressive strength was determined by the DTU-900 MNH equipment (loading speed 3 kN / min). Chemical composition was analyzed using the X-ray Fluorescence method by the XRF-Thermo ARL ADVANT'X spectrometer. The pH was determined daily during curing time of 28 days according to ASTM D3987-12 (2020) 14 with the Hanna Instruments HI221 pH meter. The sample group with the best mechanical strength (10% WG) was selected for FTIR analysis (Bruker Tensor 27 spectrometer) and the microstructure was analyzed by scanning electron microscopy (Hitachi S-4800 FE-SEM) for determination of bonds in the sample.  Figure 2.     Figure 3 showed the compressive strength of the samples under different curing conditions after 28 days of age.

Physicomechanical properties of unbaked materials
The compressive strength of the test samples ( Figure 3 ) under both curing conditions showed that the compressive strength of the 10% FA and 40% FA samples were higher than 3.5 MPa. The compressive strength of the 40% FA sample was higher than the 10% FA sample when the WG content was 6-8%. However, with the 10-14 % WGs, the compressive strength of the 40% FA sample was lower than that of the 10% FA sample. The compressive strength of the 10% FA and 10% WG samples was the highest. Therefore, the compressive strength, in this case, depends on the ability to form polymer bonding circuits in the tested samples.  The volumetric density of the samples cured at room temperature was higher than that dried at 110 • C. The compressive strength of the samples cured at room temperature with a 10 -20% FA content was also greater than that of the samples cured at 110 o C (Figure 4). This result pointed out that the rate of water evaporation too fast did not increase the compressive strength.

The change of pH according to curing time
The pH changes of the samples according to curing time are plotted in Figure 5. The plots of the pH change over curing time in Figure 5 indicated that the pH increased with an increase in the FA content. That can be explained by the higher pH of FA (11.5) than that of WS (7.5). Besides, the results also showed that the pH of the samples decreased over the curing time. The 10% FA sample at 28 days of age had the lowest pH, such as the pH value of the (10FA+90WS)+10WG samples treated 110 o C only is 9.1 ( Figure 5). The pH value decreased over curing time because of the formation of a product layer on the surface, which prevents diffusion of Na + ions and the reaction of alkaline NaOH solution with CO 2 in the air 15,16 : Figure 6 illustrate the FTIR spectra of raw material and 10% WG sample cured at 110 • C for 28 days. Several characteristic bonds are indicated on these FTIR spectra.

Analysis of bond formation by FTIR and microstructure by SEM
On the FTIR spectrum of FA, the band in the 1033-538 cm −1 wavenumber corresponds to the oscillating region of the T-O-T bond (T is the Si and Al tetrahedra) of the aluminosilicate minerals 15,17,18 . The band of 3441-3696 cm −1 corresponds to the oscillation of the H -O -H and -OH bond 18 . The band in the 469.2 cm −1 is characteristic of quartz 12,19 , and the band in the 2360 cm −1 corresponds to the oscillation of CO 2 15 . The existence of CO 2 groups on FTIR spectra is evidence of the possibility of reaction alkaline NaOH solution and CO 2 in the air. On the spectrum of WG, the band in the 3440-2700 cm −1 wavenumber region characterizes the O-H oscillation of free water. On the FTIR spectra of the alkaline-activated samples, this band has narrowed. It is indicated that the dehydration was occurred in the geopolymer bond-forming process. The band in the 1569 cm −1 characterizes the OH − group in structure, and 1080 cm −1 characterizes the groups -Si -O -Si -with bridging oxygens of the geopolymer materials 16,19 .
Comparing the characteristic bands on the FTIR spectra of WS and 10% WG activated samples cured in a dryer at 110 • C indicated that they are almost identical. That means the weak activity of WS in the geopolymerization reaction. In other words, the WS acts as a filler. Meanwhile, the characteristic peaks on the FTIR spectra of the WG and FA changed very clearly: the range from 3440 to 2700 cm −1 on the WG spectrum no longer appears on the spectrum of all 10% activated samples. That are (10FA + 90WS) + 10WG, (40FA + 60WS)) + 10WG, and (70FA + 30WS) + 10WG. Thus, the WG has lost water, condensed, and cured to form bonds in the material 20 . The characteristic peaks of the Si -O -Si bond at 1080 cm −1 on the WG spectrum shift to 950 -980 cm −1 in the FTIR spectrum of 10% WG activated samples. The microstructure of the samples investigated using the SEM. Figure 7 shows SEM images of WGactivated samples and WS-free samples under different curing conditions. Comparing the SEM images of the WG-activated and WG-free samples in Figure 7 exhibits the roundshaped FA particles in the WG-activated sample completely deformed, almost participated in the geopolymerization reaction. The non-sphere fuzzy regions show the gel structure of the geopolymers that consisted of a WG-activated sample.