Characterizing a Local Sewage Sludge Ash for Developing Alkali-Activated Geopolymer Mortar

Abstract

Production of ordinary Portland cement (OPC) has significant adverse effects on the environment and accounts for 5%–7% of CO₂ emissions globally. Geopolymer cement is potentially a sustainable alternative to OPC and is created by the activation of aluminosilicate materials with an alkaline solution that induces geopolymerization, causing gelation of the materials. Sewage sludge ash (SSA) contains relatively high contents of aluminosilicate materials and can be a precursor for geopolymers. This study aims to characterize a local SSA as a precursor to develop geopolymer mortar. Chemical and leaching analysis found no significant adverse environmental implications of the SSA. Strength activity index tests showed that the SSA has a moderate pozzolanic activity. The SSA was then utilized to develop a geopolymer mortar in conjunction with an alkaline solution consisting of sodium hydroxide and sodium silicate. Factors that influence the geopolymerization and strength gain of geopolymers were examined, including the molar ratio between silicon dioxide and sodium oxide and the activator/binder ratio. An optimal activator/binder ratio of 0.22 and a SiO₂/Na₂O molar ratio of 0.80 were determined to yield the highest compressive strength of the geopolymer mortar.

Keywords

geopolymers alkali-activated materials sewage sludge ash industrial by-products

Concrete is the most widely used construction material. However, the production of ordinary Portland cement (OPC) has significant adverse effects on the global environment and accounts for 5% to 7% of carbon emissions globally ( 1 ). Geopolymers have attracted much interest as a promising greener alternative to OPC. Geopolymer is an inorganic polymer and synthesized in alkaline medium conditions from various aluminosilicate materials containing mostly silica (SiO₂) and alumina (Al₂O₃). Most of the alumina silicate materials required can be industrial byproduct or waste, such as fly ash from the burning of coal in power plants, mine tailing from ore in the mining process, blast-furnace slag, or rice husk ash ( 2 – 4 ). The alkaline solutions used for the activation of the byproducts or wastes are typically soluble sodium or potassium silicates or sodium hydroxide. The combination of the alumina silicate precursor and the activator induces geopolymerization, which forms calcium aluminate silicate hydrate gels (C-A-S-H) ( 5 , 6 ). The gelation process allows the geopolymer to bind and harden to create the end product. Also, importantly, synthesis of geopolymers through the geopolymerization process can occur at room temperature or slightly elevated temperature with little carbon emission ( 7 ).

Sewage sludge ash (SSA), which results from the incineration of sludge in wastewater treatment facilities, is a good source of aluminosilicate content. Sewage sludge is the byproduct that remains from wastewater treatment facilities. Sewage systems carry household wastewater and stormwater to the wastewater treatment facilities, where the solid and liquid wastes are separated. The solid waste, which is processed and treated, is what is referred to as sewage sludge. The sewage sludge at this stage typically contains high water content and is difficult to dewater, owing to its complex colloidal nature and strong hydrophilicity. The sewage sludge is thus conditioned, often with such chemicals as ferric chloride or ferrous sulfate, to release trapped water and improve its dewaterability. The sludge may subsequently be pressed mechanically to reduce its water content, typically to less than 50%. The dewatered sludge is incinerated into SSA, reducing the volume of sewage sludge by 70%. About 70 million tons of SSA are produced every year in the U.S. and the majority of SSA ends up in landfills ( 8 ). SSA has been previously studied for beneficial use in such applications as concrete, brick, and tile, and applications in soft soil stabilization ( 9 – 13 ). Past studies have suggested that SSA demonstrates pozzolanic activity to various degrees ( 14 – 16 ). This characteristic enables SSA to be potentially used as a partial replacement of OPC. As in other cementitious materials, the three primary oxides are found in SSA, with average SiO₂, CaO, and Al₂O₃ contents of 32.8%, 14.3%, and 14.2%, respectively ( 15 ). It is noted that the CaO content in SSA is much lower than the typical content of 64% in Portland cement, while the Al₂O₃ content in SSA is significantly higher than the typical content of 5% in OPC ( 17 ).

Economic savings is another incentive to investigate the potential beneficial reuse of SSA. Although significantly less than municipal solid waste production, the quantity of SSA is still significant at a local level. Coupled with environmental implications, SSA disposal can be a significant portion of a wastewater treatment facility’s operation costs. For example, the Delaware County Regional Waste Quality Control Authority (DELCORA), in southeast Pennsylvania, reported a disposal cost of $396,699 in 2017 for the disposal of 4,454 tons of SSA ( 18 ). Finding an alternative to landfill disposal of SSA may offer a more economical and sustainable option for wastewater treatment plants and potentially mitigate varying degrees of environmental impact.

A fairly limited number of studies using SSA as the precursor for geopolymer concrete have been conducted, mostly to examine such factors as activator/binder ratio and the SiO₂/Na₂O molar ratio of the activator, which influence the strength of a geopolymer mixture. A previous study in geopolymer mix designs using SSA and ground granulated blast-furnace slag (GGBS) as precursors found 4.0% Na₂O content in the activator to be the optimum point, after which the compressive strength of the concrete dropped. X-ray diffraction (XRD) analyses showed that the quartz and hematite in the SSA were transformed to amorphous forms after the geopolymerization process, showing the involvement of SSA in geopolymerization ( 19 ). Another study examined the effect of SSA on the properties of metakaolin geopolymers, in which metakaolin and SSA were used as precursors, and sodium hydroxide and sodium silicate solutions were used for the activator solution ( 20 ). Samples with 1.6 SiO₂/Na₂O molar ratio activator had a higher compressive strength than 0.8 molar ratio samples. Most of the samples with 10% SSA had lower compressive strength than the 100% metakaolin samples, except for the 1.6 molar ratio samples.

One of the obstacles in the use of SSA in geopolymers is that the type of SSA can differ greatly, based on the waste facility that produces it, the production method, and the incineration process that it goes through. The aim of this study is to investigate the potential use of a local SSA from DELCORA as a precursor to develop geopolymers. The SSA was first tested and characterized to determine its physical and mineralogical features. Chemical and leaching analyses were also carried out to determine the SSA’s environmental implications. The SSA was then utilized as the precursor to develop geopolymer mortar. Since geopolymerization may yield products with different physical and mechanical properties, depending on the synthesis conditions, this study examined various influencing factors, such as activator/binder ratio and silica modulus in the activators to determine an optimal set of factors that yield a higher strength.

Materials and Methods

Materials

The SSA used in this study was collected from DELCORA’s incineration facility in Chester, Pennsylvania. The physical characterization of SSA is variable, owing to the irregularity of wastewater and different treatment methods. Depending on the nature of the sewage sludge processing, SSA’s color may generally vary from yellow to red to brown to gray. As seen in Figure 1, DELCORA’s SSA was observed to exhibit a slightly reddish tint, indicating a high content of iron oxide (Fe₂O₃). This is probably because of the use of ferric chloride or ferrous sulfate for improving the thickening and dewatering properties of the raw sludge before incineration. It should be noted that the iron oxide might not affect the geopolymer’s strength development as it does not typically participate in the reaction of geopolymerization ( 21 , 22 ).

Figure 1.

Delaware County Regional Waste Quality Control Authority’s sewage sludge ash.

Tests were conducted to examine physical characteristics of the SSA, such as specific gravity and grain size distribution. Surface topography of the SSA particles was also examined using a scanning electron microscope (SEM) (Quanta600 eSEM) located at the University of Pennsylvania’s Singh Center for Nanotechnology. In addition, XRD tests were performed to investigate the mineralogy of the SSA. To quantify the SSA’s pozzolanic activity, strength activity index (SAI) tests were conducted in accordance with ASTM C168 ( 23 ). The SAI was determined by dividing the average compressive strength of the SSA specimens (f_mSSA) by the average compressive strength of the control cubes (f_mc): SAI (%) = f_mSSA/f_mc × 100.

The SSA was chemically activated using alkaline compound to generate geopolymers. The activators consisted of two solutions: reagent-grade sodium silicate (Na₂SiO₃), also known as water glass, and sodium hydroxide (NaOH). The sodium silicate solution was acquired commercially (Sigma-Aldrich, St. Louis, MO). The sodium silicate solution has 10.6% of Na₂O and 26.5% of SiO₂ and a density of 1.39 g/mL at 25°C. Sodium hydroxide solution was prepared in the laboratory with solid powder sodium hydroxide and distilled water.

Tests of SSA’s Chemical and Leaching Characteristics

In compliance with the Resource Conservation and Recovery Act (RCRA), the SSA needs to be analyzed to determine whether it is characteristically hazardous and whether it may be accepted in a municipal landfill facility. DELCORA carries out quarterly environmental laboratory tests on its SSA. Wet chemistry tests are performed to determine the volatile matter, moisture content, and pH of the SSA. Using inductively coupled plasma atomic emission spectrometry (ICP-AES), the EPA method 6010C of SW-846 ( 24 ), metal elements are identified in DELCORA’s SSA. The toxicity characteristic leaching procedure (TCLP) has been performed, in which a sample extraction method for chemical analysis is employed as an analytical method to simulate leaching through a landfill. The leachate is analyzed using ICP-AES also according to method 6010C in SW846 ( 24 ).

Preparation and Testing of SSA Geopolymer Mortar

Cubic geopolymer mortar specimens (50 mm × 50 mm × 50 mm) were created by following the mixture design listed in Table 1. Mortar mixtures contained one-part binder (SSA) to 2.75 parts of sand by mass. The sand was fine aggregate meeting the specifications of ASTM C33/C33M-18 ( 25 ). A total of seven different mix designs were created to systematically examine influencing factors, such as SiO₂/Na₂O ratio, water/binder ratio, and activator/binder ratio. It is noted that the mixes are designated by the aforementioned ratios and their respective values enclosed in parentheses: AB (activator/binder ratio), SN (SiO₂/Na₂O molar ratio), and WB (water/binder ratio), as shown in Table 1.

Table 1.

Geopolymer Mortar Mixes

Mix designations	Amount of activator		Activator/binder ratio	SiO₂:Na₂O molar ratio	Water/binder ratio
Mix designations	Na₂SiO₃ (g)	NaOH (g)	Activator/binder ratio	SiO₂:Na₂O molar ratio	Water/binder ratio
0.21AB-0.6SN-0.8WB	228.7	358.7	0.21	0.60	0.80
0.20AB-0.8SN-0.8WB	287.0	308.0	0.20	0.80	0.80
0.23AB-0.6SN-0.9WB	257.0	403.1	0.23	0.60	0.90
0.22AB-0.8SN-0.9WB	322.5	346.1	0.22	0.80	0.90
0.22AB-0.8SN-0.8WB	318.0	295.7	0.22	0.80	0.80
0.20AB-1.0SN-0.8WB	370.0	246.7	0.20	1.00	0.80
0.26AB-0.6SN-0.9WB	288.3	397.7	0.26	0.60	0.90

The procedure for the preparation and testing of the cubic specimens was based on ASTM C-109 ( 26 ). The specimens were cast and compacted in two layers in 50-mm cubic steel molds and cured at room temperature for 24 h (Figure 2a). The specimens were subsequently removed from the molds and cured at ambient room temperature in the air (Figure 2b). The compressive strength of all test specimens was determined at Day 3, Day 7, and Day 28 after casting (Figure 2c). Three replicate cubes of each mortar mixture were used during each compressive strength testing period. A load was applied at a rate of 517 kPa/s to each specimen and the peak load recorded was treated as the load at failure. It should be noted that shrinkage tests were not carried out at this point, although it is known that there is a tradeoff between controlling shrinkage and gaining strength.

Figure 2.

Specimen preparation and testing: (a) compaction in cubic steel molds; (b) curing of cubic specimens in air; and (c) compressive strength testing.

Results and Discussions

Physical, Mineralogical, and Pozzolanic Characteristics of SSA

Figure 3 shows scanning electron micrographs of the SSA and a Type I OPC. In comparison to OPC particles’ fine angularity (Figure 3b), the SSA particles exhibit a mostly irregular particle shape, as depicted in Figure 3a. A particle size analysis determined that 70% of the SSA particles range between 75 µm and 425 µm.

Figure 3.

Scanning electron micrographs of: (a) sewage sludge ash and (b) ordinary Portland cement.

The specific gravity of the SSA was found to be 2.39, which is comparable to that of silica fume (2.10–2.40) and less than the typical Portland cement’s specific gravity, ranging from 3.12 to 3.19. The specific gravity of SSA may increase along with the incineration temperature ( 12 , 13 ). The density of SSA may approach that of the heavyweight aggregate when the incineration temperature is increased to 1,200°C ( 13 ).

XRD tests were conducted to examine the SSA’s mineral phases, as shown in Figure 4. The XRD spectrum reveals sharp peaks superimposed on humps, which represent the main crystalline phases. Aside from these crystalline phases, the amorphous content of SSA provides a useful measure of the reactivity of the material. The amorphous content of SSA generally ranges from 35% to 75%, suggesting that the material is somewhat reactive ( 27 ). Silica is the primary mineral found in DELCORA’s SSA; however, the exact amorphous silica content was not determined. Thus, it appears that SSA could exhibit pozzolanicity, based on the silica content.

Figure 4.

X-ray diffraction spectrum of Delaware County Regional Waste Quality Control Authority’s sewage slush ash.

Figure 5 shows SAIs for specimens with various SSA dosages. Mortar mixtures with 5% SSA dosages showed SAIs ranging from 85% to 93%, with no obvious trend related to curing time. SAIs for mortar mixtures with 10% SSA dosages range from 61% to 75%, also with no obvious trend related to curing time. ASTM C618 ( 23 ) specifies that, for coal fly ash to be used as a mineral admixture in Portland cement concrete, fly ash-modified mortar samples must have a minimum 28-day strength of 75% of the control mortar specimens, that is, a minimum SAI of 75%. This indicates that DELCORA’s SSA is a somewhat capable cementitious material and can be potentially used as an admixture in cement concrete with a replacement dosage up to 10%.

Figure 5.

Strength activity index (SAI) of mortar mixtures with sewage slush ash (SSA) dosages varying between 5% and 30%.

Chemical and Leaching Characteristics of SSA

Volatile matter may negatively affect the density and integrity of the SSA materials and their subsequent use as supplementary cementitious materials. According to DELCORA’s quarterly environmental laboratory test results from 2017, there is generally no detectable volatile matter in DELCORA’s SSA (Table 2). Moisture content of the SSA is also minimal or non-detectable. The pH is an influential factor on the solubility of heavy metals, with acidic or alkaline conditions generally leading to a higher solubility of heavy metals. As shown in Table 2, the pH of DELCORA’s SSA was found to range from 7.24 and 8.31, with an average of 7.91.

Table 2.

Results of Quarterly Wet Chemistry Tests in 2017 (by DELCORA)

Parameter	Quarter 1	Quarter 2	Quarter 3	Quarter 4
Volatile matter (%)	1.1	ND	ND	ND
Moisture content (%)	0.2	0.2	ND	ND
pH	8.31	7.24	8.25	7.96

Note: DELCORA = Delaware County Regional Waste Quality Control Authority; ND = not detected.

Metal elements were identified, as shown in Table 3. The most abundant metal elements are aluminum (Al) and iron (Fe). Zinc (Zn), copper (Cu), and barium (Ba) are the next most abundant metal elements in DELCORA’s SSA. Some harmful heavy metals, such as lead (Pb) and arsenic (As) are noticeably low, while other heavy metals, such as cadmium (Cd) and mercury (Hg) are not detectable. It is noted that testing results for the second quarter of 2017 varied significantly compared with results of other quarters.

Table 3.

Metal Elements in DELCORA’s SSA Tested in 2017 (mg/kg) (by DELCORA)

Element	Quarter 1	Quarter 2	Quarter 3	Quarter 4
Aluminum (Al)	45,200	ND	38,400	52,900
Iron (Fe)	34,700	ND	42,200	36,800
Zinc (Zn)	841	0.46	471.0	655
Copper (Cu)	763	1.3	930.0	863
Barium (Ba)	627	ND	790.0	809
Lead (Pb)	82.2	ND	98.2	98.5
Nickel (Ni)	56.2	0.14	66.4	97.9
Chromium (Cr)	41	ND	33.4	39.5
Antimony (Sb)	26.8	0.37	37.6	36.4
Molybdenum (Mo)	17.3	0.39	24.9	25.3
Arsenic (As)	18.4	0.21	18.3	15.7
Silver (Ag)	4.8	ND	6.0	6.5
Cadmium (Cd)	1.4	ND	ND	9.3
Beryllium (Be)	ND	ND	ND	ND
Mercury (Hg)	ND	ND	ND	ND
Selenium (Se)	ND	ND	ND	ND
Thallium (TI)	ND	ND	ND	ND

Note: DELCORA = Delaware County Regional Waste Quality Control Authority; SSA = sewage sludge ash; ND = not detected.

Table 4 lists the metal traces detected in the leachates of TCLP conducted in all four quarters of 2017. It can be seen that none of the eight heavy metals, known as RCRA-8 (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver) was detected, except for a small concentration of arsenic in the second quarter, which was well below the EPA allowable limit of 5.00 mg/L. As shown in Table 4, small concentrations of copper and molybdenum have been consistently detected in all four quarterly tests, while small concentrations of antimony, nickel, and zinc were occasionally detected in the leachate.

Table 4.

Results of Quarterly TCLP Leaching Tests in 2017 (mg/L) (by DELCORA)

Metal	Quarter 1	Quarter 2	Quarter 3	Quarter 4
Antimony	ND	0.37	ND	ND
Arsenic	ND	0.21	ND	ND
Copper	1.3	1.3	1.2	0.75
Molybdenum	0.3	0.39	0.45	0.28
Nickel	ND	0.14	ND	ND
Zinc	0.61	ND	0.35	ND

Note: DELCORA = Delaware County Regional Waste Quality Control Authority; TCLP = toxicity characteristic leaching procedure; ND = not detected.

Another type of leaching test was performed, with extraction of solid wastes with water according to ASTM D3987 ( 28 ). The intention in using this test is that the final pH of the extract reflects the interaction of the extract with the buffering capacity of the solid waste. The pH of the leachates was found to range between 7.24 and 9.01.

By using the gas chromatography method in accordance with the 8082A method ( 29 ), tests were conducted to identify another toxic substance, known as polychlorinated biphenyl (PCB). No PCBs were detected in any of the quarterly tests.

Compressive Strength of SSA Geopolymer Mortar and Influencing Factors

Figure 6 presents the compressive strength of the various mixtures after different curing times. The results for compressive strength suggest that the mixes can be grouped into three tiers, according to their compressive strengths. Initial explanations for this grouping in results are that the mixes in Tier 3 both had a relatively smaller activator/binder ratio, 0.20 and 0.21 (Table 1). Mixes 0.22AB-0.8SN-0.9WB and 0.22AB-0.8SN-0.8WB, in Tier 1, only differ in water/binder ratio, while Mix-0.26AB-0.6SN-0.9WB has the highest activator/binder ratio of all the mix designs. The samples in Tier 2 are not similar with respect to mix design. Another observation of the tiers is that the discrepancy between the three tiers becomes more prominent after 7 days. Tier 1 mixes continued geopolymerization and strength gain to Day 28 while other mixes’ strength development rapidly slowed or ceased after Day 7.

Figure 6.

Compressive strength of geopolymer mortar at different curing times.

The activator/binder ratio is the ratio of the two activators (Na₂SiO₃ and NaOH) and the binder (SSA) by mass. This ratio is a measure of how much activator in total is used in each mix design, relative to the amount of SSA. The activator/binder ratio for the seven mix designs ranged from 0.20 to 0.26. Figure 7 shows the variation of geopolymer strength with activator/binder ratio. Increasing the activator/binder ratio resulted in increased compressive strength of the geopolymer mortar until the activator/binder ratio reached 0.22. The increase in compressive strength resulted from the availability of more alkali to activate the precursors (SSA), producing more dissolved ion species, which in turn enhanced geopolymerization of the mortar ( 30 ). An activator/binder ratio greater than 0.22, however, led to a decreased compressive strength of geopolymer. This was probably the result of early precipitation of some geopolymer gels, caused by the accelerated reaction, which blocked potential formation of more gel ( 31 ). Thus, an optimal value of 0.22 for the activator/binder ratio provided sufficient alkali to activate the precursors and yet did not hinder further formation of more gel.

Figure 7.

Effects of activator/binder ratio on strength of geopolymer mortar.

The total silica modulus (molar ratio of SiO₂/Na₂O) in the mixed alkaline activators (Na₂SiO₃ and NaOH) plays an important role because the two alkaline activators could have different degrees of effectiveness in inducing geopolymerization with the precursor material. Figure 8 presents the variation of geopolymer mortar strength, along with different silica moduli. The SiO₂/Na₂O molar ratio accounts for differing amounts of each of two activators; the ratio increases with more Na₂SiO₃ and decreases with more NaOH. A higher SiO₂/Na₂O molar ratio indicates a higher Si concentration, which enhances the reaction with calcium (Ca) to form C-A-S-H gel and Ca-rich geopolymer gel. The 28-day compressive strength of geopolymer mortar, however, increased along with the SiO₂/Na₂O molar ratio up to 0.80 and dropped thereafter. The decrease in 28-day strength may be attributed to excessive Na₂SiO₃, which blocks contact between the precursor materials and the alkaline activator solution and hinders the release of air bubbles, evaporation of water, and the formation of geopolymers ( 4 ). A similar observation on the effects of SiO₂/Na₂O molar ratio on the strength of geopolymers has been made in another study ( 19 ).

Figure 8.

Effects of activator modulus on strength of geopolymer mortar.

Summary and Conclusions

This study is intended to characterize a local SSA for potential use as a precursor to develop geopolymers. Tests were conducted to examine the SSA’s physical, mineralogical, and pozzolanic characteristics. The SSA was found to be lighter than OPC and exhibits a mostly irregular particle shape. SAI tests suggest that the SSA is a somewhat capable cementitious material and has a moderate pozzolanic activity. Chemical and leaching analysis showed that no significant adverse environmental implications of SSA mortars were found.

To examine the SSA’s potential as a precursor of geopolymers, seven mix designs were created to examine a range of values for influencing factors, such as activator/binder ratio and SiO₂/Na₂O molar ratio in the activator. Cubic mortar samples were created for each mix and subjected to compressive strength testing on Day 3, Day 7, and Day 28. Results suggested three tiers of mixes, grouped according to compressive strength. Some mixes continued geopolymerization and strength gain to Day 28, while other mixes slowed or ceased strength development after Day 7. The geopolymer’s strength development indicates that activator/binder ratio and SiO₂/Na₂O molar ratio play important roles in geopolymerization and strength gain. Both the activator/binder ratio and SiO₂/Na₂O molar ratio showed an optimal value at which a greater strength was achieved. Geopolymers prepared at an activator/binder ratio of 0.22 and a SiO₂/Na₂O molar ratio of 0.80 had the highest compressive strength. Although the SSA-based geopolymer showed substantial strength gain, the geopolymer mortar’s 28-day compressive strength was significantly less than that of OPC mortar.

Footnotes

Acknowledgements

The authors thank Mr. Michael Sweeney of Delaware County Regional Water Quality Control Authority for providing the SSA resources and Mr. Carlos M. Ruiz for help with laboratory testing. This work was carried out in part at the Singh Center for Nanotechnology, part of the National Nanotechnology Coordinated Infrastructure Program, which is supported by the National Science Foundation (grant NNCI-1542153).

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: X.T.; data collection: I.A.; analysis and interpretation of results: X.T., I.A.; draft manuscript preparation: X.T., I.A., J.P. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Ibrahim Alhassan

Junboum Park

Xiaochao Tang

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