Recovery of Sewage Sludge in the Cement Industry

Abstract

This paper presents an analysis of the literature that studies the possibility of sewage sludge being used in the cement industry to reduce carbon dioxide emissions from cement production and thus solve the problem of disposing of sewage sludge so that it is no longer stored, avoiding soil pollution with heavy metals, and reducing pressure on the environment. The ash of sewage sludge is a good pozzolanic material, because when it is finely ground, it can be used as a partial substitute for Portland cement. This reduces waste storage costs. Sewage sludge ash was mixed with cement, and it was analyzed to determine whether the paste obtained could be used as a raw material in the cement industry. The presented results are on the hydration characteristics of the sewage sludge ash, the compressive strength of the cement determined after different days, the workability of the cement, and the porosity of the cement paste and the ash.

1. Introduction

The fundamental principle of the economy was “resource-production-waste”, in a linear direction, making a structural change necessary and vital. This change now exists as a concept and refers to the circular economy, in which waste must become a resource [1]. The sludge from the municipal water treatment process has characteristics that show that it can be used as a minor additive in the composition of cement. Given the situation regarding the poor management of sewage sludge, a good treatment of it could create an expansion for its correct recovery by using it in the cement industry [2]. This would reduce carbon dioxide emissions and solve the management of sewage sludge [3].

The development of urbanization implies an increased demand for cement, and implicitly, an increase in carbon dioxide emissions [4,5,6]. This paper highlights the positive impact of ash from sewage sludge in the construction industry. According to the CEMBUREAU Activity report 2020 (“Cementing the European GreenDeal”), the cement production in EU28 from 2019 was approx. 182 million tons [7]. If it contained a 1.5% minor addition of sewage sludge, it would have avoided the storage of this waste (made by EU28, in 2019), and would have led to avoiding emissions of about 1.8 million tons of CO₂ [7].

In the process of industrialization, pollution generating activities surpassed the self-cleaning and self-regulation capacity of the environment. It has become a high priority worldwide to produce environmentally friendly low carbon imprint products, including building materials [8,9,10,11].

An important and topical issue is environmental pollution. Thus, as the amount of industrial sludge residue increases, the management of excess sludge becomes problematic, and thus endangers the environment. It creates problems such as: (i) water pollution (underground by infiltration, and surface by runoff); (ii) air pollution (aerobic fermentation by gas release); and (iii) soil pollution (infestation by uncontrolled storage) [3,9,12,13].

From the cement industry about 900 kg of CO₂ are emitted, for every ton of cement produced, and anthropogenic CO₂ emissions worldwide are about 5% [10,11,14]. The manufacturing of one cubic meter of concrete (~2400 kg) is responsible for the emission of ~540 kg of CO₂ into the atmosphere [3,12].

There are seven cement factories in Romania, all with rotary clinker kilns and a dry process for cement production. If each cement plant in Romania would use 10,000 tons of dry sewage sludge as alternative fuel, for one year, approximately 25,000 tons of coal would be substituted and 60,000 tons of CO₂ emissions would be saved [15,16]. The amount of waste must be reduced by 50% before 2050; this category also includes sludge from wastewater treatment plants [5,13].

During periods of economic development, the increase in demand for cement is mainly due to infrastructure, construction, and industry projects. Globally, between 2000 and 2018, cement production increased, with an average annual growth rate of 6.2%, reaching 4200 million tons in 2018, compared to 1600 million tons produced in 2000, as seen in Figure 1 [7].

According to Figure 2, the quantities of sludge generated by wastewater treatment plants in Romania during 2005–2018 are increasing (from 134 tons of dry matter/year recorded in 2005, to 520 tons of dry matter/year in 2018).

In this paper, sewage sludge ash as raw material for the cement industry has been analyzed [8,17,18,19]. The ash of sewage sludge comes from the incineration of sewage sludge from wastewater treatment plants, and has a high porosity, and irregular sand-like shape. It is a reactive material, which increases the strength of the mortar due to these pozzolanic properties [17,20]. The sewage sludge is dehydrated in the incineration plant up to 30% before burning in a fluidized bed oven at 800–900 °C.

Through the incineration process, the amount of waste is reduced; the ash can replace the cement [21].

By incineration, the volume of sewage sludge is reduced, this being a method of managing sewage sludge [22]. After incineration at a high temperature, the components of SiO₂, CaO, Al₂O₃ sludge are similar to those of cement [23]. Therefore, the use of sewage sludge ash in the cement industry reduces environmental pollution; reduces the amount of cement required; and has an economic, ecological, and energy saving impact [24,25,26]. When the ash of the sewage sludge is used as an additive to the cement, the pozzolanic activity is lower than that of the cement, determining a lower resistance and a high addition of water, which can be avoided by grinding the ash [27].

2. Chemical Properties of Sewage Sludge Ash

2.1. Chemical Composition

It can be seen from Table 1, where the composition of the sewage sludge from a municipal waste treatment plant in Romania is presented, that it contains heavy metals such as cadmium; copper; lead; mercury; and chromium [3].

In Table 2, the oxide contents of dry sludge and Portland cement are shown.

According to Table 2, it can be observed that the common elements in Portland cement and ash from sewage sludge are: Ca, Si, Al, and Fe.

It is observed that SiO₂, Fe₂O₃, and Al₂O₃ are predominate in the composition of the incinerated sewage sludge; these being oxides involved in the pozzolanic reaction [20,23,28]. A high content of P₂O₅ in the ash can have a negative impact on the hardening of the paste when used as a cement additive, longer setting times and slow development of strength [16,32].

The contents of MnO, K₂O, MgO, and SO₃ from the sewage sludge ash are similar to Portland cement. Other oxides contained are: CaO, SO₃, K₂O, MgO, and P₂O₅ [31].

The SO₃ contents of Portland cement and sewage sludge ash are less than 4%, and comply with the provisions of the European Standard, SR EN 197-1 [33].

In Figure 3a,b, scanning electron microscopy SEM images of ash sewage sludge are presented; this is a granular material with an irregular shape and high porosity.

The use of ash as a raw material aims to reduce the cement content, reduce costs, obtain a low hydration temperature, obtain higher strengths of concrete at older ages of hardening of the paste, and improve durability [34,35].

In Table 3, the properties of Portland cement CEM I are presented according to the European Standard, SR EN 197-1.

The European Standard, SR EN 197-1, defines five classes of common cement that comprise Portland cement as a main constituent, and these are presented in Table 4.

In this paper, we analyze pastes containing 5%, 10%, 15%, and 20% sewage sludge ash.

2.2. Hydration Characteristics of Ash

Hydration of the material with sewage sludge ash increases; their resistance is developed by using a large amount of Ca(OH)₂ [22].

As the ash content increases, the capillary network of cement has changed, and the coefficient of water absorption by capillarity has doubled [24,35,36].

Paste with cement and ash has a shorter preparation time than Portland cement paste because the hydration rate of aluminum oxide in the ash of sewage sludge is higher than that of silicate in cement [21,22]. At a content of 10%, Portland cement-ash sewage sludge at 72 h, has the heat of hydration of 672.94 [J/g]; at a content of 20%, Portland cement-ash sewage sludge at 72 h has the heat of hydration of 505.90 [J/g]; at a content of 30%, Portland cement-ash of sewage sludge at 72 h has the cumulative heat of 443.13 [J/g]. The heat of hydration of the Portland cement sewage sludge ash paste decreases with the increase of the ash content of the Portland cement sewage sludge [23]. Porous particles of sewage sludge ash absorb more water, increasing the heat release rate of Portland cement.

By adding 5% ash to the cement, a low absorption capacity, a longer material life, and a high durability are obtained [5].

3. Mechanical Properties of Sewage Sludge Ash

3.1. Compression Strength

To see the strength of the cement, a compressive strength test was performed.

The compressive strength of the cement determined after 1, 7, 28, and 90 days for different pastes with sewage sludge ash added at concentrations of 5%, 10%, 15%, and 20% compared to Portland cement (0%) is shown in Table 5. This shows that the ash of sewage sludge added in proportions of 5% and 10% at 28 days had values similar to the compressive strength of Portland cement [28,31,37].

The compressive strength decreased proportionally with an increase in the concentration of added ash compared to cement strength, and increased the curing time of cement [6,24,32,36]. The compressive strength of the paste with cement and the addition of ash of 5% and 10% had values similar to the compressive strength of cement [28].

The compressive strength of cement paste and ash increased with the increasing binder aging due to cement hydration [38,39].

Sewage sludge ash has a lower pozzolanic reactivity; its porous nature determines the absorption of a larger amount of water from the mixtures, thus reducing the effect of diluting the cement [27].

Higher strength values were recorded when the ash had a small grain size compared to the coarse grain at the same ash content due to its higher reactivity; the increase in the strength of the material was also influenced by the hydroscopic nature of the ash, which absorbed more water [22,28,32,40].

In the material to which 5% and 10% ash were added, the compressive strength had higher values, with the increase of the hardening period [30,40].

Table 5

Compressive strength for different concentrations [6,27,28,29,30].

Sample		0%	5%	10%	15%	20%
	[27]	21.00	-	-	-	25.00
1 day, [N/mm2]	[29]	10.00	-	9.00	-	8.00
	[30]	7.43	-	2.11	-	-
	[6]	7.95	7.39	6.85	6.07	5.84
7 day, [N/mm2]	[27]	34.00	-	-	-	33.00
	[29]	20.00	-	19.00	-	18.00
	[30]	23.39	-	22.75	-	-
	[6]	23.21	21.22	19.62	17.03	14.73
	[28]	34.00	-	-	-	27.00
28 day, [N/mm2]	[27]	45.00	-	-	-	43.00
	[30]	35.02	-	35.53	-	-
	[29]	48.00	-	42.00	-	40.00
	[6]	30.07	33.09	25.24	23.16	15.68
	[28]	42.00	-	-	-	38.00
90 day, [N/mm2]	[28]	47.00	-	-	-	46.00
	[30]	42.06	-	44.30	-	-
	[29]	55.00	-	50.00	-	45.00
	[27]	49.00	-	-	-	47.00

3.2. Workability

In the case of the material to which 20% ash was added, the setting time increased; the 10% ash recipe had a setting time similar to that of cement [27,41]. By ensuring a water/cement ratio for the material with sewage sludge ash, a stable volume was obtained, and the workability was slightly reduced compared to Portland cement paste [37].

The setting time of the material increased with the increase of the gray granulation [20].

Table 6 shows the setting times for different concentrations of ash added to the cement.

3.3. The Porosity

The porosity of the cement paste and ash influenced the physical properties if this porous paste was hydrated; it was compact and can be used as a substitute for cement [41,42].The porosity was higher for the material to which a larger amount of ash had been added (Figure 4 and Table 7) [41,42].

4. Conclusions

The sewage sludge ash analyzed in this paper meets the standard conditions to be used as an additive in the cement industry: the content of SO₃ of ash is less than or equal to 4%, the compressive strength at 28 days is greater than or equal to 42.5 MPa, and the initial setting time is greater than or equal to 60 min [15,17].

Using sewage sludge ash as a cement substitute reduces the pollutants resulting from the manufacturing of cement and reduces the storage space of the cement [6]. Its use in the cement industry aims to reduce the cement content, reduce costs, obtain a low hydration temperature, improve workability, and obtain higher strengths of concrete at higher curing ages (higher number of days). The oxides contained in sewage sludge ash are similar to those in Portland cement, so the ash can be used as an additive in the manufacturing of cement [6]. Sewage sludge ash has a porous, irregular structure, and must be hydrated in order to be used as an additive in the cement industry [39,42].

The compressive strength of cement paste was similar to that of cement for adding a small amount of ash (5% and 10%) [28].

The setting time of the material increased with the increase of the gray granulation [20].

Given the commitments of major cement producers to become CO₂-neutral by 2050, the inclusion of this waste in cement outlines a perspective that can contribute to meeting the targets set [43].

The use of ash as an additive in the cement industry is a good choice to reduce the pressure on the environment, but it has to be considered that when the ash of the sewage sludge is used as an additive to the cement, the pozzolanic activity is lower than that of the cement. This causes a lower resistance and a high addition of water, which can be avoided by grinding the ash [44,45].

Author Contributions

Conceptualization: C.O.R. and L.P.; methodology: C.O.R., G.P., I.C.P. and G.V.; investigation: C.O.R., G.P., G.V., M.B.; validation: C.O.R., E.V.S., M.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Global cement production in the years 2000–2018 [7].

Figure 2. The amount of sludge generated in Romania [14].

Figure 3. Images of sewage sludge ash (a) magnification 1000× (b) magnification 4000× by scanning electron microscopy (SEM) [25].

Figure 4. Total porosity [41,42].

Table 1

Sewage sludge composition from a municipal waste treatment installation in Romania.

Indicatory	Unit Measure	Value
Net calorific value	GJ/t	8.92
Humidity	%	12.20
Mercury	mg/kg	0.809
Cadmium	mg/kg	2.17
Plumb	mg/kg	42.50
Crom	mg/kg	73.00
Cupru	mg/kg	370.00
Mangan	mg/kg	460.00
Arsen	mg/kg	6.14
Zinc	mg/kg	985.00
Petroleum products	mg/kg	7780.00

Table 2

Oxide content [27,28,29,30,31].

The Component Element	Sewage Sludge Ash [%]					Portland Cement [%]
	[29]	[30]	[27]	[28]	[31]	[29]	[30]	[27]	[28]	[31]
SiO2	16.60	17.10	28.60	28.30	37.04	19.50	20.10	19.50	19.80	20.33
Al2O3	5.10	5.10	17.60	12.50	15.24	6.00	4.91	4.40	3.90	5.21
Fe2O3	9.10	15.70	4.40	18.60	14.03	3.10	5.43	2.60	3.20	3.13
MnO	-	0.09	-	0.20	-	-	0.04	-	0.10	-
P2O5	15.00	20.20	1.90	0.50	9.12	-	0.23	-	0.90	0.20
CaO	12.90	23.80	20.10	10.60	6.91	62.10	65.70	60.50	65.20	64.00
K2O	2.80	1.57	1.90	1.90	2.77	-	0.81	0.90	0.70	0.63
MgO	3.80	2.32	2.30	3.20	2.80	1.70	0.53	2.90	1.50	1.62
SO3	2.10	2.02	2.00	6.20	3.66	2.60	4.74	3.63	5.50	4.17
Na2O	3.50	1.15	1.23	7.40	7.11	0.80	0.67	0.24	-	-
TiO2	-	0.83	1.50	0.50	0.38	-	0.35	0.20	0.30	0.27
ZnO	-	-	0.52	-	-	-	-	-	-	-
Cl	0.01	0.01	-	-	-	0.03	0.10	-	-	-
Loss on ignition (LOI)	-	-	0.70	-	-	-	-	2.70	-	-

Table 3

CEM I characteristics [33].

Type of Cement CEM I	Characteristics	Conditions
CEM I 42.5 R	Initial setting time	≥60 min
	Compressive strength 2 days	≥20 MPa
	Compressive strength 28 days	≥42.5 MPa…≤62.5 MPa
	Loss on ignition	≤5%
	Sulfate content (as SO3)	≤4%
	Chloride content	≤0.10%
CEM I 52.5 R	Initial setting time	≥100 min ≤ 140 min
	Compressive strength 2 days	≥30 MPa
	Compressive strength 28 days	≥52.5 MPa
	Loss on ignition	≤5%
	Sulfate content (as SO3)	≤4%
	Chloride content	≤0.10%

Table 4

Types of cement [33].

Class	Description	Constituents
CEM I	Portland cement	Comprising Portland cement and up to 5% of minor additional constituents
CEM II	Portland-composite cement	Portland cement and up to 35% of other single constituents
CEM III	Blast furnace cement	Portland cement and higher percentages of blast furnace slag.
CEM IV	Pozzolanic cement	Portland cement and up to 55% of pozzolanic constituents.
CEM V	Composite cement	Portland cement, blast furnace slag, or fly ash and pozzolana.

Table 6

Portland cement setting time and sewage sludge paste [29,41].

Sample	Initial Time [29,41] [h:min]		Final Time [29,41] [h:min]		Water /Paste [36]	Volume Stability [Expansion, mm]
Cement Portland	1:50	3:30	3:10	4:16	0.34	0
10 % SSA	2:0	3:30	4:45	4:58	0.38	0
20 % SSA	2:45	2:80	6:00	6:00	-	0

SSA: sewage sludge ash.

Table 7

Total porosity.

Sample	Total Porosity [%] [41,42]
Cement Portland	16	17
10 % SSA	19	18
20 % SSA	20	19
30% SSA	21	24

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