1. Introduction
The construction materials industry, which was once based on a wide range of materials appropriate to local conditions and/or to specific needs, has evolved into an industry based on Portland cement-based materials [
1]. Such a change of trend has turned Portland cement-based materials into the most heavily consumed manufactured material in the world in terms of volume, being responsible for about 7% of the total anthropogenic emissions of carbon dioxide (CO
2) [
2]. Thus, worldwide concerns about CO
2 emissions have increased interest in alternative binding technologies and materials to Portland cement (PC) [
3,
4], such as carbonated-based materials [
4], mainly due to their capacity to capture and store CO
2 in their matrices [
5]. It has been shown to be feasible to produce CO
2-cured binders with high strength results from electric arc furnace slag (EAF slag)-, PC- [
6], and carbonated reactive magnesia cement (CRMC)-based materials [
7], especially if the accelerated carbonation curing conditions are controlled [
6] as well as the mixture designs [
7]. It was also shown to be practicable to use waste materials in CRMC-based materials [
8,
9,
10,
11,
12] which may reduce the environmental impacts and production cost of the designed materials [
13].
In addition to that, the ash produced by the thermal transformation of sewage sludge is a raw material that may also be used in the production of construction binders, allowing cement consumption to be reduced. Due to its properties, the waste sludge generated in sewage treatment processes is a serious environmental threat. This is because of the difficulties involved in its management and disposal. One developing trend in sludge processing is thermal treatment. The basic chemical composition of sewage sludge ash (SSA) includes oxides of silicon, calcium, phosphorus, and aluminium. The amounts of individual components vary strongly depending on the region, degree of urbanisation, and the method of sewage treatment [
14]. Moreover, SSA also holds heavy metals, the amount of which primarily depends on the type of sewage supplied to the treatment plant. Data available in the literature show heavy metal content in mg/kg dry weight of sludge or ash. Unfortunately, the qualitative range of the elements in question is variable, resulting in a difficult comparative analysis. From the available literature, it is possible to undertake a basic analysis of the heavy metal content of ash from laboratory-scale combustion in an electric furnace and industrial-scale combustion in a fluidised bed furnace. Copper and zinc are found in the highest amounts regardless of the region or method of generating the ash. The elements most commonly reported in the literature are also cadmium, chromium, nickel, and lead. The content of heavy metals varies widely, regardless of how the ash was obtained [
15,
16,
17].
Therefore, heavy metals should be appropriately stabilised and disposed of due to their very high toxicity levels. One method is to immobilise them—this involves initiating physical and chemical processes leading to their permanent inactivation and neutralisation. Some construction materials can act as a “matrix” and demonstrate the ability to immobilise heavy metal ions [
18,
19], as was recently demonstrated in geopolymer-based sewage sludge ash (SSA) mortars [
20]. However, such materials may include, but are not limited to, cementitious binders [
18,
19].
Thus, in this study, accelerated carbonation curing technology was employed to produce CO2-cured mortars incorporating SSA with the aim of developing mortars with a reduced carbon footprint and heavy metal immobilisation properties. Therefore, this study analyses the degree of immobilisation of heavy metals in SSA-based CO2-cured mortars, evaluates the compressive strength of the thus-designed mortars, and through thermogravimetry and derivative thermogravimetry (TG-DTG) and Fourier transform infrared spectroscopy (FT-IR) analyses provides pieces of evidence of the carbonation throughout the specimen’s volume.
2. Materials and Methods
2.1. Materials
Sewage sludge ash (SSA) used as a filler in the CO2-cured mortars was obtained from the sewage treatment plant in Płaszów in Krakow, Poland, which is a result of the combustion of sewage sludge at a temperature of around 800 °C in a fluidised bed furnace.
The CO
2-reactive compounds consist of Portland cement (PC) containing a limestone additive provided by Secil under the trade name CEM II/B-L 32.5N and reactive magnesium oxide (r-MgO), known under the trade name “Magal P”, provided by Invivo-nSA, which is, in fact, calcinated magnesia mainly used as an additive to animal feed. Before being used, the r-MgO material received was ground in a ball mill grinder and sieved to obtain particles with a diameter of less than 125 μm. Also included was electric arc furnace slag (EAFS) obtained from the national steel industry, located at Maia and Aldeia de Paio Pires, Seixal, Portugal. Before being used, the EAFS material received was dried in an oven at 60 ˚C; afterwards it was ground in a ball mill grinder and subsequently sieved to obtain particles with a diameter of less than 45 μm. These particle sizes were used following previous research works [
6,
11,
12], in which CO
2-cured binders were produced.
River sand (RS) used as fine aggregate was provided by Tabal-Sepor Areias e Argamassas LDA, which was obtained from screening natural materials from river dredging.
The estimated oxide compositions of SSA, PC, r-MgO, and EAFS, were determined by energy-dispersive X-ray spectroscopy (SEM-EDX) tests on a Hitachi S-3400N apparatus in which three different locations were randomly selected in each raw material sample to obtain their chemical compositions. The true powder densities of SSA, PC, r-MgO, and EAFS were determined using a gas displacement pycnometer on a Micromeritics AccuPyc 1340 apparatus. In addition, loss on ignition (LOI) of the raw materials was obtained by TG-DTG analysis, which was carried out on an SDT Q-50 (TA Instrument). These are presented in
Table 1.
2.2. Mix Compositions
The mix compositions of the CO
2-cured mortars proposed in this study are detailed in
Table 2. The mixtures were designed with a volume proportion of 3 to 1, three parts of RS to one part of SSA binder. Reactive compounds, i.e., PC, r-MgO, and EAFS, were kept constant at 10% of the overall solid mass. The water to solid (w/s) ratios adopted were 0.10 for the PC and EAFS mix designs [
6] and 0.15 for the r-MgO mix design due to their carbonation curing behaviour, since HMCs contain water in their composition whereas calcium carbonates do not, and due to preliminary experimental trials that presented dry and uncarbonated core in SSA.M when w/s ratio of 0.10 was used.
2.3. Specimens’ Preparation
The specimen preparation procedure of each mix was divided into three main steps, i.e., mortar preparation, casting of specimens, and accelerated carbonation curing. To prepare the mortar, the solid materials in the planned mix compositions were mechanically blended until the mixture was homogeneous and soon afterwards, tap water was gradually added at the defined w/s ratio resulting in a uniform mortar. To cast the specimens, the prepared mortar was placed in a prismatic mould to be compacted into a cubic format of edges of 40 mm using a static compaction pressure of 30 MPa. Once the pressure reached 30 MPa, it was maintained for more 60 seconds in order to provide a better particle adjustment to the specimens. Afterwards, the moulded specimens were extruded from the mould and placed in a moisture-saturated chamber to prevent water evaporation until three other cubes were prepared. In addition to this, during the specimen casting step, it was observed that a small quantity of water carrying fine particles was expelled. Hence, this resulted in a reduction of the water content in specimens and of fine particles.
Shortly after four specimens were cast, they were placed inside a pressurized carbonation chamber for an accelerated carbonation curing (ACC) period of 24 h at controlled atmosphere of CO
2 concentration > 99%, partial pressure of 0.7 bar, temperature of 50 ± 2 °C, and relative humidity (RH) > 99%. Thereafter, the specimens were taken out of the carbonation chamber and were stored in room conditions (RC) for 24 h at a temperature of 20 ± 2 °C and RH of about 60% in order to cool them down. In addition, the specimen’s mass and dimensions were taken before the ACC period and after the RC period, allowing their bulk density, mass loss, and possible change of the specimen’s dimensions to be determined. In addition, control specimens, SSA.P (
Figure 1a), were prepared to provide a comparison with specimens that contained r-MgO and EAFS as reactive compound, SSA.M (
Figure 1b) and SSA.E (
Figure 1c), respectively.
2.4. Compressive Strength
Compressive strength tests were undertaken soon after the RC period had elapsed. The test was carried out on a uniaxial loading basis on three samples. The equipment used was an ADR Touch 3000 BS EN Compression Machine with digital readout and self-centring platens, operating at a constant loading rate of 2.4 kN/s.
2.5. TG-DTG
Thermogravimetry and derivative thermogravimetry (TG-DTG) analysis of the specimens was carried out on a SDT Q-50 (TA Instrument). This was performed in the temperature range of ambient to 1000 °C, at a heating rate of 20 °C/min, using helium as the purge gas and platinum crucibles. The tested material consists of about 5 mg of powdered specimen’s core with a diameter of less than 63 μm, in order to confirm that the accelerated carbonation process took place through the whole specimen’s volume.
2.6. FT-IR
The Fourier transform infrared spectroscopy (FT-IR) data were recorded from 600 to 4000 cm−1 using Nicolet iS10 FT-IR Spectrometer (Thermo Scientific), Smart iTR accessory instrument by diamond HATR crystal. The material used consists of particles passing throughout a 63 µm aperture sieve of specimen’s core.
2.7. Leachability of Heavy Metals
The tests for leaching of heavy metals were conducted in accordance with the standard EN 12457-4 [
21], whereby in the case of both cements and waste materials, samples reflecting their natural grain size were selected for testing. For tests of total heavy metal content, all materials were mineralised with aqua regia in the ratio of 3 mL HCl + 1 mL HNO
3 + 0.5 g MO under closed microwave furnace conditions at 180 °C for 45 minutes. The process was carried out based on the indications of the standard PN EN 13346 [
22].
Testing for heavy metal content, i.e., Sb, As, Se, was performed by the ICP method according to PN-EN ISO 11885 [
23]. For chromium, the ASA method was used according to EN 1233 [
24], whereas in the case of other heavy metals, the tests were performed according to PN-ISO 8288 [
25].
4. Conclusions
This study has proposed sewage sludge ash heavy metals immobilisation in CO2-cured mortars based on Portland cement, reactive magnesia, or electric arc furnace slag. Therefore, the results of this study are summarised below:
The accelerated carbonation curing period led to a reduction of CO2-cured mortars bulk density at about 5%.
The compressive strength results achieved reached up to 12.7 MPa. Due to the high carbonation ability of CaO and MgO, the quantity of these compounds seems to be directly related to the strength development in the CO2-cured mortars. Although SSA.E specimens presented the lowest compressive strength, they are nevertheless promising since the paste in this mortar composition is produced only from waste materials (sewage sludge ash and electric arc furnace slag).
The TG-DTG and FT-IR analyses indicated that the carbonation reached the inner parts of these forming carbonate products, such as calcium carbonates (CaCO3) and hydrated magnesium carbonates (HMCs), which are mainly responsible for the binding properties of the CO2-cured mortar compositions devised.
The observed CO2-cured mortars’ heavy metals leachability was at a very good level, generally not exceeding 5%. Moreover, although the low compressive strength results observed in SSA.E mortar, it presented the highest heavy metal immobilisation degree among the three designed mortars, especially regarding arsenic, chromium, copper, nickel, and mercury. Finally, the criteria for inert waste and post-industrial wastewater according to Polish ministerial regulations, based on European Commission directives was met.
Therefore, the CO2-cured mortars may be considered a promising cementitious material for immobilising heavy metals in which the compressive strength is related to the success in the accelerated carbonation curing process. Furthermore, this may represent an alternative way of making hybrid binders incorporating SSA and a reactive compound. Finally, the mortars produced may represent an effective way of not only incorporating SSA, but also a wide variety of wastes, and at the same time, capturing and storing CO2 in cementitious materials. However, future studies on the microstructure field are needed for a better understanding of the products developed in these composites.