Decarbonising the Portland and Other Cements—Via Simultaneous Feedstock Recycling and Carbon Conversions Sans External Catalysts

The current overarching global environmental crisis relates to high carbon footprint in cement production, waste plastic accumulation, and growing future energy demands. A simultaneous solution to the above crises was examined in this work. The present study focused on decarbonizing the calcination process of the cement making using waste plastics and biowastes as the reactants or the feedstock, to reduce the carbon footprint and to simultaneously convert it into clean energy, which were never reported before. Other studies reported the use of waste plastics and biowastes as fuel in cement kilns, applicable to the entire cement making process. Calcination of calcium carbonate and magnesium carbonate is the most emission intensive process in cement making in Portland cements and Novacem-like cements. In the Novacem process, which is based on magnesium oxide and magnesium carbonates systems, the carbon dioxide generated is recycled to carbonate magnesium silicates at elevated temperatures and pressures. The present study examined the Novacem-like cement system but in the presence of waste plastics and biomass during the calcination. The carbon dioxide and the methane produced during calcination were converted into syngas or hydrogen in Novacem-like cements. It was established that carbon dioxide and methane emissions were reduced by approximately 99% when plastics and biowastes were added as additives or feedstock during the calcination, which were converted into syngas and/or hydrogen. The reaction intermediates of calcination reactions (calcium carbonate–calcium oxide or magnesium carbonate–magnesium oxide systems) can facilitate the endothermic carbon conversion reactions to syngas or hydrogen acting as non-soot forming catalysts. The conventional catalysts used in carbon conversion reactions are expensive and susceptible to carbon fouling. Two criteria were established in this study: first, to reduce the carbon dioxide/methane emissions during calcination; second, to simultaneously convert the carbon dioxide and methane to hydrogen. Reduction and conversion of carbon dioxide and methane emissions were facilitated by co-gasification of plastics and bio-wastes.


Introduction
Current environmental challenges relate to meeting global CO 2 emission targets, managing tons of plastics waste, and meeting the future energy needs. Cement production presents a major opportunity for addressing concerns related to waste plastics and biowastes as energy sources and chemical feedstock. This work identified and explored issues associated with this opportunity. These include: Review of emission specifications and energy requirements for lime and clinker production Comparative tonnage of present day energy sources vs. coal Available plastics/energy sources Feed-stock recycling of tires in cement production Emission and toxicity concerns Reactions of Engineers and contractors did not embrace these alternate cements due to high costs [11], and they prefer strong materials and strict building standards [9,12,13]. Geopolymer cement (USD 161.00) costs nearly thrice as much as the Portland cement (USD 51.00) [14,15]. High costs and lack of field testing prohibit the use of these new cements.

Simultaneous Decarbonization, Wastes Management, and Clean Energy Production in Portland Cements
The present study offers a novel step-change process to decarbonize the cements during the calcination via co-gasification of biomass and waste plastics, to cut down on emissions and energy requirement. In this section, energy requirement and source of emissions in cement making, roles of waste plastics and biowastes in reducing these emissions, and energy requirements are discussed.

Emissions from Calcination of Carbonates, the Raw Materials Used in Cements
CaCO 3 , MgCO 3 , and dolomites are the raw materials used in cement making [16,17]. Calcination of calcite (limestone, CaCO 3 ) requires higher energy than the calcination of dolomite and magnesite (MgCO 3 ). The incipient evolutions of CO 2 for magnesites, dolomites, and calcites occur at 640 • C, 730 • C, and 906 • C, respectively. In total, 1.092 kg CO 2 is released per kg magnesia (MgO) and 0.477 kg CO 2 per kg dolomite. The energy demand for MgO production ranges between 5 and 12 GJ/t MgO [18].
A cement clinker is made by calcining a homogeneous mixture of limestone (CaCO 3 ) and clay or sand (silica and alumina source) in a rotary kiln at~1450 • C (reaction 1) [9]. 3CaCO 3 + SiO 2 → CaSiO 5 + 3CO 2 (1) Subsequently, Portland cement is produced by grinding the clinker with~5% of gypsum (calcium sulphate). There are two sources of CO 2 emissions in Portland cement: (1) burning the coal as fuel and (2) calcination of limestone to lime. The focus of the present study was to reduce CO 2 produced during the calcination of limestone to lime (reaction 2) as well as during the calcination of magnesite to MgO, a Novacem-like cement system. About 65% of CO 2 emissions are due to the calcination of raw materials, mainly from reaction 2. Energy consumption for lime (CaO) production is 4.25 GJ/t of quicklime [19]. The remaining 35% is due to fuel combustion. The amount of CO 2 released is 1 kg/kg cement during calcination. Almost equal amounts of CO 2 are released from heating up the required amount of coal. Coal consumption is 0.2 t/t cement. To produce 1 kg of clinker, 1.16 kg of limestone is required [20], of which CaO content is 0.65 kg/kg clinker. Emissions from 1 t clinker production are calculated as: 1 t × 65% × 0.79 = 0.51 t CO 2 from CaCO 3 calcination [21]. CaCO 3 → CaO + CO 2 (2)

Role of Waste Plastics in Reducing the Emissions in Cement Processing
About~104 Mt of waste plastics are projected to enter our environments by 2030 [22]. Wrong waste management practices of end-of-life (EOL) plastics pose huge environmental challenges. Plastics production and incineration will account for 56 Gg.t of carbon emissions between now and 2050 [23,24]. Halogenated and PVC plastics release dioxins, polychlorinated biphenyls, HBr, and furans into the environment. Harsh HCl gas from PVC can damage treatment plants and incinerators. Demand for silicones in electrical, electronics, medical, and other industries led to their increased land filling and the loss of valuable resources [25,26]. Toxic odors and severe temperatures constrain silicones repurposing. Net emissions factors for plastics for different materials management options are given in Table 2 [27,28]. The driving forces of plastics recycling schemes are energy recovery and cutting emissions, penalties, energy consumption, non-renewable resources, and manufacturing costs [29,30]. Energy recovery from waste plastics depends on their calorific values (kJ/kg): coke~25,000-30,000, PE~44,800, PP~42,700, PS~41,900, PET~23,200, PVC~1800, and epoxy (resin)~32,000 [31,32]. More than 90% of the plastics produced (300 Mt/y) are not recycled [33].

Waste Plastics as Fuel
A cement plant with 1 Mt capacity can consume between 10,000 and 30,000 Mt of plastics as fuel per year. About 50,000 t of waste plastics can be treated as fuel with 3000 to 4000 t of lime in a shaft kiln to generate syngas, which can support high temperature processes, such as glass foundries and iron and steel production replacing the fossil fuels [38,39].

Waste Plastics as Chemical Feedstock
Waste plastics are used as fuel in the cement industry but not as chemical feedstock (as raw material) to reduce the CO 2 emissions thus far. Feedstock recycling of plastics is a sustainable solution to manage the plastic wastes such as mixed and halogenated plastic wastes and silicone wastes not suitable for recycling. Chemical feedstock recycling processes can extract valuable resources, e.g., C, H, Cl, and Si, from waste plastics, silicones, and biomass without considerable pre-treatment or depolymerization. During chemical feed stock recycling, plastics decompose without burning, producing chemically useful materials, and can convert to syngas at cement making temperatures [34,40]. Syngas is a renewable fuel with similar properties to natural gas that contains H 2 and CO and has many applications, as seen in Figure 1 [41]. It is a precursor for liquid fuel production via Fischer-Tropsch process and a main source of H 2 in the refineries [42]. Waste plastics as chemical feedstock in iron and steel industry reduce CO 2 emissions, acting as reductants and as the source of syngas [29][30][31]43]. Up to 30% reduction in CO 2 emissions is demonstrated in iron ore reduction using waste plastics as feedstock to partially replace coke as the reductant. Blast furnace and coke ovens treat waste plastics as chemical feedstock to produce syngases. The advantage of feedstock recycling in blast furnace approximates to 50 GJ/t of mixed plastics [44].

Biomass/Biowastes
Biomass/biowastes are generally considered carbon neutral because the CO 2 emitted to the atmosphere during combustion is absorbed while growing the replacement biomass. However, emissions accrue during farming, harvesting, processing, and delivering the fuel. A "carbon neutral" emissions factor for biomass is 0.04 kgCO 2 e/kWh (net CO 2 e emissions assuming carbon sequestration) and 0.39 kgCO 2 e/kWh when all emissions accrued at the point of consumption are considered [45]. The biomass has the following composition: cellulose 42%, lignin 29%, and hemicellulose 7% [46]. At high pyrolysis temperatures, biomass exhibits increased amounts of H 2 and CO and decreased amounts of CO 2 in the gases [47]. Carbon neutral natural rubber components in tires contribute to lower CO 2 emissions [48].

Decarbonizing Cement via Chemical Feedstock Recycling of Wastes
This study focused on decarbonizing the calcination phase (reaction 2) of Portland cement and Novacem-like cements and converting the CO 2 generated during the calcination to hydrogen and/or syngas. This was achieved by calcining the mixture of plastic wastes and biowastes and the carbonates (calcium carbonate or magnesium carbonate).
The author's previous studies formed the basis for the proposed decarbonization in Portland and Novacem-like cement processing. The author's earlier studies similar to the Novacem process detail the low temperature and the high pressure carbonation (−13 • C and 6 bar) of silicate rich magnesites dumped as wastes and calcination reactions of MgCO 3 to MgO producing CO 2 [49][50][51][52][53]. Novacem production involves carbonation of magnesium silicates under elevated temperature and pressure (180 • C/150 bar). The carbonates produced are heated up to 700 • C to produce MgO, where the CO 2 generated is recycled back to carbonate the silicates [11].
Author's earlier studies were extended by the author to incorporate plastics and biowaste during the calcination of MgCO 3 to MgO, resulting in a great reduction in the carbon footprint and simultaneous production of hydrogen [54]. Author's research on plastic degradation and use of plastics and forestry wastes in materials processing was the inspiration to extend the application of organic waste material to cement processing [29,30,[55][56][57][58]. The author's work on non-soot forming the catalytic ability of calcine intermediates, e.g., the MgCO 3 -MgO system in carbon conversion reactions, the dry reforming reaction to produce syngas and/or enriched hydrogen [59,60], underpins the application of waste organic materials in reducing the emissions in cement making.
The present study adopted the strategies discussed in the earlier works of the author, i.e., the use of organic wastes to decarbonize the calcination reactions of CaCO 3 to CaO (reaction 2) of Portland cements and calcination reactions of MgCO 3 to MgO of Novacemlike cements ("green" alternatives to Portland cement), which, to date, remain high carbon footprint processes. Similar chemistries between MgCO 3 -MgO and CaCO 3 -CaO systems enabled the author to extrapolate the results from one system to the other, accounting for slight differences in calcination temperatures and the amounts of CO 2 released during the calcination reactions. The catalytic ability of these systems is exploited in carbon conversion reactions (dry reforming reactions) to produce syngas and hydrogen during the calcination phase. The authors' study on replacing silica and coke with silicone wastes in ferrosilicone production formed the basis for recommending the use of silicone wastes in place of expensive silica in cement clinker production (reaction 1) [61].

Objectives
This study sought to resolve the high carbon footprint associated with Portland and Novacem-like cements and unsuitable plastic waste management strategies simultaneously. The CaCO 3 -CaO system of Portland cement and the MgCO 3 -MgO system of Novacemlike cements are reported with the overarching aim to minimize emissions, energy, and pollution. As the calcination phase (reaction 2) is the major emitter of global CO 2 , this study aimed to minimize the emission during this phase by introducing waste plastics and biowastes as chemical feedstock. This work did not focus on the use of plastic and bio wastes as fuel sources. Specific objectives included establishing the criteria for suppressing the CO 2 (mainly from calcination of carbonates and gasification of wastes) and CH 4 emissions (from co gasification of wastes) during the calcination and increasing the H 2 generation during the calcination.

Experimental
The experiments involved the study of calcination reactions of CaCO 3 (reaction 2) and MgCO 3 , responsible for the most emissions in cement making, in the presence of plastics and/or biomass. These experiments were designed to establish the criteria for emissions reduction and emission conversions to hydrogen, the source of green energy. The study included monitoring the off-gas composition from calcination experiments.
Pinus radiata was vacuum dried at 80 • C for 2 h and packed to a density of 400 kg m -3 in a furnace. The proximate and the ultimate analyses details of the Pinus radiata and the plastics are given in Table 3 [62,63].
Calcination experiments were carried out in a laboratory setup with furnaces [53,54,61]. Isothermal calcination of CaCO 3 samples was carried out at 1250 • C and 1450 • C in an electricity operated horizontal tube furnace in an inert (argon) atmosphere at a flow rate of 1.0 L/min. The MgCO 3 samples were subjected to isothermal calcination at 1000 • C in an IR image gold furnace and an arrangement of internals for heating. MgCO 3 samples (20 to 50 mg) packed inside the silica tube were introduced at 1000 • C in the middle of a graphite heating element. Helium at~50 mL/min was maintained. Total sulfur/% 0.08

Off-Gas Compositions
A gas chromatographic (GC) analyzer (SRI8610C Chromatograph Multiple Gas #3 GC) configuration equipped with a thermal conductivity conductor (TCD) and a continuous IR gas analyzer were used to measure off-gases, CH 4, and CO 2 periodically for CaCO 3 and CaCO 3 + resin studies. The amounts of H 2 could not be monitored during the calcination studies due to the limitation in the IR used.
The volatiles from the MgCO 3 +resin +biomass system were measured with MTI Activon M200 series micro gas chromatograph (GC) instrument. The thermal conductivity detectors with a 5A molecular sieve column at 60 • C was used to measure H 2 and CO. A Poraplot U column at 40 • C was used to measure CO 2 , CO, CH 4 , C 2 H 4, and C 2 H 6 . The evolution rate was determined as the wt.% of initial Wt. of sample/min.

X-ray Diffraction (XRD)
XRD with a copper Kα source operated at 45 kV and 40 mA and scanned at a step size of 0.026 • and a scan rate of 1 • /min and X'pert High score software were used for phase identification of calcined MgCO 3 with epoxy resin at 1200 • C. Figure 2 illustrates the results from isothermal calcination reactions of CaCO 3 with and without the plastic resin (resin). Figure 2 shows calcination of CaCO 3 (2.36 g) at 1450 • C without the resin (test 1), calcination of CaCO 3 (2.36 g) with the resin (2.37g) at 1450 • C (test 2), and calcination of CaCO 3 (2.36 g) with the resin (2.06 g) at 1250 • C (test 3). Test 2 showed almost no traces of CO 2 but only CH 4 , while test 3 showed reduced amounts of CO 2 and almost equal amounts of CH 4 . These figures show the effects of the resin and the temperatures in suppressing the CO 2 emissions to almost zero at high temperatures during the calcination. In these tests (tests 3 and 4), the resin quantity was kept almost equal or slightly less than the CaCO 3 at 2.36 g. To summarize, calcination of CaCO 3 in test 2 showed more CH 4 and negligible amounts of CO 2 at 1450 • C; test 3 showed almost equal amounts of CO 2 and CH 4 at 1250 • C, demonstrating the effect of temperatures. Additionally, test 2 had slightly higher resin content than in test 3. Both the high temperature and the higher resin content could be responsible for suppressing the CO 2 significantly. Hydrogen content was not measured during these tests. Biomass effect was not studied during calcination of CaCO 3 .      A summary of test details and results of calcination reactions of MgCO 3 ·xH2O and CaCO 3 with various ratios of biomass and plastics and at different temperatures is shown in Table 4, including the experimentally observed (y%) and the expected values for the gas evolution. The expected gas composition was calculated based on the mass% of different components (carbonates, biomass, and resin) present in each sample [54,60]. Cumulative gas compositions determined by GC are shown in Table 5. Resin amount about three quarters that used in test 7; approximately equal amounts of resin, MgCO 3 , and biomass 7

Calcination of MgCO 3
Test 8, Table 4, 1000 • C Note: In MgCO 3 tests, the amounts of resin + biomass was greater than MgCO 3 . All tests were at 1000 • C. High temperatures and high plastic (resin) content favored suppression of CO 2 above 95%, as seen from the test results of tests 5, 6, 7, and 8 (Tables 4 and 5), whereas biomass contributed to less suppression, e.g., up to 82% reduction in CO 2 but up to 230% increase in hydrogen. A higher resin content than the biomass (test 7) during the calcination resulted in CO 2 reduction up to 99%, and in CH 4 , there was a reduction up to 97% accompanied by 360% increase in H 2 compared to the expected value. Test 7 could be an ideal scenario to produce H 2 enriched gas. Resin content approximately equal to or less than the biomass during the calcination resulted in 76% reduction in CO 2 and~63% reduction in CH 4 but 4684% increase in H 2 compared to the expected value (Test 8).

Discussion
Note: biomass was not used in CaCO 3 tests. During calcination of MgCO 3 ·xH 2 O, the effect of temperature was not studied. Another interesting observation was the absence or the negligible amounts of CO, contrary to what was expected.

Low Carbon Portland Cement and Novacem-Like Cement
The calcination phase of the cement production is the most emission intensive process. Attempts to reduce CO 2 emissions during the using waste plastics as the chemical feedstock were never reported before. Cement kilns use shredded waste plastics as fuel but not as chemical feedstock. The current study demonstrated the CO 2 reductions during the calcination reactions of calcium carbonate (Portland cements) and magnesium carbonate (Novacem-like cements) in the presence of resin and/or biomass (Figures 2 and 4-7), indicating the feasibility of using cements/clinkers production as waste plastic conversion facilities. Owing to their similar chemistries, the results from MgCO 3 studies can be extrapolated to CaCO 3 (taking into consideration the higher calcination temperature of CaCO 3 (906 • C), which is close to the iron ore reduction temperatures.
The role of plastics and biomass as feedstock in greatly reducing the carbon footprint of calcination reactions in cement making as well as the conversion of carbon from calcination reaction to syngas/hydrogen are explained in the following sections.

Chemical Feed Stock Recycling of Plastics
Plastics pyrolysis shows two phases, solid carbon and gas, namely CH 4 and H 2 , which are thermodynamically stable at 1100 • C [64]. CH 4 and the solid carbon further undergo catalytic transformation to syngases. The following reactions characterize the chemical feed stock recycling of waste plastics: Plastics decomposition (pyrolysis) results in reaction 3: Pyrolysis product from reaction 3 undergoes methane cracking (greater than 557 • C) (reaction 4): C n H m (g) → nC (s) + H 2 (g); CH 4 = C (s) + 2H 2 ; ∆H = 75.6 kJ/mol (4) Syngas production is governed by the following reactions: The Boudouard reaction (reaction 5,~701 • C): Water gas shift reaction (reaction 6): Water gas reaction or char reforming (reaction 7, greater than 700 • C): C + H 2 O → H 2 (g) + CO; ∆H = 131 kJ/mol (7) Dry reforming reaction (~700 • C in presence of catalysts) (reaction 8): Methane reforming reaction (reaction 9) CH 4 + H 2 O → 3H 2 + CO; ∆H = 206 kJ/mol Reactions 7 (water gas reaction), 8 (dry reforming reaction), and 9 (methane reforming reaction) result in various ratios of syngas. These reactions are endothermic and high temperature reactions requiring catalytic support. The main benefit of CO 2 reforming methane (reaction 8, where CO 2 acts as the oxidizing agent) is, when H 2 /CO is~1, it is suitable for synthesizing oxygenated chemicals, e.g., methanol, acetic acid, aldehydes, ethanol, a wide variety of alcohols, olefins, and gasoline [65]. Oxygenates facilitate easy and safe storage and transport of energy. Methanol mixed with dimethyl ether (DME) is an excellent fuel for diesel engines with a high cetane number and beneficial combustion characteristics. The energy input for the CH 4 dry forming reaction (reaction 8) is 20% higher than the steam reforming (or the methane reforming) reaction 9, resulting in syngas of varying H 2 /CO molar ratios. The drawback of the methane reforming (reaction 9) is that the H 2 /CO ratio 3:1 is greater than that required for the Fischer-Tropsch process.

Chemical Feedstock Recycling of Biomass
Biomass undergoes similar reactions as waste plastics during pyrolysis (refer to Section 5.2). Gases generated during the pyrolysis of biomass are CO, H 2 , CH 4 , and CO 2 ; other products of pyrolysis include H 2 O and char depending upon the ambience [66]. Steam gasification/reduction chemical processes of biomass often occur at temperatures above 700 • C governed by: methane cracking (reaction 4), Boudouard (reaction 5), water gas shift (reaction 6), char reforming (reaction 7), dry reforming reaction (reaction 8), and methane reforming (reaction 9). During the gasification of biomass in an inert environment at 900 • C, cellulose contributes to CO, hemicellulose promotes CO 2 generation, while lignin aids H 2 and CH 4 generation.

Reduction in CO 2 Emissions during Calcination
Calcination of inorganic carbonates in reducing atmosphere (reactions 10 and 11) serves to capture or utilize CO 2 , the chemical H 2 storage system for CH 4 , and the fuels from syngas [66][67][68][69][70]. Plastics and the biomass provide the reductive atmosphere to reduce the CO 2 emissions during calcination. H 2 produced in the reductive calcination can be a means to produce CH 4 or CO/syngas from the CO 2 emitted [71].
The methane cracking reaction (reaction 4) reduces the CO 2 generated during calcination of MgCO 3 and CaCO 3 in the presence of plastic/biomass. Reduced CO 2 emissions (reaction 10 and 12) in the presence of a reductive atmosphere of H 2 and N 2 mixtures was reported [71]. The H 2 and the C, the products of reaction 4, react with MgCO 3 (reactions 10 and 12) and CaCO 3 (reactions 11 and 13), resulting in reduced CO 2 emissions (reactions 10 and 11).
MgCO 3 + C → MgO + 2CO (12) Calcination of magnesite in a reductive hydrogen atmosphere results in decreased CH 4 and increased CO content. Amounts of CH 4 formed in reactions 10 and 11 depend upon MgCO 3 content, i.e., the amount decreases as MgCO 3 content decreases. The CO increases as the MgO content increases. MgO calcined reductively catalyzes the reverse water gas shift (reaction 14), leading to CO generation. This results in reduced CO 2 emissions below 820 • C, which means H 2 increases above 820 • C [71][72][73]. However, reaction 14 was reported to occur above 1000 • C during iron oxides reduction [74].
The reduction in CO 2 emission is greater than the reduction in CH 4 if no carbon deposition occurs during the dry reforming reaction (reaction 8) [42,75].

Methane Conversions
Resin to carbonates ratio during calcination governs the CO 2 /CH 4 ratios. When resin/CaCO 3 is equal to or less than one, CH 4 /CO 2 emission is high (Figure 2). When resin/MgCO 3 ratio is high, both CH 4 and CO 2 emissions are reduced by 94% (test 6, Figure 5). The presence of biomass during MgCO 3 calcination results in higher CH 4 /CO 2 , while the CO 2 is reduced up to 82% (Figure 4).
Increase in CH 4 can be attributed to reaction 3. In total, 100% of the CO 2 from MgCO 3 calcination can be transformed to CH 4 in the presence of H 2 and the catalysts Co/Ca/CoO (reaction 15) [76].
CH 4 conversion (reduction in CH 4 ) at high temp0eratures is ascribed to reactions 4, 8, and 9, leading to H 2 generation (tests 5, 7, and 8). Reduction in CH 4 (reaction 8) depends on CO 2 /CH 4 ratios as well as the temperatures. A high CO 2 /CH 4 ratio (reaction 8) results in high conversion of CH 4 , demonstrating the positive effect of CO 2 as a soft oxidant at temperatures greater than 700 • C.
The dry reforming reaction (reaction 8) requires a cheap and pure source of CH 4 and CO 2 . Pure CO 2 is released during cement production from calcination of MgCO 3 or CaCO 3 , and the CO 2 from pyrolysis of biomass and the plastics ensure CO 2 is greater than CH 4 (reaction 2) (Tables 4 and 5). It should be noted that more CO 2 is released from MgCO 3 (52%) compared to the CaCO 3 (44%) stoichiometrically during calcination. Under the experimental conditions, MgCO 3 calcination can result in sudden copious amounts of CO 2 (calcination temperature~700 • C) compared to that from CaCO 3 (calcination temperaturẽ 900 • C). Increasing the amount of CH 4 (from biomass and plastics) can increase H 2 generation from reaction 4. Figures 6 and 7 show increased H 2 and greatly reduced CO 2 during the calcination of MgCO 3 in the presence of plastics and biomass. It is anticipated CaCO 3 follows a similar trend owing to its similar chemistry to MgCO 3 . This is attributed to reactions 4, 7, 8, and 9 directly contributing to increased H 2 and syngas (H 2 and CO).

Co-Pyro-Gasification of Waste Plastics and Biomass vs. Individual Gasification of Wastes
Hydrogen enriched syngas production is attributed to several factors. Co-pyrogasification of plastics and biomass blends increases the quality and the composition of syngas (H 2 /CO ratio) [66,77]. The present study showed the biomass and plastics blend enhanced the hydrogen generation while reducing CO 2 and CH 4 emissions. Using only plastics greatly reduced the CO 2 emissions with negligible gen-eration of hydrogen; the biomass use only decreased CO 2 emissions to an extent but fa-cilitated the generation of both H 2 and CH 4 (Figures 6 and 7, Tables 4 and 5). Co-pyro gasification of plastic wastes and biomass converts wastes predominantly to gas rather than to char and tar [77].
Increasing the CO 2 promotes a high yield of syngas [73]. CaCO 3 and MgCO 3 , plas-tic wastes and biomass, were the main sources of excess CO 2 in the present study. When steam is present, the water gas shift reaction (reaction 6) shows reduction in CO and increase in H 2 yields [72]. It should be noted that, in the present study, CO was not observed. If the water gas shift reaction 6 is not present, soot formation through me-thane cracking can occur (reaction 4).

Temperature Effects
In this stud, calcination of CaCO 3 at 1450 • C (test 2) showed almost no CO 2 content compared to calcination at 1250 • C (test 3) and increased methane in the absence of any external catalysts (Figure 2). The reductive H 2 atmosphere can lower the calcination temperature by more than 150 • C compared to a non-reducing atmosphere [71]. The sudden spike in CH 4 seen in Figure 2 may be attributed to reaction 11 from increased H 2 .
Increasing the gasification temperature of the biomass usually promotes syngas production while concurrently inhibiting the biochar production [78]. A slight decline in the syngas at temperature above 800 • C is ascribed to the reverse water gas shift reaction (reaction 14). Conditions for high H 2 yield are discussed below.
High calcination temperatures increase H 2 and CO contents, simultaneously decreasing the CO 2 content by facilitating the hydrocarbons cracking (reaction 4) [47]. Reaction 4 favors higher H 2 generation at temperatures above 900 • C [73]. CH 4 formation is favored at low temperature and elevated pressure. There is a decrease in CO content at temperatures above 800 • C. High H 2 yield is attributed to low amounts of CH 4 in reaction 8 and high temperature when CO 2 /CH 4 is high [60].

Char Formation
Char formation has immediate relevance to the endothermic carbon reforming reactions (reactions 8 and 9), which require catalysts. These catalysts also catalyze soot formation (reaction 4) [42] resulting in catalytic fouling, affecting the stability of the catalysts and increasing the costs of the dry reforming process (reaction 8), which hinders its commercialization. Reducing the char formation is important in carbon reforming reactions (reactions 8 and 9).
In the present study, decreasing amounts of CH 4 and CO 2 and hydrogen generation during the calcination of MgCO 3 confirmed the occurrence of carbon reforming reactions (reactions 8 and 9) without the aid of external catalysts. The XRD trace of the calcined MgCO 3 at 1250 • C in the presence of plastics showed no carbon formation (Figure 8). This was attributed to the high temperatures (cement making temperatures) and the high amounts of pure CO 2 generated from the calcination reactions (reaction 2) as well as from the co-pyro gasification of biomass and plastics. Lignin present in biomass contributes to high amounts of char [79]. To suppress biochar production, it is necessary to increase the temperature and the heating rate, which can promote the syngas production [78]. Thermodynamic calculations indicate the required temperature to be 1035 • C for 50% CO 2 conversion in reaction 8 without the catalyst. High CH 4 and CO 2 conversions at temperatures 700 • C require catalytic systems such as metal oxides, monometallic and bimetallic catalysts, and supported metal catalysts [80][81][82]. Steam reforming reactions (reactions 9 and 16) are favored at temperatures above 900 • C and 15-30 atm using nickel-based catalysts. However, carbon fouling of the catalysts is a serious issue.
Steam can eliminate the carbon formed as quickly as its formed. Alkali compounds improve the water gas reaction or the char reforming reaction (reaction 7) at temperatures above 700 • C [83,84]. Though reaction 5, the Boudouard reaction, can be a source of char formation, it does not occur above 700 • C [73]. When CO 2 /CH 4 is high and temperature is above 700 • C, the coke deposition is diminished due to the oxidation reaction of CO 2 with the surface carbon (reaction 5) [84].
Conversion of CO 2 and CH 4 is determined by the ratio of CO 2 /CH 4 and the carbon or the soot formation [42]. Presenting CO 2 to the catalytic dry reforming process (reaction 8) reduces the soot deposition. The CO 2 from the calcination reactions of carbonates during cement making ensures CO 2 is greater than CH 4 , thus reducing the carbon formation (Tables 4 and 5). In the present study, more than 70% CO 2 conversions were achieved without an external catalyst.

MgCO3-MgO and CaCO3-CaO Catalytic Systems Generated In Situ
The hydrogen generation in calcination reactions is governed by the steam (reactions 9 and 14) and the dry reforming reactions (reaction 8). The efficacy of these reactions relies on external catalytic systems, e.g., nickel-based catalysts. A 90% CO 2 conversion was achieved for an MgO promoted catalytic system [75] promoting a partial reduction of CO 2 . In the present study, high conversions up to 99% were realized for both CH 4 and CO 2 , accompanied by H 2 generation without the use of external catalysts (Table 4). This was attributed to the MgCO 3 -MgO and the CaCO 3 -CaO systems acting as catalysts generated in situ.
Freshly prepared MgO and CaO on their own or in combination act as catalysts for the carbon conversion reactions [85]. The catalytic ability of CaO is better than MgO during the biomass conversions [86]. The characteristics that make MgCO 3 -MgO or CaCO 3 -CaO desirable catalysts for carbon reforming reactions include: Lewis basicity, mesoporosity, high reactivity and stability, small crystal size, high specific surface, high adsorption, and reduced carbon formation, promoting both steam forming and dry forming of CH 4 . Lewis bases considerably improve CO 2 reforming of the CH 4 reaction 8, resulting in values higher than the equilibrium values of H 2 . Freshly formed MgO from basic MgCO 3 has a high specific surface, mesoporosity, low bulk density, low crystallite size, and nitrogen adsorption up to 100 cm 3 /g, making it catalytically active [50][51][52]87,88].
MgO calcined reductively catalyzes the reverse water gas shift reaction, leading to decreases in CO 2 (reaction 14). The catalytic effect of CaO increases syngas production from mixed plastic wastes and from the halogenated plastics and the PVC fractions. Lime serves as a passage for fuel and gas and simultaneously binds halogen and other harmful pollutants [38,89]. CaO's catalytic action prevents formation of dioxins and furan and tar containing cleavage products and oil at temperatures greater than 900 • C, hence facilitating the use of halogenated plastic waste streams. CaCO 3-CaO suppresses the release of toxins such as C 6 H 6 and HBr [90]. It was demonstrated that Portland cement making can effectively be treated as a plastic/biowaste and carbon conversion facility without the use of any costly external catalysts.

Syngas Production-Proposed Mechanism
It is proposed that one of the major reactions taking place during calcination in the presence of waste plastics and/or biomass is the reaction between CH 4 from pyrolysis (reaction 3) and MgCO 3 or CaCO 3 to produce MgO or CaO and syngas (reactions 17 and 18). As calcination of carbonates progresses, the CO 2 released reacts with CH 4 , resulting in increased amounts of H 2 and CO (as MgO content increases, the amount of CH 4 decreases, and the amounts of H 2 and CO increase). Hence, it was concluded the MgCO 3 -MgO system or the CaCO 3 -CaO systems generated in situ effectively catalyzed the dry reforming reaction (reaction 8) without coke deposition (Figure 8).
However, CO (Figures 4-7) was not detected in the present study. It is possible that high temperatures, composition of reactants, and CO 2 /CH 4 could effectively suppress CO emissions. It was proposed that the catalytic actions of MgCO 3 -MgO and CaCO 3 -CaO systems not only catalyzed reactions 8 and 9 to produce H 2 /CO but also catalyzed the subsequent conversion of syngas to hydrogen and other, smaller hydrocarbon molecules, which could be building blocks to other useful fuels and chemicals. Composition of the reactants (e.g., MgCO 3 , resin, biomass) controlled the product gas distribution, e.g., as in selective production of H 2 (test 7), or the mixed distribution of CO 2 :CH 4 :H 2 in the product mixture in test 8.

Applications
Potential applications of the present study include extending similar strategies to more problematic materials, such as using halogenated waste materials in Sorel cements and Alinite clinker, and using silicones to replace sand, a costly commodity in Portland cement clinkers, and feedstock recycling of tires as sources of both plastics and biomass in cement making to combat high carbon footprint.

Decarbonising Sorel Cements and Alinite Clinker Using Halogenated Waste Plastics
Developing environmentally safe processes to handle halogenated plastic wastes is vital due to stringent environmental regulations. CaCO 3 and MgCO 3 inhibit the release of toxins such as C 6 H 6 , HBr, and dioxins, enhancing the pyrolysis process [90,91]. Hence, cement making is an ideal platform to repurpose halogenated plastic wastes.

Silicones for Eco-Efficient Clinker Production
Concrete and cement clinker production use a significant amount of sand, the world's second most consumed natural resource [95]. Silicones can replace high pure silica and coke in ferrosilicon production at cement making temperatures [61]. Silicone polymers possess organic and inorganic moieties with valuable resources such as silica, methane, carbon, and hydrogen. The organic moiety of silicones can reduce the emissions, while the inorganic moiety contributes silica. If co-pyro-gasified in the presence of biomass, the carbon emitted can be converted to hydrogen.
SiO 2 from silicones can better replace the sand in cement clinkers in addition to offering similar emission and energy benefits derived from the waste plastics. Use of virgin silicones, siloxanes, and silanes in energy enabling technologies and as energy and materials results in energy savings and greenhouse gas (GHG) emission reductions. The CO 2 emission cuts realized in Japan, North America, and Europe using virgin silicone products amount to~54 Mt/y [96].
A pathway for the direct production of clinker (calcium silicates) from calcite and waste silicones to eliminate the use of silica is shown in reaction 21. This reaction demonstrates reduction in CO 2 and energy consumption and simultaneous production of syngas and H 2 . Waste or virgin silicones can replace silica in reaction 1. CaCO 3 calcined in the presence of silicones (polydimethylsiloxane (C 2 H 6 OSi) (Figure 9)) at cement making temperatures can directly produce clinker (CaSiO 5 ) and syngas with a great reduction in CO 2 emissions (reaction 21) while simultaneously facilitating silicone waste management. CaCO 3 + (C 2 H 6 OSi)n → CaSiO 5 + CO + H 2 (21) Figure 9. Structure of a linear silicone polymer (polydimethylsiloxane).

Tires as Source of Both Plastic and Biomass
The present study indicates the potential use of tires as a chemical feedstock to gain the benefits of emission and energy as well as to use the rubber ash generated in situ during the pyrolysis/gasification as sand replacement in the cement system. Scientists are working on ways to replace sand in concrete with other materials, e.g., rubber tire ash. Rubber tires that comprise both synthetic rubbers (plastics) and natural rubbers (carbon neutral biomass) can be the ideal candidates to reduce the GHG emissions (CO 2 and CH 4 ) and to generate H 2 in cement making, as proposed in the present study.
It should be noted that ash content from biomass and waste plastics is negligible and is unlikely to alter the material properties (ash content from plastics and wood-LDPE, HDPE, PP, and PVC-less than 0.05%; wood 0.45%; rubber tires 5.7%; and coke/coal 18.4%) [97].

Other Industrial Applications
This study has relevance not only to cement industries but also to iron and steel industries (where CaO and MgO are used as fluxes) in regard to dead burned magnesia production, carbothermic reduction of magnesium, carbon conversions, waste valorization, and emission and energy reduction while supporting the hydrogen economy and the generation of precursors for new materials. Use of plastics and biowastes can result in considerable reductions by about 200 • C in reaction temperatures (~1600 • C) during dead burned magnesia (DBM), fused magnesia (FM) production, and carbothermic reduction of Mg. Dead burned magnesia (DBM) currently makes up the largest portion of produced magnesia intermediate products, and there is growing demand and market share for FM [98].

Conclusions
IEA's Sustainable Development Scenario (SDS) aim is to stay below 1.5 • C global warming, by adopting carbon mitigation strategies in cement sector. The findings of present study on decarbonsing the cement production using waste streams as chemical feedstock, and to simultaneously convert the CO 2 produced during cement production to clean energy, are most relevant to the IEA's SDS aim. The cement sector as a potential waste plastics/rubber tires treatment facility to simultaneously meet the emission targets, convert the GHG emissions to hydrogen, and maximize the recovery of resources present in waste materials, e.g., Si, H, CH 4 , and C, was discussed.
The study focused on developing Novacem-like low carbon cements and decarbonizing Portland cements. Use of waste plastics and biomass as chemical feedstock (co-pyrogasification) to reduce the carbon footprint in the calcination step of cement making was demonstrated, which was never reported before. It should be noted the use of wastes as fuel in cement making was not considered in this study. Therefore, emission and energy benefits reported in this study were in addition to the benefits from using the wastes as fuel. Up to 99% reduction in GHG in Portland cement and Novacem-like cements production was established in this study.
The effects of temperature, the ratio of the plastics: biomass: carbonates in controlling GHG emissions, H 2 production, and catalytic ability, and carbon fouling of the calcine intermediates are examined. High temperatures and high plastic content favored suppression of CO 2 more than 95%, whereas biomass contributed to less suppression, i.e., up to 82% reduction in CO 2 but up to 230% increase in hydrogen. A higher resin content than the biomass during calcination resulted in CO 2 reduction up to~99% and CH 4 reduction up to~97%, accompanied by 360% increase in H 2 compared to the expected value. A higher biomass content than the resin during calcination resulted in 76% reduction in CO 2 and 63% reduction in CH 4 but 4684% increase in H 2 compared to the expected value. When CO 2 /CH 4 were high and the temperature was above 700 • C, the coke deposition was diminished, thus preventing the carbon fouling of the catalytic calcine intermediates. Increasing the gasification temperature of the biomass also suppressed biochar formation. It was concluded that the catalytic actions of MgCO 3 -MgO and CaCO 3 -CaO systems not only catalyzed reactions 8 and 9, the carbon conversion reactions to produce H 2 /CO, but could catalyze subsequent conversion of syngas to hydrogen and other smaller hydrocarbon molecules as well.
Use of mixed plastics, including halogenated plastics, silicones, and biomass from the waste inventory as chemical feedstock in cement making was examined. CaCO 3 minimizes the negative impacts of dioxins and toxic emissions from halogenated waste plastics during feedstock recycling and syngas production. The strategies presented in the present study can be applied to Alinite clinkers and Sorel cements production using halogenated plastic wastes with similar emissions and energy benefits.
Recommendations for direct clinker production from silicone/silicone wastes (as sand replacement), solid residues from tires (ash), and silicones (silica) from rubber tires or silicone polymers used as the feedstock can offer emission and energy benefits. They can replace sand in direct production of a cement clinker (CaSiO 5 ).