Introduction

Portland cement is crucial in the development of modern society. The growing need for infrastructure and housing has led to cement production outpacing other construction materials and population growth (Miller et al., 2018; Statista, 2020; United Nations, 2019). In 2020, cement production reached nearly 4.5 gigatons (Statista, 2020), raising environmental concerns once the cement industry accounts for 7-8% of global anthropogenic CO₂ emissions (Scrivener; John; Gartner, 2018). In response to these challenges, the industry has adopted the partial substitution of cement with supplementary cementitious materials (SCMs) (Lothenbach; Scrivener; Hooton, 2011) as a key strategy to mitigate its environmental impact. Each year, the cement sector recycles over 400 million tons of industrial waste or by-products as SCMs, making it one of the largest recyclers in the global economy. Most of these materials come from other industries with limited and their full utilization potential has already been fully explored. Thus far, the cement industry has recycled little of its own waste in cement production and does not have a clear strategy for the circular economy of its own waste.

The literature shows that concrete waste fines can be reused as raw materials for clinker production (Zhutovsky; Shishkin, 2021), filler without any additional procedure (Lima et al., 2024; Wu et al., 2023), filler-pozzolan after enforced carbonation (Teune et al., 2023; Zajac et al., 2020, 2023), or converted into thermoactivated recycled Portland cement (RC) by thermal treatment (Shui et al., 2008; Splittgerber; Mueller, 2003). RC refers to cementitious waste (e.g., cement paste or concrete waste fines) originally made with Portland cement, which regains its binding properties after undergoing thermal treatment, namely herein as dehydration once the target calcination temperatures below 600 degrees release water. The characteristics of RC largely depend on the waste composition and calcination temperature used (Carriço; Bogas; Guedes, 2020). In this context, researchers have adopted laboratory-made Portland cement pastes as the precursor material. A patented process suggests achieving 90 vol% purity in separating waste cement from concrete waste fines (Carriço et al., 2021); other specific mills as Loesche claim almost 70% in mass of recovery of cement waste into fines (Loesche, 2014). When subjected to thermal treatment, hydrated products (e.g., ettringite, other calcium aluminates, portlandite and C-S-H phases) have firstly their free and chemically combined water partially or totally removed (Castellote et al., 2004; Vyšvařil et al., 2014; Real et al., 2020; Angulo et al., 2022; Bogas et al., 2022; Lü; He; Hu, 2008). Two critical temperatures and the associated processes were identified (Alonso; Fernandez, 2004):

1. up to 500 °C, ettringite decomposition, water removal from C-S-H, and partial dehydration of portlandite, which may form quicklime (a high water reactive compound that is not always desirable as a cementitious material); and

2. above 500 ºC, total portlandite decomposition, CO₂ emissions due to calcite decomposition (Zhang; Ye, 2013), and an increasing formation of intermediate calcium silicates (belite-like) as the calcination temperature rises.

Therefore, recycled cement may contain partially dehydrated phases, residual hydrated compounds, and reactive (intermediate calcium silicates) particles, depending on the waste origin, calcination temperature, among other factors (Shui et al., 2009). Further fundamental understanding of cement paste phases and various types of cement wastes after dehydration is still necessary (Xu et al., 2022).

When dehydrated fines are exposed to water again, they rapidly regain chemically bound water (Xinwei; Zhaoxiang; Xueying, 2010; Angulo et al., 2015; Lima Pacheco et al., 2021). RC releases substantial heat during the initial 10 minutes of rehydration, a different behavior from OPC (Angulo et al., 2015). Combined water of C-S-H is recovered (Angulo et al., 2022). The morphologies of the hydrated and rehydrated phases are significantly different (Wang; Mu; Liu, 2018). RC requires more mixing water than OPC to produce a workable mixture (Xuan; Shui, 2011). It is associated with a high surface area and internal porosity, which include thermal cracks during dehydration (Bogas; Carriço; Tenza-Abril, 2020; Zhang; Ye; Koenders, 2013), and a tendency to agglomerate (Yu; Shui, 2013). RC exhibits a rapid and elevated reactivity compared to conventional SCMs (Zanovello et al., 2024). Therefore, recycled Portland cement waste can be a circular economic alternative to the cement industry.

This study aims to address two primary research questions:

1. what is the optimal temperature for the thermal activation (dehydration) of RC to achieve superior mechanical and environmental performance?

2. is the dehydration of cement waste necessary or can its utilization as an inert filler compete effectively with RC?

To address these questions, a literature review was conducted, integrating data from 23 studies to assess the physicochemical, morphological, microstructural, mechanical, and environmental properties of RC under dehydration and subsequent rehydration. In addition to various characterization techniques, this work employed the combined water fraction (cwf) (Abrão; Cardoso; John, 2020) and CO₂ intensity (Damineli et al., 2010) indices to evaluate binder efficiency and environmental performance, respectively. This multifaceted approach offers valuable insights into the engineering of recycled cement and its potential role in a circular economy.

Methods

Figure 1 illustrates the methodology for data collection and analysis employed in this study. We created a database using information from 23 peer review papers (Table 1). Our criteria for selection included dehydration/calcination temperatures up to 1000 °C, a minimum annealing time of 2 hours, and a preference for RC derived from laboratory-made pure Portland cement pastes to minimize variations in the analysis. The references used in each analysis are provided in the figure subtitles. Over 70 references were reviewed, with 23 being suitable for data collection.

This study comprises three main sections. The first section analyzed the decomposition of recycled cements as a function of dehydration/calcination temperatures and the resulting physicochemical changes. Initially, volumetric composition graphs were constructed, considering phase density and pore volume, using Quantitative X-ray diffraction using the Rietveld method (QXRD) and mercury intrusion porosimetry (MIP) techniques. The relationship between the remaining combined water (measured by thermogravimetry – TG) in recycled cement grain and the dehydration temperature was established based on the literature data. The physicochemical characterizations analyzed were density (measured by Helium pycnometry – He Pyc), grain porosity (measured by MIP), and surface area (measured by Nitrogen adsorption and quantified by the BET method, SSA_BET).

The second section established correlations for the kinetics and reactivity of recycled cement dehydrated at different temperatures and ages, using combined water (measured by TG) and heat release (measured by isothermal calorimetry – IC). The water demand relative to OPC was also determined, which was fundamental for calculating the combined water fraction (cwf) index. Compressive strength was evaluated to determine mechanical properties and to establish correlations with cwf and CO₂ intensity, using TG data as input. CO₂ emissions were calculated based on the method proposed by Quattrone, Angulo and John (2014).

In the third section, analyses were conducted for compressive strength, CO₂ emissions, and CO₂ savings, focusing on the prominent results of blended recycled cement reported in the literature.

To account for variations in shape and composition (paste or mortar) reported in the literature, we standardized the compressive strength testing using cylindrical paste samples of 27 × 54 mm. Additionally, CO₂ emissions and intensity were calculated to support the environmental analysis. The mechanical performance of the blended recycled cement was examined.

Graphical data were extracted using WebPlotDigitizer (automeris.io), a web-based tool designed for the precise conversion of visual representations into quantitative datasets.

Volumetric composition of RC grain

QXRD data from Angulo et al. (2022) and Xu et al. (2023b) were improved by converting the diagram to a volume basis and including internal grain porosity. For volumetric conversion, the densities of each phase utilized in the analysis are provided in Table 2 (Balonis; Glasser, 2009). The internal grain porosity followed the trend shown in the ‘Physical and Morphological Changes’ section.

Combined water content and combined water fraction (cwf

Equation 1 was used to calculate the remaining chemically bound water in dehydrated cement.

The chemically combined water (w_n) content was calculated based on De Weerdt et al. (2011).

Where Wx corresponds to the percentage of mass lost (% g/g) at temperature X (°C) by TG analysis, w_n (g/g).

Effective combined water (w_{n effective}) was obtained by Equation 2. This correction is needed because the combined water in the recycled cement as it does not contribute to the reactivity and strength evolution of the rehydrated paste.

Where:

Wn_paste is the rehydrated paste combined water (g/g); and

Wn_cement is the recycled cement combined water (g/g).

The combined water fraction (Equation 3) is an index that reflects binder efficiency by correlating the reactivity (chemical effect) and water demand (physical effect) of Portland cement (John et al., 2019). Moreover, cwf strongly correlates with cement pastes’ porosity and compressive strength of pastes and mortars (Abrão; Cardoso; John, 2020). Zanovello and coauthors showed that those concepts are also valid for recycled cements (Zanovello et al., 2023) when recycled cement grain porosity is considered additionally to the water demanded for the mixture to achieve a given consistency. Part of those pores may become saturated when porous cement paste waste fines are used (Bouarroudj et al., 2021). However, only controlled experiments can more precisely determine their contributions and ameliorate this equation.

Where:

w_{n effective} is effective combined water of the material at the age under analysis (g/g);

w_mix is the amount of water used in the mixture (g/g);

V_RC is the volume of RC in the cementitious material (cm³/g); and

P_grain is the RC grain porosity (%) measured by MIP or estimated by the trend shown in the ‘Physical and Morphological Changes’ section.

Compressive strength standardization

As the shapes of specimens varied in the literature, the compressive strength of the pastes was standardized as cylinder of 27 x 54mm. First, the cubic samples were converted to 27 mm cubes using a scale factor index. For this purpose, a trend derived from Zanovello et al. (2023) using Oliveira et al. (2021) data was utilized (Equation 4).

Where “SF_index” is the scale factor index and “d” is the side of the cube (mm).

After the first conversion, the height/diameter coefficient (h/d_coef) was applied to transform the cubes into a 27 x 54 mm cylinder. For this, the coefficient was considered to be 1.15 (Neville; Brooks, 1987; Yi; Yang; Choi, 2006). Equation 5 was used to obtain the compressive strength of the 27 x 54 mm cylinders (MPa).

CO₂ emissions and CO₂ intensity

The CO₂ emissions of the binders were calculated to assess their environmental impact. A mathematical approach was employed to ensure consistency in process variables such as the furnace, fuel, and accounting (Quattrone; Angulo; John, 2014). For simplicity, the CO₂ emissions from the crushing and grinding processes were excluded in the evaluation of recycled and ordinary cements. To estimate the CO₂ emissions during binder production, three key parameters were used:

1. CO₂ emissions from fuel combustion to heat the system;

2. the release of CO₂ due to the decarbonation process; and

3. CO₂ emissions from supplementary cementitious materials. Together, these parameters provide the total CO₂ emissions per binder (kg CO₂/ t of binder) (Equation 6).

To calculate the CO₂ emissions from fuel combustion for the heating process, several equations are needed: the energy required to heat the system (furnace) (Equations 7 and 8), the mass of fuel needed (Equation 9), and the CO₂ emissions produced (Equation 10). This method follows the approach proposed by Quattrone; Angulo; John (2014). Petroleum coke was used as the fuel in this analysis.

Where:

Qb is the heat required to raise the binder’s temperature (kJ);

C is the binder’s specific heat, fixed at 0.80 kJ/kg°C for OPC and 1.85 kJ/kg°C for RC (Wang, 2016);

M is the mass of material at the start of the process to ensure a ton of binder at the end (kg);

θ_t is the treatment operating temperature (°C); and

θ_amb is the ambient temperature, fixed at 25 °C. The TG result for a conventional OPC paste at 28 days with a water-to-binder (w/b) ratio of 0.50 g/g was utilized to evaluate the m values, see Figure 2.

For ordinary cement, the initial mass of materials used in clinker production includes limestone, clay, sand, and iron ore, with a total fixed at 1600 kg per ton of clinker (Huntzinger; Eatmon, 2009). The mass needed for blended cement was determined proportionally to the clinker content. The operating temperature for clinker production was set to 1500 °C.

For recycled cement, the initial mass of paste required to produce one ton of recycled cement was calculated by multiplying the recycled cement mass (1000 kg) by the mass loss of the predecessor material at 25 °C, divided by the mass loss at dehydration temperature. If SCMs are added after the dehydration process, the mass of recycled cement must be adjusted to reflect their proportion in the final binder composition. Mass loss was determined using TG analysis, and the treatment’s operating temperature was set equal to the dehydration/calcination temperature.

Where:

Es is the heat required by the system (furnace) to heat the binder (kJ); and

η_ƒf is the furnace efficiency, set at 0.55 (Trinks et al., 2003).

Where:

m_fuel is the mass of fuel (kg); and

H is the net heating value of the fuel used in the heating system.

For petroleum coke, this value is 34,200 kJ/kg (Guadagni, 2003).

Where:

CO_{2 fuel} is the amount of CO₂ emitted per ton of binder produced during the heating step (kgCO₂/t of binder); and

CO_{2 factor} is the CO₂ emission factor from burning the fuel, 3.17 kgCO₂/kg fuel.

Equation 11 was used to calculate the CO₂ emissions from the decarbonation process for both clinker and recycled cement. For OPC, 1200 kg of limestone was used as the basis for the calculation (Huntzinger; Eatmon, 2009). In the case of recycled cement, m_limestone was calculated based on the percentage mass loss of the precursor material between 550 °C and its dehydration temperature, multiplied by the initial mass (m). No decarbonation occurs at dehydration temperatures below 550 °C.

Where:

Mx is the molar mass of the component; and

M_CO2 and M_CaC3 are equal to 44 and 100 g/mol, respectively.

Equation 12 was employed to calculate the CO₂ emissions (kg/m³) associated with the production of all cementitious paste raw materials.

Where B_m is the binder mass needed to produce one cubic meter of paste. The binder mass was the sum of the cement and SCM masses. The CO_2paste unit is kgCO₂. m^-3 paste.

For an overall analysis, the CO₂ intensity index was estimated for 300L of paste (Equation 13). This paste volume is commonly used in concrete.

Where the unit of CO₂ intensity is kgCO₂. m^-3. MPa^-1 and f_paste (MPa) is the compressive strength of cement paste (27x54 mm cylinders).

For literature data with blended cements, the CO_{2 SCM} emissions associated with limestone filler, fly ash, granular blast furnace slag, were 8, 42, and 89 kgCO₂/t of SCM, respectively (Miller et al., 2018). For metakaolin, 376 kgCO₂/t (Heath; Paine; McManus, 2014; Jones; McCarthy; Newlands, 2011). The percentage of CO₂ savings compared to OPC was calculated using Equation 14.

Results and discussions

Dehydration of cementitious waste powders

Phases decomposition

Dehydration refers to the loss of water molecules from hydrated cement compounds within specific temperature ranges. This process triggers a series of transformations in the hydrated phases of cement (Alonso; Fernandez, 2004; Farage; Sercombe; Gallé, 2003; Guilge, 2011; Shui et al., 2008).

Figure 3 shows the influence of treatment temperature on the composition of recycled cement during dehydration/calcination from 25 °C to 850 °C. Between 25°C and 300 °C, ettringite breaks down into calcium aluminate compounds and bassanite, while porosity increases from 18٪ to 27٪. There are minimal structural changes within this range (Shui et al., 2008; Zhang; Ye, 2013).

From 300 °C to 500 °C, portlandite decomposes into quicklime (CaO). The dehydration of C-S-H up to 400 °C does not lead to significant changes in crystalline composition, which is consistent with findings reported by Zhang, Ye and Koenders (2013).

However, from 500 °C to 650 °C, more noticeable changes occur. At 500 °C, C-S-H can partially transform into β-C₂S, suggesting that the long silicate chains in C-S-H may have been partially decomposed. As dehydration continues, calcium silicates become increasingly crystalline. Additionally, aluminates and brownmillerite emerge, with porosity in the grain increasing from 35% to 42%.

From 650 °C onwards, the content of C₂S polymorphs increases significantly, along with porosity, which rises from 42% to 55% at 850 °C. Only clinkerization and partial fusion of phases can eliminate the pores. According to Serpell and Zunino (2017), it is predicted that the β-C₂S content increases while α-C₂S and γ-C₂S decreases in the 650–850 °C range. Additionally, the amorphous content decreases, resulting in the near-total water removal.

This gradual dehydration and increase in porosity demonstrate the intricate relationship between temperature, chemical phase transitions, and microstructural changes in cementitious materials.

Physical and morphological changes

The physical and morphological characteristics of RC grains according to treatment temperature are shown in Figure 4. Real et al. (2020) and Angulo et al. (2015) findings demonstrate a strong exponential fit , consistent with data from other studies (Figure 4a). Real’s non-thermoactivated cement (NTC) showed lower values compared to Angulo and Zanovello, likely due to its curing procedure of 7 days underwater and the remainder in open air, versus 28 days at 100% RH. As the dehydration temperature rises, more chemically bound water in the phases is removed, resulting in a denser structure that follows a linear trend (Figure 4b).

During dehydration, water removal from the phases causes thermal cracks (Bogas; Carriço; Tenza-Abril, 2020; Zhang; Ye; Koenders, 2013) and internal porosity, which can be quantified using MIP for the grains, as shown in Figure 4c. Hydrated cement paste (RC0) or NTC exhibits a porosity of 26% (Zanovello et al., 2023). For RCs, the porosity increases to 27%, 35%, and 45% when dehydrated at 300 °C, 500 °C, and 700 °C, respectively. The morphologies of hydrated and rehydrated phases are notably different (Wang; Mu; Liu, 2018). The impact of dehydration on the morphology of RC grains is assessed by measuring the specific surface area using BET analysis (Figure 4d). Zhang, Ye and Koenders (2013) and Gholizadeh-Vayghan et al. (2024) reported distinct trends, with literature data showing closer alignment to the trend presented by Zhang, Ye and Koenders (2013). No significant changes in SSA are observed between 200 °C and 400 °C, as no major chemical transformations happen in this range. However, from 400 °C to 600 °C, SSA increases due to the decomposition of C-S-H and portlandite. Above 600 °C, decarbonation leads to a decrease in SSA, or even partial fusion of silicate phases. Further studies are needed for clarification.

Based on the aforementioned characteristics, RC has a higher water demand compared to Portland cement.

Rehydration of cementitious waste powders

Kinetics and reactivity

The rehydration of RC shows different behavior compared to OPC hydration. Several researches observed significant heat release peaks within the first few minutes of hydration for 450-550 °C (see Figure 5a) (Angulo et al., 2015; Baldusco et al., 2019; Xu et al., 2023a; Zanovello, 2023). This initial peak is about 8-10 times greater than the acceleration peak observed in OPC. The early peak is linked to the wetting and rapid reformation of dehydrated phases. Wang, Mu and Liu (2018) and Xu et al. (2023a) evaluated the heat release during calorimetry for RC produced at different dehydration temperatures. The heat release magnitude in the first minutes followed the order: 550 °C > 450 °C > 650 °C > 750 °C > 850 °C > 1100 °C, corresponding to the treatment temperatures used in producing RC. Dehydration at temperatures above 650 °C resulted in a noticeable reduction in initial reactivity.

Angulo et al. (2022) and Serpell and Zunino (2017) demonstrated a strong correlation between heat release and combined water content (or compressive strength for a fixed water-to-binder ratio). Figure 5b shows that this exponential correlation is valid across various literature data, considering different dehydration temperatures and (re)hydration times (ages), based on the effective combined water content.

To understand in-depth the reactivity of recycled cement, we reviewed three different sources (Angulo et al., 2022; Wang; Mu; Liu, 2018; Xu et al., 2023a) that examined various dehydration temperatures and used OPC as a reference binder (See Figure 5c). The results show that RCs dehydrated at around 500 °C exhibit higher reactivity than OPC for up to 12 hours. However, after 24 hours, OPC presented higher reactivity than all recycled cement studied. Differently, Wang's data revealed that RC450 maintained reactivity levels comparable to or exceeding those of OPC from 6 hours to 28 days (Wang; Mu; Liu, 2018). RC dehydrated below 300 °C exhibited negligible reactivity, suggesting that non-thermoactivated cement (i.e., hardened cement paste powder) is an inert material that does not significantly contribute to strength development. Additionally, RC750 and RC850 exhibited different reaction patterns, showing increased water reactivity between 7 and 28 days.

Figure 5d illustrates the effective combined water content of RCs dehydrated at various temperatures during the early ages of 1 and 7 days. The results indicate that the highest initial reactivity occurs in RC dehydrated at 450 °C and 550 °C (RC450, RC500, and RC550), potentially due to the higher surface area in this range. This elevated reactivity at early ages is a unique property for RC’s application as an SCM.

A benchmark of effective combined water at 28 days (Figure 5e) reveals that RCs dehydrated at higher temperatures tend to combine more water at this age. Xu et al. (2023a) and Real et al. (2020) reported similar linear trends starting at 400 °C. Furthermore, Zanovello et al. (2023) demonstrate that using a dispersant to prevent agglomeration allows RC to combine more water, increasing the effective combined water content by 55% (from 0.11 to 0.17g/g).

Water demand

Figure 6a illustrates the relationship between the water demand of RC and OPC for the same consistency. The relative water demand increases as a function of treatment temperature. The cement waste without thermal treatment (0 °C) requires 30٪ more water than OPC. From 400 °C of dehydration, the water demand becomes twice that of OPC, reaching 2.5 for RC treated at 800 °C. The main factor associated with this behavior is the internal porosity of the RC grain.

Figure 6b illustrates the logarithmic relationship between the literature data trends of RC grain porosity (Figure 4c) and relative water demand (Figure 6a). Although the shape factor could potentially serve as an index to correlate water demand and grain morphology and porosity, grain morphology was not explored due to the absence of necessary data in the literature (SSA by BET and granulometry by laser diffraction).

Combined water fraction (cwf) and Mechanical performance

Figure 7a presents literature values for cwf of RC produced at various treatment temperatures, with compressive strength results measured or converted for cement pastes in cylindrical samples of 27x54 mm. A strong linear correlation was observed, confirming that cwf is valid for all RCs. The highest binder efficiency reported in the literature was for RC500, achieved by reducing water demand through dispersant use (Zanovello et al., 2023). The highest compressive strength was obtained by RC450 from Wang; Mu; Liu (2018). This corroborates with the trend observed for conventional Portland cements (Abrão; Cardoso; John, 2020) and that RC respects the strength-porosity correlation as presented by Zanovello et al. (2023).

Figure 7b shows the chemical (28d-effective combined water, Figure 5e) and physical (water demand – Figure 6a, considering a w/s OPC of 0.30g/g) effects of RC considering a standard consistency paste, both variables are used in cwf determination. Thermoactivation up to 100 °C does not contribute to reactivity and requires a higher water demand (considering internal grain porosity) than OPC (1.6 to 1.8 times).

Figures 7c and 7d illustrates a curve model based on the literature trend for cwf and 28d-compressive strength as a function of dehydration temperature. This trend reflects RC pastes with similar workability and no use of dispersants. The results suggest that cwf and strength stabilize beyond 500 °C, indicating that RC500 to RC1000 mixtures exhibit similar binder efficiency and compressive strength when prepared with the same consistency. Without water demand reduction, the maximum cwf predicted by the trend is 0.17 g/g for RC700. For literature RC pastes with reduced water content, binder efficiency was improved by 30٪ (0.22 g/g) (Zanovello et al., 2023).

Environmental analysis

Table 3 presents the CO₂ emissions calculated for RCs submitted to different dehydration temperatures and OPC.

Figure 8a presents the model curve associated with cwf and compressive strength trends (Figure 7a) and the CO₂ emissions for 300L of paste. The compressive strength is capped at maximum compressive strength for the RCs dehydrated in the temperature range of 550 °C to 900 °C. A stabilization mechanism similar to that observed for cwf is evident for CO₂ emissions. A similar stabilization mechanism to the cwf is seen for CO₂ emissions, expected due to the linear trend between cwf and strength.

Figure 8b shows the relationship between CO₂ intensity and compressive strength. The optimal environmental performance (i.e., minimum CO₂ intensity) was presented by RC 300, 2.4 kgCO₂. m^-3. MPa^-1. However, RC 300 presents a limited compressive strength, which affects the possibilities of replacement in blended cement. Therefore, considering environmental and mechanical performance, RCs dehydrated between 400 and 600 °C presented the best results.

Blended recycled cements

The combination of RC with SCMs and OPC is an optimization strategy used in the literature. Figure 9a shows the relative compressive strength of neat and blended RCs compared to OPC, based on the proportion of clinker replaced by RC and SCM. The RC-based materials analyzed had the same water-to-solids ratio as the OPC. The dashed line represents the dilution line of the cement, indicating a decrease in strength as the proportion of clinker is replaced by an inert material. NTC and RC120 performed below the dilution effect due to their high specific surface area and inability to combine water effectively. RC450, RC500, and RC700 showed performance above the dilution effect (Carriço et al., 2022; Wang; Mu; Liu, 2018).

Binary mixtures of RC and ground granulated blast-furnace slag (GBFS) demonstrated remarkable performance, achieving between 76% and 109% of the OPC strength. Metakaolin (MK) also helped NTC surpass the dilution line, equating its performance with that of RC700. Zanovello (2023) developed an engineered recycled cement by incorporating micronized Portland cement and implementing a water reduction strategy based on rheological principles and dispersant use. This approach achieved standard consistency (100 mm mini-slump) with similar performance to OPC. Most literature studies do not consider consistency, which can result in poor workability and artificially higher strength values. Additional studies with fixed consistency and variable water-to-binder ratios are needed to fully understand the performance improvements from blending RC with SCMs and OPC. Blended recycled cements represent a promising circular economy pathway. The synergy between RC and materials such as slag or pozzolans improves porosity refinement and enables the development of engineered recycled cements. The best results have been achieved with RC and slag.

Figure 9b shows a trend similar to that of Figure 9a, highlighting the significant contribution of clinker to the CO₂ emissions of binders. Only RC with slag, neat RC450, neat RC750, and NTC + MK exceeded the dilution curve, with the engineered RC achieving the same strength as OPC while reducing CO₂ emissions by 55% compared to OPC. RC + GBFS achieves even greater CO₂ savings. However, the global availability of slag is insufficient to support the levels used. While acknowledging the critical role of locality in sustainable development, the results remain highly relevant. Nevertheless, concerns regarding the scalability of the technology must be addressed, particularly considering the declining availability of SCMs like fly ash and slag. Exploring alternative and more readily available SCMs, such as metakaolin, fillers, and calcined clay, will be essential for the production of blended recycled cements.

Conclusions

This work evaluates thermoactivated recycled Portland cement subjected to various dehydration/calcination temperatures. A literature review was conducted, encompassing 23 references. The study analyzed the physical-chemical characteristics of RC grains, combined water (reactivity), water demand, binder efficiency (cwf index), environmental impact (CO₂ emissions), and performance (CO₂ intensity index) of rehydrated RC pastes. The findings provide valuable insights into RC engineering and the development of mix designs. The following conclusions were drawn:

1. the recycled cements dehydrated between 450 °C and 550 °C demonstrated the highest levels of effective combined water content at 1 and 7 days. This high reactivity of RC at early ages makes it particularly advantageous for use as an SCM. The effective combined water content at 28 days increased as the RC was subjected to higher treatment temperatures;

2. when modeling cwf and dehydration temperature for mixtures with same consistency at 28 days, it is observed that RC dehydrated between 500 °C and 1000 °C exhibits similar binder efficiency and compressive strength, indicating stabilization. Considering both environmental and mechanical performance, the recycled cements dehydrated between 400 °C and 600 °C presented the best results while minimizing additional CO₂ associated with decarbonation;

3. non-treated cement (ground-hardened cementitious material) lacks reactivity and requires 30% more water than OPC. Using untreated cement as a filler increases CO₂ emissions or reduces mechanical performance. Its mechanical performance, even when combined with pozzolans, is comparable to the dilution effect;

4. engineered RC achieves significantly higher performance due to the combined water ability for almost the same surface area of non-treated cement waste, highlighting dehydration as a superior approach by restoring binder capacity. Blending RC with SCMs and/or OPC improves performance by refining pore size and reducing porosity, making it a promising strategy for a circular economy.

The authors emphasize the need to delve into the following points:

1. engineering blended recycled cement binders for a standard consistency;

2. integrating recycled cement with emerging technologies such as carbonated concrete fines;

3. evaluating the impact of real waste sources, including concrete waste fines and slurry from the ready-mix industry; e.g. the dilution effect caused by aggregate impurity in the powders;

4. durability concerns of RC-based cementitious materials; and

5. deepening the understanding of the fundamental science behind thermoactivation.

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