ABSTRACT:

In this study, low-carbon recycled cement (RC) was produced from thermally activated concrete and cement paste wastes and its incorporation in mortars, as cement replacement, was investigated. The calcination temperatures were defined based on thermogravimetric and X-ray diffraction analysis. Mortars were characterized in terms of their main fresh and mechanical properties (compressive and flexural strength, ultrasonic pulse velocity and dynamic modulus of elasticity). The influence of RC replacement ratio (20%, 50%, 75% and 100%), RC treatment temperature and RC fineness were analyzed. In all tested properties, RC from concrete waste showed significant lower efficiency than that from cement paste waste. The RC rehydration capacity was improved by increasing its fineness and preheating temperature. Mortars produced with 100% of 650 pre-heated RC were able to attain as high as 8.3 MPa of compressive strength at 28 days, corresponding to about 20% of that found in reference mortars without RC. In addition, mechanical properties were little affected by RC replacement ratios up to 20%. This study shows the efficient rehydration capacity of thermal activated RC and suggests its high potential for the construction industry.

ABSTRACT:

In the last decades, the growing concern with sustainability and the emergence of new construction techniques, namely mechanical and chemical stabilisation, to improve their performance and address some of their main issues (reduced water resistance), has brought a new momentum to earth construction (Riza et al., 2010). Chemical stabilisation of compressed earth blocks (CEB) is usually achieved with the incorporation of hydraulic binders, namely ordinary Portland cement (PC). Though it is effective in improving the overall performance of CEB (Walker, 2004; Riza et al., 2010), PC is a carbon intensive building material, associated with the consumption of large quantities of natural resources and energy, as well as, with the emission of copious amounts of carbon dioxide and other harmful green-house gases (Real et al., 2022).

Recently, a new promising binder has been developed as a more eco-efficient alternative to PC, thermoactivated recycled cement (RC), which is produced from recycled construction and demolition waste materials subjected to thermal activation at lower temperatures than PC (about 600-700ºC), thus, not only avoiding natural resource consumption and recycling waste, but also having reduced CO2 emissions during production (Real et al., 2022). RC has been successfully incorporated in other building materials, such as pastes, mortars and concrete (Real et al., 2020; Carriço et al., 2022; Real et al., 2021; Carriço et al., 2021). However, the effect of its use as a stabiliser on the performance of CEB has yet to have been assessed. Therefore, this study aims to analyse the influence of the incorporation of RC in the thermal conductivity of CEB. To this end, CEB were produced with different types of RC (recycled cement produced from cement paste waste treated at 650ºC) and PC and amounts of stabiliser (5-10%), as well as unstabilised CEB (UCEB). The tests were performed using through a modified transient pulse method (ASTM D5334, 2014; ASTM D5930, 2009), resorting to an ISOMET 2114 heat transfer analyser with a surface probe. The CEB were tested under varying moisture conditions, namely in laboratory conditions and in the dry and saturated states.

The thermal conductivity of the CEB in the saturated state was more than twice as high as those in the dry state, displaying a significant loss of thermal insulation capacity. In fact, the effect of the moisture content was more relevant to the thermal conductivity than other parameters, such as the type and amount of stabiliser. The CEB stabilised with RC displayed lower thermal conductivities than CEB stabilised with PC or UCEB. This should be due to the higher porosity associated to the CEB with RC, which were produced with a higher amount of water to account for the higher water demand and porosity of this stabiliser.

ABSTRACT:

Compressed earth blocks (CEB) is a technique that allows to improve earth performance, due to the action of high-pressure compaction, increasing the cohesion between particles and their bear strength (Cid-Falceto et al. 2012, González-López et al. 2018). In addition, to overcome the high variability and low integrity of earth, especially in humid environment, chemical stabilization is absolutely necessary (Mansour et al. 2016). In this regard, ordinary Portland cement (OPC) is the most efficient stabilizer, ensuring adequate mechanical and durability performance of CEB for an incorporation amount of 5-10% by weight of cement (Reddy 2012). However, the use of up to circa 200 kg/m3 of OPC significantly increases the embodied energy and environmental impact of CEB, eliminating the green nature of earth construction. In fact, the clinker production is responsible for over 7% of the CO2 world emissions (Sousa and Bogas 2021).

In this context, alternative low-carbon recycled cement (RC), retrieved from concrete waste, has been explored in Instituto Superior Técnico, University of Lisbon (IST-UL) during the last 4 years. The idea is to recover the binding properties of hydrated cement waste, through its thermoactivation at 600-700ºC (Carriço et al. 2020), and obtaining for the first time a hydraulic binder that is 100% recycled and 100% circular. RC directly contributes to the reduction of CO2 emissions, the saving of natural resources and the effective reuse of construction and demolition waste (C&DW). Moreover, it has been demonstrated the high rehydration capacity of RC and its comparable performance to OPC when incorporated in cement-based materials, namely in mortars and concrete (Carriço et al. 2020b, Bogas et al. 2020, Carriço et al. 2021).

Therefore, this study explores, for the first time, the use of RC as a more eco-efficient solution for CEB stabilization. In this case, CEB produced with different contents (5%, 10%) and types of stabilizer (100% OPC, 100% RC, and blended binders with partial replacement of OPC with 20% or 50% RC) were characterized in terms of their main physical and mechanical properties, namely density, compressive strength, splitting tensile strength, flexural strength, ultrasound pulse velocity, modulus of elasticity, pendulum sclerometer, elasticity modulus, shrinkage and thermal conductivity (Figure 1). CEB were characterized considering different curing (air and wet curing) and wetting conditions (dry, lab conditions, saturated). Unstabilized CEB were also considered for comparison purposes, allowing a better assessment of the potential contribution of RC as a stabilizer.

In general, for the analysed properties, although the performance of CEB decreased with the replacement of OPC with RC, the improvement was significant in relation to unstabilized CEB, especially in humid conditions. The lower performance of RC than OPC in stabilizing the CEB was attributed to the development of a lower amount of long-term hydration products and, especially, to the higher initial total porosity of CEB. The greater water demand of RC led to a higher amount of mixing water, increasing the water/binder ratio and the total porosity of CEB, which was up to 13% higher than that of OPC CEB. Therefore, under lab conditions of intermediate humidity, RC CSEB showed 25% less compressive strength and 24% less modulus of elasticity than reference OPC CSEB with equal amount of stabilizer. However, the reduction of compressive strength was less than 15% for up to 50% replacement of OPC with RC. A good correlation was found between compressive strength and other mechanical properties, showing the same trend regarding the influence of the type and amount of stabilizer. Long term shrinkage was lower in RC CEB of greater porosity and lower stiffness than in OPC CEB, but early shrinkage was similar in both CEB. The thermal conductivity was up to 26% lower in RC CEB due to their higher total porosity than OPC CEB. Under extreme saturation or drying conditions, the compressive strength reduction in RC CEB was 43% and 12%, respectively, underlying the lower stabilization capacity of RC than OPC.

Nevertheless, RC was an efficient stabilizer, leading to a significant improvement of tested properties, compared to unstabilized CEB. Stabilization with 10% RC was able to almost double the mechanical strength of unstabilized blocks, under lab conditions, and to ensure the integrity of CEB under saturation conditions, for a compressive strength over 2 MPa. Unstabilized CEB were highly sensitive to water, degrading fast after saturation.

ABSTRACT:

This study aims at characterizing the mechanical and shrinkage behaviour of mortars produced with different percentage replacement of Portland cement (PC) with Recycled cement (RC) retrieved from unrealistic lab-made old cement pastes (RCP) or directly obtained from hardened concrete waste (RCC). RCC was obtained from a new patented method of concrete waste separation, which allows recovering cement waste with circa 75 wt% purity. RCP was considered to better analyse the RC behaviour, without the influence of other contaminating constituents. Mortars were produced with bout 5, 15, 30, 50 and 100% RCP and up to 30% RCC. Due to the high RC water demand, they were designed with similar flowability or water/binder (w/b). Then, mortars were tested in terms of fresh properties (flowability, density), compressive strength, capillary absorption and drying shrinkage. Compared to reference PC mortars of equal w/b, the reduction of compressive strength in mortars with up to 100% RCP was lower than 25%. Moreover, the compressive strength was not significantly altered for up to 15% RCP. For the same consistency, mortars with RCP and higher w/b showed higher capillary absorption and higher shrinkage. However, for the same w/b, the absorption coefficient decreased with RCP content, because the interparticle porosity was lower than that of PC mortars. On the other hand, even for the same w/b, shrinkage increased with the replacement of PC with RCP. This was attributed to the denser interparticle microstructure of RC mortars and the higher porosity and lower stiffness of RC particles. Nevertheless, for up to 50% RCP, the 90 days shrinkage increased less than 17%. The replacement of the same amount of RCP with up to 30% RCC did not significantly affect the capillary absorption and slightly reduced compressive strength and long-term shrinkage. Overall, it was demonstrated the good efficiency of the new separation method and the high potential of RC as a supplementary cementitious material of high eco-efficiency.

ABSTRACT:

Amongst the many challenges faced by mankind nowadays, excessive consumption of non-renewable resources and pollution are two of the most relevant. These two challenges are, in many cases, directly connected, with the consumption of non-renewable resources being, in most cases, also a source of pollution. One of such cases is the construction industry in general, and the concrete industry in particular. With an estimated consumption of over 30 billion tons per year, concrete ranks second in terms of the most consumed materials in the world after water (WBCSD 2009). This means that it is the most consumed artificial material worldwide, implying the consumption of large amounts of non-renewable resources in its production. In addition to the pollution during the production stage, concrete makes up a significant portion of the more than the 3 (Akhtar and Sarmah 2018) to 10 (Wu et al. 2019) billion tons of construction and demolition waste generated annually. The energy consumption of the cement industry represents 2% of global primary energy consumption or about 5% of total global industrial energy consumption (Hendriks et al. 1998). At the same time, the resulting carbon emissions makes cement production the third largest source of anthropogenic emissions of carbon dioxide (CO2), after fossil fuels and land-use changes, being responsible for 8% of the total emissions worldwide (Andrew 2018). Carbon emissions are, therefore, one of the most significant, if not the most significant, environmental impact directly associated with cement production and, indirectly, to the use of concrete (BIO 2011). Recycling cement from construction and demolition waste allows tackling both these challenges simultaneously by creating a closed-loop-recycling (ECRA 2017), contributing significantly towards the circular economy plan devised in the EU (EC 2020). In Europe, this solution would also contribute towards: i) meeting the goal of reusing, recycling or recovering a minimum of 70% (by weight) of non-hazardous construction and demolition waste, excluding naturally occurring material (article 11.2 of the Waste Framework Directive (EC 2008)); and ii) meeting the green deal targets (CEMBUREAU 2020) by the partial or total replacement of Portlant clinker by recycled cement (RC). This research effort builds on an ongoing research line carried out at IST, in particular the energy consumption necessary for RC production assessed by Sousa and Bogas (2021). The results obtained reveal that the need for washing and drying the material before separating the cement paste from the aggregates is responsible for the consumption of the largest portion of the total thermal energy required for producing recycled cement. As such, alternative solutions were explored to avoid the need for this washing and drying, namely by using air to remove the dust from the particles and increasing the power of the magnetic separator. The results presented in tTable reveal that the energy consumption of the RC air-blowing cleaning production method is roughly 25% of the RC wet cleaning production method and the emissions even less.

ABSTRACT:

Stringent targets have been defined by the concrete industry in order to reduce the greenhouse gas emissions, the concrete waste disposal and the excessive consumption of natural resources. Among other strategies, the reuse of concrete waste and the development of alternative cements have been focus of intensive research, towards more sustainable concretes. In this regard, relevant research have been carried out in the department of civil engineering of Instituto Superior Técnico, aiming the development of more eco-efficient concrete produced with recycled cement (RC) retrieved from hardened concrete debris. In the last decade various investigations have been conducted concerning the mechanical characterisation of pastes or mortars with RC obtained from laboratory cement waste pastes. However, the characterisation of concretes produced with recycled cement, as well as the retrievement of recycled cement directly from concrete waste, have been barely studied. This communication presents the recent findings on the production and behavior of concrete with recycled cement obtained from concrete waste (RCC) and from cement paste waste (RCP). RCC was obtained from the cement fraction of concrete waste according to a novel separation method patented by the authors. With this method, the cement fraction can be recovered with nearly 90 vol% purity. The consideration of RCP allows the more accurate analysis of the maximum potential of recycled cement. The concretes were characterised in terms of mechanical strength (compressive and tensile strength), modulus of elasticity, shrinkage, transport properties (oxygen permeability, capillary absorption), chloride migration and carbonation resistance. Concretes were produced with different w/b (0.35- 0.65), and 5 to 100% replacement of Portland cement (PC) with RC. In general, the mechanical strength was only little affected by the RC incorporation, showing a maximum strength reduction of only 17% for 100% replacement of PC with RC. Due to the high water demand of RC, the addition of super plasticizer (SP) greatly improved the RC efficiency. The addition of SP had a significant effect on the dispersion of cementitious particles and in improving the concrete compactness. Concrete with RC had comparable durability to that of PC concrete, regardless of the RC content. Durability and mechanical properties were more affected by the w/b than by the type of binder (RC or PC). Moreover, RC may refine the microstructure due to the reduction of the interparticle space. However, the total open porosity tends to be lower in PC concrete. A significant improvement was found in the mechanical and durability performance of RC concrete compared to that with the same content of non-treated concrete waste or commercial filler. RC proved to be a very effective supplementary material, reducing the carbon footprint of concrete without significantly affect their mechanical and durability proprieties. Concrete with RCC showed similar mechanical and durability behavior to that with RCP, for up to 30% incorporation. Concrete with only RC could be produced with strength class C25/30 and similar to higher durability of PC concrete of the same strength class. This indicates that 100% recycled concrete can be attained towards a truly circular economy of the concrete industry.

ABSTRACT:

In the present study, concretes produced with recycled cement (RC) obtained from the thermoactivation of concrete waste (RCCW) or laboratory cement paste (RCPW) were analysed in terms of some of their main transport properties (capillary absorption, oxygen permeability) and carbonation and chloride penetration resistance. Recycled cement was incorporated at various percentages, between 5 and 100%, taking into account concretes of different w/b. RCCW was obtained from an innovative process of concrete waste separation recently patented by the authors, allowing the retrievement of cement waste with almost 90 vol% purity. RCPW and RCCW showed high rehydration capacity and concrete produced with them reached comparable durability to that of reference ordinary Portland cement (OPC) concrete. For up to 15% RCCW replacement the concrete durability was not significantly affected and the cement retrievement from concrete waste was very effective. Overall, RCPW and RCCW actively contributed to the densification of the microstructure and the improvement of concrete durability. The recycled cement, as obtained in the present study, showed high potential to be used as an eco-efficient clinker substitute.

ABSTRACT:

The concrete industry is currently facing the serious challenge of reducing its significant environmental impact, regarding the extensive extraction of raw materials, large generation of construction and demolition waste and relevant CO2 emissions. In this context, the low-carbon thermoactivated recycled cement (RC) has been emerged as a potential lever for the mitigation of these 3 vectors. However, due to the early stage of research in this domain, most studies have been only focused on the production of RC and its incorporation on cement pastes or mortars, without exploring its application in concrete production. In this study, it is characterized the mechanical behaviour of concrete produced with RC, either from waste cement paste or old concrete. To this end, a comprehensive experimental campaign involving the fresh, physical and mechanical characterization (compressive and tensile strength, modulus of elasticity) of concrete with RC and water/binder between 0.35 and 0.65 was carried out. It was found that the mechanical strength is little affected by the increase replacement of ordinary Portland cement (PC) with RC. Even considering 100% incorporation of RC, the mechanical strength was over 83% of that of the reference PC concrete with the same w/b. The incorporation of SP greatly improved the RC efficiency. Moreover, up to 15% incorporation of RC the mechanical strength of concrete was slightly improved, without either compromising its workability. From this study, it is shown the high potential of RC as an effective supplementary cementitious material. Up to 30% replacement, recycled cement directly retrieved from waste concrete according to a new patented method of the authors showed to be almost as effective as RC from waste paste.

ABSTRACT:

The cement industry is presently facing the demanding challenge of reducing its large amount of carbon emissions in order to meet the targets set to fight climate changes. One recent, and very promising, approach to reduce the carbon footprint is the production of more eco-efficient recycled cement from cement-based waste materials. This study aims at comparing the difference in terms of energy consumption and carbon dioxide emissions between recycled cement and conventional clinker production. The results demonstrate that overall carbon dioxide emissions, considering both direct and indirect emissions, of the recycled cement are 23% lower than the conventional clinker cement.

ABSTRACT:

This paper discusses the effect of the thermal activation temperature on the rehydration of recycled cement and mechanical strength of pastes produced with this more eco-efficient binder. For this purpose, various tests were performed in order to characterise the thermoactivated recycled cement, namely thermogravimetry, X-ray diffraction, and 29Si nuclear magnetic resonance spectroscopy, as well as water demand, setting time and isothermal calorimetry. The mechanical strength of the recycled cement pastes was also determined. Overall, the dehydration and hydration of the thermoactivated recycled cement were significantly affected by the thermoactivation temperature, leading to the formation of distinct C2S polymorphs. The compressive strength of the thermoactivated recycled cement pastes was influenced by the different reactivity of these polymorphs. These binders presented longer setting times and their pastes displayed lower compressive strength than those with the reference CEM I. This was essentially attributed to the high water demand and pre-hydration phenomena exhibited by the thermoactivated recycled cement. The optimal thermal activation temperature was 700 °C.

ABSTRACT:

Este estudo analisou a influência da incorporação de cimento reciclado (CR), resultante da ativação térmica de resíduos cimentícios, no comportamento mecânico de betões. O cimento reciclado foi obtido de pastas de cimento hidratadas e da fração cimentícia de betão recuperada através de um novo método de separação. Estes resíduos cimentícios foram posteriormente sujeitos a um tratamento térmico a temperaturas muito inferiores às necessárias na obtenção de clínquer a partir de matérias-primas tradicionais. Foram analisadas diferentes percentagens de incorporação de CR (0-40%) em substituição de cimento Portland. Os ensaios abrangeram a resistência à compressão, o módulo de elasticidade, a velocidade de propagação de ultrassons e a retração. De um modo geral, a resistência mecânica e a velocidade de propagação de ultrassons não foram significativamente influenciadas pela incorporação de CR. O módulo de elasticidade diminuiu e a retração teve tendência a aumentar para teores elevados de CR. Até 15% de incorporação o cimento reciclado obtido da fração cimentícia de betão apresentou um comportamento semelhante ao cimento reciclado obtido de pasta.

ABSTRACT:

O cimento reciclado (CR) é um novo ligante de baixo carbono obtido a partir da ativação térmica a baixas temperaturas de resíduos de materiais cimentícios hidratados. Este trabalho teve como foco a absorção e a retração de argamassas produzidas com CR proveniente de resíduos de pastas de cimento e betão de composição conhecida. Foram também analisadas diferentes percentagens de incorporação e a influência da finura e composição do material precursor. Adicionalmente, o efeito da combinação de cinzas volantes com CR em argamassas também foi analisado. Para tal, foram avaliadas as principais propriedades no estado fresco, a resistência à compressão, a absorção capilar e a retração por secagem. Devido à elevada exigência de água destes novos ligantes, foram produzidas argamassas de igual espalhamento e de igual a/c, para possibilitar a comparação em misturas de igual trabalhabilidade e de igual composição, respetivamente. Em argamassas de igual trabalhabilidade, os CR analisados neste estudo apresentaram uma exigência de água superior para atingir a fluidez pretendida, tendo-se verificado um crescimento linear com o aumento da percentagem de incorporação de CR. A mesma tendência foi observada em argamassas de igual a/c onde o espalhamento reduziu cerca de 100 mm nas argamassas com 50% CR face às argamassas de referência. A resistência à compressão de argamassas com 30% de CR de igual trabalhabilidade e igual a/c foi 70% e 90% da das argamassas de referência, respetivamente. Para 100% de CR verificou-se um decréscimo de 20% na resistência à compressão de argamassas de igual a/c. Para argamassas de igual composição, o coeficiente de absorção capilar das argamassas com 100% CR foi inferior ao das argamassas de referência. Para tal terá contribuído a natureza porosa das partículas de CR que, para um mesmo a/c, ao absorver parte da água de mistura permite que haja uma maior proximidade entre partículas adjacentes, contribuindo para a densificação da microestrutura. A retração por secagem das argamassas com CR foi superior à das argamassas de referência e principalmente influenciada pela menor dureza das partículas de CR e pelas ligações entre produtos de hidratação mais fracas do que as desenvolvidas numa matriz de cimento Portland Tipo I.

ABSTRACT:

O cimento reciclado (CR) é um novo ligante de baixo carbono obtido a partir da ativação térmica a baixas temperaturas de resíduos de materiais cimentícios hidratados. Este trabalho teve como foco a absorção e a retração de argamassas produzidas com CR proveniente de resíduos de pastas de cimento e betão de composição conhecida. Foram também analisadas diferentes percentagens de incorporação e a influência da finura e composição do material precursor. Adicionalmente, o efeito da combinação de cinzas volantes com CR em argamassas também foi analisado. Para tal, foram avaliadas as principais propriedades no estado fresco, a resistência à compressão, a absorção capilar e a retração por secagem. Devido à elevada exigência de água destes novos ligantes, foram produzidas argamassas de igual espalhamento e de igual a/c, para possibilitar a comparação em misturas de igual trabalhabilidade e de igual composição, respetivamente. Em argamassas de igual trabalhabilidade, os CR analisados neste estudo apresentaram uma exigência de água superior para atingir a fluidez pretendida, tendo-se verificado um crescimento linear com o aumento da percentagem de incorporação de CR. A mesma tendência foi observada em argamassas de igual a/c onde o espalhamento reduziu cerca de 100 mm nas argamassas com 50% CR face às argamassas de referência. A resistência à compressão de argamassas com 30% de CR de igual trabalhabilidade e igual a/c foi 70% e 90% da das argamassas de referência, respetivamente. Para 100% de CR verificou-se um decréscimo de 20% na resistência à compressão de argamassas de igual a/c. Para argamassas de igual composição, o coeficiente de absorção capilar das argamassas com 100% CR foi inferior ao das argamassas de referência. Para tal terá contribuído a natureza porosa das partículas de CR que, para um mesmo a/c, ao absorver parte da água de mistura permite que haja uma maior proximidade entre partículas adjacentes, contribuindo para a densificação da microestrutura. A retração por secagem das argamassas com CR foi superior à das argamassas de referência e principalmente influenciada pela menor dureza das partículas de CR e pelas ligações entre produtos de hidratação mais fracas do que as desenvolvidas numa matriz de cimento Portland Tipo I.