ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2023 25 belite-rich Portland cement clinkers, belite-ye’elimite-ferrite clinkers, carbonatable calcium silicate clinkers, and magnesium oxides derived from magnesium silicates may be considered to reduce global CO2 originating from concrete production[10]. Alkali-activated binders also emerge as a potential alternative, particularly in some Asian countries. In other countries the availability and the costs of these latter materials make their actual use quite limited. Furthermore, some aspects concerning the durability, especially the alkali-aggregate reaction on a long-term basis still remains to be clarified[11]. The rapid technology evolution of the chemical additives and their increased performance is also an aspect to be widely considered in the sustainability of cement. Adequate type and dosage of super plasticizers or strength enhancers and the addition of mineral additives may significantly reduce the cement content of concrete. Limits for cement content by weight of concrete can be lowered down to 260 kg/m3 without adversely compromising the properties, provided that an appropriate mix design is achieved. THE REUSE/RECYCLING ISSUE The reuse of reinforced concrete components is a challenging task and is feasible in some contexts. The direct reuse of elements requires a deep knowledge of their state of conservation. Pollutant contaminations, carbonation fronts, and steel rebar corrosion state are some of the parameters to be controlled. In addition, the micro- cracking pattern needs to be carefully investigated to assess reuse for structural components. This is a decisive parameter that may partially hinder direct reuse of the components. Precast elements substitution is a fast and applicable potential way for engineering infrastructures, such as bridge renovation or replacement. In this type of application, it appears necessary to produce new cast elements, and the addition of recycled concrete may be considered. Recycled concrete aggregates (Fig. 2a) have been a concern since World War II, due to the high amount of debris accumulated[12]. A relatively high amount of construction and demolition waste, up to 35%, is still sent to landfills[13]. To ensure adequate quality of separated cementitious material, a selective demolition must be done. This is a crucial aspect that increases the costs and the demolition time, but also enables a more direct recycling of the components, especially the concrete. In fact, one of the primary issues is dishomogeneity of the material. Poor quality recycled aggregates from buildings or supporting walls are mixed with high-quality infrastructure concrete. The latter generally exhibits higher mechanical and physical parameters that are not seen in other structures. The porosity of the old mortar that surrounds the aggregates is a major problem to be solved. Consequently, water absorption may be lowered from 4-5% to 1-2%. Several methods to improve the concrete’s recycled aggregate properties are under investigation, although only some of them are applicable. Mechanical crushing to eliminate the old mortar paste and obtain the original natural aggregate may be one way. This process is also used with reclaimed asphalt pavement aggregates to reduce the bituminous component of the coarse aggregates (Fig. 2b). This is done when not directly reused within asphalt pavements. Another procedure uses the accelerated carbonation of the old residual cementitious paste around the aggregates and the treatment can be seen as a CO2 sink. Monitoring and control of the recycled aggregate quality, in particular the water adsorption, helps to classify the material to be used for building or infrastructure. Furthermore, the content of pollutants, such as chloride, sulphate, or gypsum must be investigated, although compared to the whole concrete mass, they barely represent a problem. Mechanical properties Fig. 2 – (a) Recycled concrete aggregates, (b) reclaimed asphalt pavement aggregates with reduced bituminous amount, (c) and mixed granulates. (a) (b) (c)
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