Alkali-activated binder (AAB) has been extensively researched in recent years due to its potential to replace portland cement (PC) and lower carbon footprint. However, major barriers to its commercialization are related to the inadequate characterization of mechanical properties and long-term durability. The mechanical and durability performance of AAB is highly influenced by its microstructure. There is minimal research on correlating the microstructural changes to the specimen-level performance of alkali-activated concrete (AAC). Among AAB's primary advantages as a building material is its superior performance at high temperatures and lower environmental impact. The performance of reinforced concrete to function as a composite at high temperatures is evaluated through its bond strength. However, there is no reported study on the bond strength of ambient cured (fly ash + slag)-based AAC. To validate the practical sustainability of AAC, life cycle assessment (LCA) can be used to evaluate the environmental impact.

Therefore, the present study evaluates the effect of varying precursor proportion (fly ash: slag varied as 100:0, 70:30, 60:40, and 50:50), activator modulus (Ms, varied as 1.0 and 1.4), and high-temperatures (538 °C, 760 °C, and 892 °C) on the mechanical properties and microstructure of AAC. The effect of varying precursor proportions and Ms on the mechanical performance of AAC is evaluated through compressive, bond, flexural, and split tensile strength testing. The performance of AAB at extremely high temperatures is assessed in terms of residual compressive and bond strength. LCA of AAC is conducted using the ReCiPe 2016 methodology. Furthermore, since the commercialization of any novel alternative material depends on cost-effectiveness, a simplified cost analysis is performed.

The results from microstructural experiments show the formation of new crystalline phases and decomposition of reaction products when exposed to high temperatures, and they correlate well with the observed mechanical performance. The 28-day compressive strength with slag content is enhanced by 151.8 - 339.7 %, depending on the mix. In ambient conditions, lower Ms improves mechanical performance. When exposed to high temperatures, specimens with a high slag content and a low Ms suffered significant deterioration. AAC with fly ash: slag ratio of 70:30 and Ms of 1.4 is proposed as optimal from the results obtained in the present study. The results reveal that the biggest impact on climate change comes from transport (45.5 – 48.2 %) and sodium silicate (26.7% – 35.6 %). The proposed optimal AAC mix has a global warming potential 42.6 % lower than PC concrete. ReCiPe midpoint approach is more efficient in analyzing all impact categories except freshwater ecotoxicity (FETP) and human toxicity potentials (HTP) for which USEtox is recommended. The proposed AAC mix has a higher cost than PC concrete in the present scenario. In contrast, if a carbon tax is enacted, the cost of the proposed AAC mix will rise by only 18.4 %, whereas PC concrete prices will rise by 81.7 %. This proposed AAC mix is an environmentally sustainable replacement for PC concrete specifically intended for applications requiring the superior high-temperature performance of reinforced concrete.

The industry of building materials and construction, despite of their obviously conservative character, quite often has to face so-called “industrial revolution of the XXI century”. New trends, new methods of experiments and research are becoming perspective foundation for creation of high-tech products and processes characterized by guaranteed reliability index, developing principles of manufacturing up-to-date “supermaterials” and are marking the start of the sixth technological wave.

An impressive breakthrough in the construction technologies in the 21st century was achieved due to the properties of modern concrete which have recently seemed unattainable. These include extremely low values of water/cement ratio and air content of the concrete mixture with long-lasting flowability, cohesion and uniformity; the fresh concrete ability to easily and completely fill in the formwork to concrete of any configuration with dense reinforcement without the use of energy with horizontal or vertical mix pouring; the concrete ability to achieve a given strength with an adjustable strength development subject to climatic factors; dense concrete structure at the nano-, micro- and macrolevel as a factor ensuring high strength, resistance and durability.

The presence of nanomaterials and nanotechnologies in the construction segment is becoming more prominent. The detailed analysis and long-term forecast for the development of research and application of nanomaterials and nanotechnologies in construction shows that the cement and concrete cover over 40 % of the nanotechnology products in construction materials (market value is about $ 5.6 billion) with a predicted annual growth more than 10 %.

Recent advances in nano-chemistry and the development of new methods for synthesis of nanoparticles are now expected to offer a new range of possibilities for improvement of concrete performance. Incorporation of nanoparticles into conventional construction materials can provide the materials with advanced or smart properties that are of specific interest for high-rise, long-span, or intelligent infrastructure systems.

Self-regulating concrete (SRС) is one the most in-demand subjects of the modern concrete science. The choice of components and the design of SRC compositions are based on a prognostic assessment of the direction of spontaneous processes to ensure high functionality at any technological and operational stage. The concept of “self-regulation” should be interpreted as the technologically predicted course of spontaneous processes in order to achieve the maximum possible functionality of the interacting components and concrete mixes, which meets the concept of defined performance concrete (DPC).

Examples of successful applications of SiO₂, TiO₂, Fe₂O₃, Al₂O₃, CaCO₃ nanoparticles, nanosized spinel MgAl₂O₃, nanoferrit ZnFe₂O₄, and nanoclays in concrete are given. The most promising contemporary developments include the synthesis and application of new forms of carbon, viz fullerene (C₆₀, C₇₀, C₅₄₀), graphene oxide (GO) and new types of carbon nanotubes.

In conditions of the planet population growth and inevitable emergence of raw material and power shortage in construction quite rapid displacement of traditional materials and technologies by energy-saving and material-efficient solutions must be a determining factor. Nano-binders and nano-engineered cement-based materials with nano-sized cementitious component or other nano-sized particles may be the next ground-breaking development.

Due to the interest to increase the durability and sustainability of concrete structures and construction techniques, a wide range of novel cementitious materials are being designed and investigated. One such recent material is a cementitious material containing superabsorbent polymers (SAPs) studied only from 1999 onwards, mainly for its internal curing purposes with mitigation of autogenous shrinkage and sealing characteristics. Other positive influences are the change in rheology, the increase in freeze-thaw resistance, amongst others. From 2010 onwards, a combination of addition of synthetic microfibers and SAPs was studied, for their improved influence on autogenous healing in cementitious materials. It was found that optimal self-healing features were possible, as the crack widths were limited and water was available during dry periods. Some of those first samples now have an age of over 10 years.

As the autogenous healing capacity is dependent on the age of the material, so will be the possible influence of added materials to promote this healing. The effects beyond one year are not omnipresent in literature. The effect of the age cannot be investigated as long as the actual specimens do not reach the required maturity. In a previous study, the age was studied up to 8 years’ time. In this study, specimens from the same batch were studied after a decade of maturing in different storage conditions.

Typical strengths and crack widths were obtained. Due to the stress initiator property of SAPs, the number of cracks increases. Due to the macro-pore formation, the strength is lowered. However, the healing ratios are always higher for SAP compared to REF samples. This is due to the water action by the SAPs during dry periods and the ability of SAPs to extract moisture from the ambient environment. This leads to better conditions for healing products to form as water is available. The main visual appearance of the healing products was the whitish calcium carbonate crystallization.

The small crack widths after 10 years are still able to be partially healed. The main visual healing product is calcium carbonate. Further hydration was less likely as most binder already hardened during storage conditions. Generally, the samples containing SAPs show more prominent healing and they are still able to swell almost completely after a decade storage in an alkaline cementitious environment. This makes them a sustainable option for the future as less maintenance and repair will be required.

When recycling reclaimed asphalt (RA) in new hot-mix asphalt (HMA), higher temperatures of the virgin aggregate allow the mobilization of a higher amount of the binder in the RA. However, this implies a more severe short-term aging of the virgin bitumen and poorer properties of the aged-virgin bitumen blend, due to the lower virgin bitumen/RA bitumen ratio. On the contrary, the adoption of lower temperatures has the opposite effect (lower mobilization of the RA binder but higher performance of the bituminous blend). In addition, the reduction of material heating results in a lower bitumen viscosity, which may determine lower compactability and lower adhesiveness. Previous studies showed that a reduction of 30 °C in the mixing temperature of AC containing RA does not imply a significant increase of the air voids content but allows improving the material performance against cracking, fatigue, and rutting. Moreover, the lower mixing temperature also preserves the effectiveness of the rejuvenating agent. To have a deeper understanding of this phenomenon, the objectives of the research were: (i) evaluating how the binder adhesive properties changes when varying the content of aged bitumen; (ii) assessing if the adhesion is higher on virgin aggregate or on RA particles, coated with aged bitumen; (iii) understanding how the blending temperature influence the binder-aggregate adhesion.

To this aim, binder bond strength (BBS) tests were carried out at 25 °C. The experimental program provided 2 types of substrates, simulating virgin limestone aggregate and RA, 3 RA/virgin binder proportions (20/80, 35/65 and 50/50), 2 types of rejuvenator in the binder (coded with the letters A and B), 2 bitumen application temperatures (140 °C and 170 °C), and 5 repetitions.

The results showed that the adhesive properties of the binder decreased when increasing the aged bitumen content from 20% to 50%. The t-test α values obtained when comparing the pull-off tensile strength (POTS) of the blends with 20% and 35% of RA bitumen and the blends with 35% and 50% of RA bitumen were respectively 1.2×10^-6 and 6.1×10^-7, confirming the decreasing trend of POTS with aged bitumen content. Moreover, for high RA bitumen contents (35% and 50%), the adhesion on the RA substrate was higher than on the limestone substrate (α = 0.004). Between the two rejuvenators, the type B allowed obtaining higher POTS values for high RA bitumen contents (35% and 50%), as confirmed by α = 0.008. Differently, the bitumen application temperature (140 °C or 170 °C) did not significantly influence the POTS (α = 0.50). This indicates that the increase of adhesiveness that can be obtained at higher temperature was approximately balanced by the more severe aging underwent by the binder. However, as in site the lower mixing temperature implies the lower mobilization of the RA binder, thus a lower RA/virgin bitumen proportion, from the experimental results it can be stated that the reduction of the mix temperature is beneficial for the adhesion between the binder and both the virgin and the pre-coated RA aggregates.

Introduction

The use of alternative materials in asphalt pavements has become a critical matter in pavement engineering due to sustainability issues. These issues include the utilisation of finite resources, the need to re-use of wastes and reduce the generation of Green House Gases emissions. To cope with these issues, two approaches can be considered. Firstly, the use of Reclaimed Asphalt (RA) in new asphalt mixtures has become a common practice in the last decades in the asphalt industry particularly in small amounts (<20%). However, there are still some concerns on the use of higher amounts (>20%), due to uncertainties in its performance. Secondly, the use of non-petroleum-based binders as alternatives to conventional bitumen is starting to gain force in this field. Recently, the combination of both approaches has been shown to be feasible and could lead to more sustainable solutions in pavement engineering. Nevertheless, more research is needed to give confidence to these innovative asphalt mixtures towards their final implementation. On this regard, the aim of this investigation is to optimise the combination of a RA source and alternative binders, made from vegetal by-products of other industries (biobinder) and targeting the maximum content of both materials in new asphalt mixtures.

Methodology

For this purpose, two sources of reclaimed asphalt (RAs) and two type of biomaterials were characterised, namely a biobinder and a bioemulsion. The cohesion and stiffness properties of the RAs were studied by means of ITS and ITSM testing 100% RA specimens manufactured at different temperatures. With this, the degree of activation of the RA binders was estimated. On the other hand, the biobinder, bioemulsion and the extracted binder from the RA were conventionally and rheologically characterised at the whole range of service temperatures of pavements.

The optimisation of the design of the sustainable asphalt mixtures was performed using the rheology and performance-related properties of the RA, the extracted binder from RA and the biomaterials, and the different results and hypothesis on the degree of blending between both binders obtained from the RAs characterisation.

Results

The results of the RAs characterisation show the potential of the testing programme used to determine the degree of binder activation of RA as an important intrinsic property to consider in the design of recycled asphalt mixtures. The characterisation of the biomaterials reveals their ability to fully replace asphalt binders in hot and cold recycled asphalt mixtures reducing global warming potential. Finally, the optimisation of the asphalt mixture design using the rheological and performance-related properties of the individual components, plus the estimation obtained of degree of blending between binders show the key role of these parameters to perform the adequate design of sustainable asphalt mixtures including alternative materials.