Each year a large amount of construction and demolition waste (CDW) is generated in the European Union. For sustainability development the CDW is recycled and re-used. To promote the use of CDW, recycled aggregates from CDW were incorporated in alkali-activated concrete (AAC), which mainly consisted of secondary materials or industrial by-products. This study investigated the influence of recycled aggregates on workability, compressive strength and drying shrinkage of slag-based AAC. Properties of conventional concrete with natural/recycled aggregates were also tested for comparison. The results showed that the pre-saturated recycled aggregates only slightly affected the workability of conventional concrete or AAC. Recycled aggregates reduced compressive strength of both conventional concrete and AAC due to extra water for pre-saturation of the recycled aggregates. The mass loss of the concrete specimens upon drying was greater for low-strength concrete than for moderate strength concrete. The recycled aggregates increased the mass loss and drying shrinkage of AAC. For conventional concrete, low-strength concrete had a higher drying shrinkage compared with moderate strength concrete. On the contrary, for AAC in this study, low-strength concrete had a lower drying shrinkage compared with moderate strength concrete.@en

This paper investigated the feasibility of using biomass fly ash (BFA) to prepare alkaliactivated slag and fly ash paste. The reference mixture was alkali-activated slag and coal fly ash (CFA) paste with a slag-to-coal fly ash ratio of 50/50. In other mixtures, coal fly ash was replaced at 40% and 100% with BFA, respectively. The results showed that the incorporation of BFA accelerated the setting of the paste, while its impact on the compressive strength was minor. XRD and FTIR results indicated that the BFA participated in the reaction process. BFA showed potential use as CFA replacement in synthesizing alkali-activated materials, which would pave a way for the valorisation of BFA.@en

Alkali activated slag (AAS) has shown promising potential to replace ordinary Portland cement as a binder material. Synthesized from industrial by-products, AAS can show high strength, thermal resistance and good durability. However, AAS has been reported to exhibit high autogenous shrinkage. Autogenous shrinkage is a critical issue for building materials since it can induce micro- or macro-cracking when the materials are under restrained conditions. Hence, this work aims at mitigating the autogenous shrinkage of AAS by means of internal curing. The influences of internal curing on microstructure formation and autogenous shrinkage are investigated. The results show that internal curing provided by superabsorbent polymers is a promising way to reduce the autogenous shrinkage of AAS.@en

In the social background of population ageing, this paper addressed the issue of social isolation and loneliness. Reflecting on the current design for social connectedness. We advocate a positive perspective to older people and consider design as mediation for social connection. Practice cases and research show the potential of design mediation connection, however, there is a lack of summarized knowledge of how to design for mediation. This paper aims to answer the research question-How does design as mediation facilitate social connection? This question was answered from two aspects. One focused on the form or features as regards to the design as a result, the other is reflecting design as an ideating process. This research follows a Research through Design (RtD) approach. A design project was launched in a design course for design students. Research materials, including the design process and design outcome, were documented and reflected. As a result, the design elements for enhancing social connection was clarified and conceptualized. It includes five categories from abstract to concrete level, namely role, partnership, encounter, facility and trigger. Design strategies were also concluded according to the five elements. The paper also proposed an ideation process of designing for social connection by linking the design elements with the ideation activities. This research criticized viewing socializing as a functional activity and advocated a new perspective of design as mediation for social connection among older people. The results provided practice guidance and reference for designers in designing intervention for social connection.@en

In this study, internal curing by superabsorbent polymers (SAP) is utilized to mitigate self-desiccation and autogenous shrinkage of alkali-activated slag (AAS) pastes. Absorption and desorption kinetics of SAP incorporated in AAS pastes were studied with X-ray tomography. Internal curing delayed the peak of the rate of heat liberation but increased the total reaction degree of AAS pastes. Internal curing by SAP mitigated effectively the drop of internal relative humidity and the self-desiccation-induced autogenous shrinkage of AAS pastes. The cracking tendency of AAS pastes undergoing shrinkage in restrained conditions was also significantly reduced with SAP. Nonetheless, adding SAP, regardless of their content, cannot eliminate the autogenous shrinkage of AAS pastes, suggesting the existence of other autogenous shrinkage mechanism(s) besides self-desiccation.

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Ferric-rich calcium sulfoaluminate (FR-CSA) cement is an eco-friendly cement. Fe₂O₃ exists in different minerals of FR-CSA clinker, e.g., Ca₄Al₂Fe₂O₁₀ (C₄AF), Ca₂Fe₂O₅ (C₂F), and Ca₄Al_6-2xFe_2xSO₁₆ (C₄A_3-xF_xS-). The mineral composition depends on the chemical composition of the raw materials and significantly determines the reactivity of FR-CSA cement. To optimize the phase composition of the FR-CSA clinker, chemical reagent raw mixtures with different amounts of CaO were used to prepare the FR-CSA clinker. X-ray diffraction (XRD) analysis, Rietveld quantitative phase analysis (RQPA), Fourier Transform Infrared spectroscopy (FT-IR), and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) were used to identify the mineralogical conditions of the FR-CSA clinker. The results indicated that the amounts of CaO in raw materials greatly affected the iron-bearing phase formation in the FR-CSA clinker. With decreasing CaO content involved in calcination reaction, the amounts of Fe₂O₃ incorporated in C₄A_3-xF_xS- increased up to 17.72 wt% (where x = 0.36). The findings make it possible to optimize the mineral composition of the FR-CSA clinker by changing the CaO content in raw materials. Furthermore, low CaO content in the raw material is beneficial to the formation of C₄A_3-xF_xS-, which enables the use of solid wastes containing low calcium for producing FR-CSA cement.

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Carbonation of hydrated cement paste (HCP) causes numerous chemo-mechanical changes in the microstructure, e.g., porosity, strength, elastic modulus, and permeability, which have a significant influence on the durability of concrete structures. Due to its complexity, much is still not understood about the process of carbonation of HCP. The current study aims to reveal the changes in porosity and micromechanical properties caused by carbonation using micro-beam specimens with a cross-section of 500 μm x 500 μm. X-ray computed tomography and micro-beam bending tests were performed on both noncarbonated and carbonated HCP micro-beams for porosity characterization and micromechanical property measurements, respectively. The experimental results show that the carbonation decreases the total porosity and increases micromechanical properties of the HCP micro-beams under the accelerated carbonation. The correlation study revealed that both the flexural strength and elastic modulus increase linearly with decreasing porosity.

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This study aims to provide a better understanding of the autogenous shrinkage of slag and fly ash-based alkali-activated materials (AAMs) cured at ambient temperature. The main reaction products in AAMs pastes are C-A-S-H type gel and the reaction rate decreases when slag is partially replaced by fly ash. Due to the chemical shrinkage and the fine pore structure of AAMs pastes, drastic drop of internal relative humidity is observed and large pore pressure is generated. The pore pressure induces not only elastic deformation but also a large creep of the paste. Besides the pore pressure, other driving forces, like the reduction of steric-hydration force due to the consumption of ions, also cause a certain amount of shrinkage, especially in the acceleration period. Based on the mechanisms revealed, a computational model is proposed to estimate the autogenous shrinkage of AAMs. The calculated autogenous shrinkage matches well with the measured results.

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Drying of cement-based overlay systems is a critical issue, because it causes differential shrinkage between the overlay material and the concrete substrate and may induce cracking or debonding of the overlay material. In this paper the mechanisms of moisture transport in hydrating cement-based overlay systems are studied. A model is proposed for simulating the moisture transport. A parameter study has been conducted to quantitatively investigate the influence of the thickness of the overlay material and the curing conditions on hydration of the overlay materials. The evolution of the moisture profile in the overlay system and the development of the degree of hydration (DOH) of the overlay material have been calculated. The change of water content in the overlay material is investigated, in terms of the water absorbed by the substrate, the water consumed by hydration of the overlay material and the water evaporated to the environment. The simulation results show that the water evaporation is a dominant factor that causes water loss of the overlay material, while the water absorption by the substrate plays only a minor role. Moist curing is much more effective than sealed curing (e.g. by using sealing agent) for hydration of the overlay material. The DOH of the overlay material is significantly increased with longer moist curing. Under the same curing condition (e.g. moist curing + drying), thinner overlay materials are more vulnerable to water loss and exhibit a lower DOH. It suggests that for proper hydration of cement-based overlay materials, moist curing is recommended rather than applying a sealing agent.@en

Chloride-induced corrosion is a critical issue for RC structures. Cement-based coatings can be used to protect concrete structures with unsatisfactory quality against chloride ingress. To evaluate the effectiveness of the coatings to extend the service life of coated concrete structures, the evolution of the chloride profile in the coated concrete structures should be determined. This paper investigated the mechanism of chloride ingress into coated concrete structures (i.e., coatings made of cement paste and concrete substrate). A numerical tool is proposed for calculating the chloride profiles in the coated concrete structures. A parametric study investigated the influence of several factors on the chloride ingress: the water:cement (w:c) ratio of the coating, the thickness of the coating, and early or late application of the coating. A preliminary cost analysis of coating materials was carried out. The results showed that the effectiveness of the coatings increased with coating thickness at a drastic increase of material cost; the effectiveness of the coatings increased with the decrease of the w:c ratio at a moderate increase of material cost. In order to extend the service life of the substrate, a coating with a low w:c ratio is recommended, and the coating thickness should be designed depending on the requirements. Moreover, the exposure history of the substrate before application of the coating also has an influence on the effectiveness of the coating. To protect an existing concrete structure exposed to a chloride environment against rapid chloride ingress, it is preferable to apply a coating as early as possible, because the effectiveness of the coating is reduced by late application.@en

Concrete is currently the most widely used material in construction field. A good performance of concrete structures is required for reaching the designed service life. However, the initial scatter in the quality of concrete is unavoidable in practice, possibly resulting from improper design, poor execution, or deterioration due to stresses during the service life of concrete structures (e.g. external load, stress induced by temperature, drying shrinkage or ingress of chemical substances, etc.). A cementitious coating can be used to protect low quality concrete structures against the ingress of aggressive substances, like CO2 and chloride ions, and to extend the service life of the reinforced concrete structures.@en

A typical cement-based exterior wall consists of a top coat, a base coat, an insulation layer, a steel frame and an interior layer. The top coat and the base coat are often made of mortar. Efflorescence at the surface of the top coat is a critical problem from the aesthetic point of view. The efflorescence can be caused by leaching of Ca(OH)2 from the top coat and the base coat. This paper aims to mitigate the efflorescence of the top coat by incorporating metakaolin.
Cement in the top coats is substituted with metakaolin at different substitution rates (i.e. 0 - 20% by mass of cement). Mechanical properties of the top coats are measured. The extent of the efflorescence of two-layer specimens (i.e., top coat + base coat) is quantified by image analysis. The content of Ca(OH)2 and the porosity of the top coats are determined for explaining the mechanism of mitigation of efflorescence by incorporating metakaolin. The results show
that the compressive strength and the flexural strength of the top coats are improved when the substitution rates of metakaolin are lower than 15%. The metakaolin results in lower porosity and lower content of Ca(OH)2 in the tops, due to the filler effect and the pozzolanic reaction of metakaolin. The substitution of metakaolin for cement can effectively mitigate the
efflorescence of the top coats, and the efficiency of metakaolin increases with the substitution rate (from 0 to 20%). The conclusion is that metakaolin is a promising material for mitigating efflorescence of cement-based exterior wall.@en

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.

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This article presents the results of an interlaboratory experimental study performed by 13 international research groups within the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available superabsorbent polymers (SAP) were tested in terms of their influence on the freeze–thaw resistance of ordinary concrete. To test the robustness of the method, all participating laboratories used locally produced materials. Furthermore, following this aim, various accelerated methods were used to estimate the resistance of the concrete to freeze–thaw cycles. The effect of adding SAP was from insignificant to considerably positive in terms of improvement in material performance as determined by reduced mass loss after freeze–thaw cycles; only one participant observed worsening of the material behaviour. At the same time, due to the addition of SAP, a much less pronounced decrease in the dynamic Young’s modulus was observed as a result of freeze–thaw testing without deicing salt.

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