International Concrete Abstracts Portal

This study focuses on evaluating the mechanical, microstructural, and durability properties of 3D printing mortar (3DPM), with a specific emphasis on the influence of incorporating recycled fine aggregates (RFA). These RFA are produced from construction and demolition waste (C&DW) in Belgium and are sieved to a maximum particle size of 2 mm [0.08 in].

Cast and printed samples of mortar containing 100% RFA, with a sand-to-cement ratio of approximately 1:1 and a water-to-cement ratio of 0.29, were subjected to mechanical tests, including flexural, compressive, and tensile strength, at 2, 7, 28, and 56 days. The possible anisotropic behavior of the printed material was also investigated. The results show that using RFA does not significantly affect the mechanical properties of the mortar, and some anisotropic behavior was observed based on the compression test results. The end goal of the project is to print non-reinforced urban furniture; in order to assess its durability, only freezing and thawing (F-T) behavior was investigated. The F-T behavior was analyzed based on the quantity of spalling particles after 7, 14, 28, 56, and 91 F-T cycles. The results show that up to 91 F-T cycles, no significant surface damage occurred.

Although limestone calcined clay cement (LC³) is a valid alternative to reduce the carbon footprint of cement production, some of its properties, like workability, still need to be investigated and fully understood. In this work, different cement pastes containing variable amounts of calcined clay with and without superplasticizer were analyzed. Measurements at the rheometer scale were performed to evaluate the superplasticizer’s effect on the samples’ workability. The amount of free water available after 1 hour of hydration in cement pastes was detected by Differential Scanning Calorimetry (DSC). Moreover, the Time-Domain Nuclear Magnetic Resonance (TD-NMR) was used to identify whether this water was contained either in capillary pores or in interhydrate spaces. The results obtained by DSC and TD-NMR revealed that pastes containing superplasticizers show a slightly higher amount of available free water, with a direct positive consequence on rheological properties. However, the amount of calcined clay (CC) in cement impacts both aspects: superplasticizer dosage to reach the target fluidity of pastes and workability retention over 60 minutes. Moreover, the confirmation of the exponential correlation between yield stress and the solid content of cementitious particles is possible when considering the detected capillary water as an indicator of the normalized concentration of solid particles.

Using particle packing models (PPMs) in combination with limestone fillers has been shown to be effective in proportioning eco-efficient concrete mixtures with reduced Portland cement content, resulting in suitable performance in fresh and short-term hardened states. However, the decrease in Portland cement and increase in limestone fillers may lower the pH of concrete, raising concerns about durability and long-term performance, potentially leading to increased corrosion of steel reinforcement in the presence of carbonation or chlorides. In this study, the performance of three eco-efficient concrete mixtures with varying cement (250, 200, and 150 kg/m³) and inert filler contents is evaluated against accelerated chloride exposure. The findings highlight the influence of the mixture proportioning and water-to-cement ratio on the resistance to chloride ingress. Ultimately, it is verified that the distance between cement particles is a major contribution towards chloride ingress.

The management of municipal solid waste incineration fly ash (MSWI FA) has become a critical issue as its generation increases rapidly along with the global population growth. In this study, MSWI FA was treated via water-washing, and then the untreated and water-treated MSWI FAs (RFA and WFA) were blended with mainstream supplementary cementitious materials (SCMs), including ground granulated blast-furnace slag (GS), silica fume (SF), and limestone powder (LS). The MSWI FASCMblends were used as a cement replacement in a mortar. The content of MSWI FAs was set at 10% (by weight of binder) for all mortar mixtures. The content of GS and LS was also set at 10%, while the SF content was 2.5%. Flowability, setting time, isothermal calorimetry, compressive strength, and free-drying shrinkage tests were performed. The results showed that mortars containing raw (untreated) fly ash (RFA) had reduced strength, whereas mortars containing water-treated fly ash (WFA) displayed comparable or even higher strength than the control mortar (made with 100% cement) after 28 days. While mortars containing RFA showed increased drying shrinkage, mortars containing WFA exhibited diminutive or no increase in drying shrinkage when compared to the control mortar. Based on the test results, the mixture with a cement:WFA:GS ratio of 80:10:10 was the optimal binder for concrete applications.

The decarbonization of the cement industry will require a versatile portfolio of different alternative cements. In contrast to other known and popular alternatives, cement based on MgO-rich hydrates remains an unexplored topic and little is known about the formation and stability of Mg-Al LDH as the main binding phase in a cement. In this work, we report on experimental work on the phase assemblage of MgO-Al₂O₃-(SiO₂)-CO₂-H₂O system via the hydration of an amorphous magnesium aluminate (AMA) in the presence of different magnesium carbonates and metakaolin. The data reveal that hydrotalcite is the main hydrate of the cement with relatively fast reaction kinetics in which AMA is fully hydrated after 7 days of curing in ambient water. Additionally, a more detailed phase assemblage will be beneficial in the better understanding and in improving the thermodynamic data for the MgO-Al₂O₃-(SiO₂)-CO₂-H₂O system and of interest to shed light on the long-term stability of this cement.