International Concrete Abstracts Portal

It is known that the use of recycled coarse aggregates (RCA) can raise a variety of problems, which are mainly due to the porosity of the old mortar contained in RCA. One of the simpler ways to solve these problems is the pre-wetting of RCA, which allows not only to minimize disadvantages but also to obtain the advantages associated with the effect of internal curing. Undoubtedly, the strongest positive effect of pre-wetted RCA is on the rheology of recycled concrete. But there are also possible positive effects of internal curing for strength and durability of blended cement concretes, which require longer curing times compared to normal Portland cement concrete. In this paper, we mostly study the influence of porous RCA on the rheology of cement paste, based on slag cement with a 75% slag content. For this purpose, the absorption properties of RCA of different sizes were studied. From this, mathematical dependences of the workability of cement systems on w/c and time could be obtained. These further underline the positive effect of pre-wetting of RCA with regard to retaining the workability of cementitious systems. This lays the basis for a broader study of pre-wetting RCA on the rheology of mixtures, strength, and durability to be covered in future publications.

Accelerated carbonation of recycled concrete aggregates (RCA) is one way to convert them into carbon stores by capturing CO₂ from cement plants. This study investigates the CO₂ captured depending on composition (paste, mortar, or concrete), origin (laboratory, platform), production process (crushing, molding, sawing), and age of RCA. The CO₂ captured is quantified by means of calcimetry (CaCO₃ content evolution). RCA studied ranged in size from 4 to 16 mm (0.16 to 0.63 in.). They were carbonated on a laboratory or semi-industrial scale. It has been shown that the CaCO₃ content of young RCA or RCA protected from natural carbonation, crushed and composed of CEMI is more likely to evolve. It was shown that the cement paste content and the duration of accelerated carbonation increase the amount of CO₂ captured. The composition of the parent aggregates affects the non-carbonated and carbonated CaCO₃ contents, which requires accurate sampling to limit bias in the results. Carbonation efficiency is more difficult to estimate on a semi-industrial scale and the assessment by calcimetric measurement is biased when the parent concrete is made of slag-based cement. The study was carried out within the framework of the French national program FastCarb.

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.

Due to its predominant soda-lime composition, most post-consumer glass processed by recycling facilities would be classified as high-alkali pozzolanic glass powder (GP). In cementitious matrices, the intrinsic alkaline pore solution induces the dissolution of both silica and alkali ions. Therefore, the GP can potentially induce two similar reactions in concrete: either a deleterious alkali-silica reaction or a pozzolanic reaction. The equilibrium of the pore solution will determine which reaction will prevail in the long term. To understand the chemical stability of GP in a cementitious system, the evolution of the solubility of key elements in an alkali-rich synthetic pore solution was studied as a function of reaction time, particle size, presence of Ca(OH)₂ and CaCO₃, and binder/solution ratio (B/S). The solution was based on the R³ method, which consists mainly of lab-grade chemicals such as KOH and K₂SO₄. Although the chemical equilibrium seems to be fully reached in the first hours of hydration, the main products, such as C-S-H, are unstable because the alkali leaching/uptake in the C-S-H chains is dynamically evolving. The experiments show that both C-S-H precipitation and alkali leaching rates increase with increasing B/S ratio and decreasing particle size, and are directly related to the presence of calcium in the solution.

Approximately 22 kg of spent pot lining is generated (SPL) per ton of aluminum produced by electrolytic cells. Untreated SPL is classified as hazardous industrial waste due to its hydroreactive nature and the presence of leachable cyanide and fluoride compounds. After treatment with the Low Caustic Leaching and Liming (LCL&L) industrial process, the refractory portion of the SPL is transformed into an inert material called LCLL. This project analyzed the use of LCLL as a cement binder. The reactivity of LCLL was studied using the compressive strength activity index, and RILEM R³ tests. The results showed that LCLL is composed of stable crystalline phases such as corundum, albite, and nepheline, and contains graphite. The improvement of LCLL reactivity was explored by calcination and the addition of synthetic fluorite, also from the LCL&L process. The results showed a significant improvement in reactivity with the formation of a larger amount of reactive amorphous phases with high silica and alumina content with optimum fluorite content of 10%. Calcined LCLL showed similar reactivity to fly ash, without retarding effect, with compressive strength equivalent to cement at 112 days and the formation of a new phase rich in carboaluminate.