International Concrete Abstracts Portal

Should concrete core samples extracted from a structure be submerged in water until testing? This month’s Q&A provides answer to that question along with a discussion on proper and improper moisture conditioning of cores to be tested per ASTM C42/C42M.

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.

This paper examines the influence of harvested fly ash on the properties of mortar and concrete. Class F and harvested fly ash were used at the substitution rate of 20% by weight of Portland cement. The investigated properties included heat release, consistency, setting time, compressive strength at different testing ages, absorption, the volume of permeable voids, surface resistivity, and drying shrinkage. The results revealed that the harvested fly ash produced the lowest released heat of hydration and longest setting times. Mixtures containing harvested fly ash displayed lower strength at all curing ages. Compared to traditional fly ash, harvested fly ash showed inferior transport properties for both absorption rate, permeable voids, and surface resistivity. Mixtures containing harvested fly ash showed comparable 120-day drying shrinkage when compared with the companion mortars made with traditional fly ash.

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.

This paper investigates the leaching behavior of magnesium phosphate cement-based materials (MPC) prepared using hard-burnt magnesia (MgO) and monopotassium phosphate (KH₂PO₄). Three different formulations of variable Mg/P molar ratios (1, 2, and 3) were tested. Upon hydration, K-struvite was the main hydrate formed in all cases. After 28 days of curing, mechanical properties were assessed on MPC mortars. Semi-dynamic leaching tests were performed on MPC paste monoliths for 28 days under well-controlled conditions and using demineralized water with a pH maintained at 7. Leaching solutions were frequently renewed. Solids were characterized before and after leaching using XRD and SEM/EDS. The cumulative element release, plotted against the square root of time, indicated that leaching was mainly controlled by diffusion in all samples. Examination of the solids revealed a zonation process involving K-struvite dissolution and cattiite precipitation. Analyses of leachates showed that fluxes of leached species increased with decreasing Mg/P molar ratios. This parameter was thus a key factor, influencing the pore network and the resistance of MPC-based materials to leaching.