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.

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.

Belite cements (BCs) have been suggested as innovative environmentally-friendly materials, since they can allow a reduction of CO₂ emissions up to about 30% with respect to normal portland cements (NPCs); furthermore, the manufacturing process of BC, compared to that of NPC, is characterized by reduced limestone requirement, lower synthesis temperatures and, consequently, reduced specific fuel consumption. The peculiar composition of BC can also be exploited for achieving valuable technical properties (e.g. better durability against sulfate and carbonation attacks as well as low heat of hydration). This paper examines the influence of temperature (20°C and 40°C) on the hydration behavior and the technical properties of a pilot-scale industrial BC (w/c=0.50) up to 180 days. An NPC class 52.5 R was employed as a reference term. It was found that, compared with NPC, BCs showed lower mechanical properties at early ages (2 days), while at longer curing periods, the compressive strength values were always greater at both 20°C and 40°C. However, stability tests displayed that BCs shrank less than NPC in the air, while they exhibited approximately the same expansion values when submerged in water. BC pastes showed the best hydration behavior for curing periods longer than 28 days.

There is no accepted test to determine the time needed for a cement paste microstructure to stabilize (and thus, for its diffusion coefficient to stabilize). This stabilization time is crucial when applying Crank's solution to Fick's law of diffusion of deleterious ions, as an important hypothesis is a constant diffusion coefficient. One potential approach involves utilizing the hyperbolic law to fit the evolution of bulk electrical resistivity, which is directly related to microstructural changes. This approach could provide a rapid means of determining the stabilization time and the final resistivity value. The objective of this work is to validate if this law is appliable for different types of cement and different types of curing and to determine the stabilization time for the different types of cement pastes: Ordinary Portland Cement (OPC) and blends with Supplementary Cementitious Materials (SCMs, i.e., slag, fly ash, silica fume, and limestone). Results show that the hyperbolic law for Portland cement allows predicting in 56 days the resistivity after 120 days with an error of less than 1%. Moreover, this law can be useful to estimate the time necessary for stabilization of the resistivity. However, it appears that this law is not applicable to every type of SCM especially silica fume and fly ash.