International Concrete Abstracts Portal

Hybrid cements contain a small amount of Portland cement; the rest of the binder is made up of pozzolanic or latent hydraulic admixtures. Another component is an alkaline activator. These binders, therefore, combine the advantages of alkaline activation and Portland cement. In this work, a combination of Portland cement, ground granulated blast furnace slag (GBFS), siliceous fly ash (FA), and ground recycled masonry (GRM) is chosen. The GRM mainly contains ground bricks - i.e., heat-treated clay with potential pozzolanic properties and usually about 7-8% CaCO3 from the original mortar. Sodium water glass modified with potassium hydroxide is used as an activator. Potassium ions improve the workability of the mixture and limit the efflorescence of the hardened mixture. The dose of cement was optimized, as well as the dose of the activator. Furthermore, mixtures with different GBFS or FA and GMR ratios were tested. With an optimal composition and a water-to-binder ratio of 0.50, it is possible to achieve compressive strengths between 15 and 20 MPa at the age of 28 days, with the expectation of further improvement.

Alkali–silica reaction (ASR) is a major concrete durability problem. The occurrence of ASR results in significant maintenance and reconstruction costs to concrete infrastructures all over the world. Current market solutions are not always sustainable, such as hauling non-reactive aggregates or fly ash from long distances; or use of a lithium-based admixture that has availability, and lead time challenges. The current article will present an ASR mitigating admixture, which is based on the abundant water-soluble calcium salts, such as Ca(NO3)2. The current ASTM C1567 test method is not suitable for evaluating this new ASR mitigating technology due to the potential for leaching of soluble Ca(NO3)2 into the soak solution. A slight modification to the ASTM C1567 test using additional Ca(NO3)2 in soak solution to buffer any potential leaching has been developed for evaluating this new ASR preventing technology. Twenty-seven aggregates have been evaluated with this innovative technology. Depending on the reactivity of the aggregate and the alkali content of the cement, the dosage of this new admixture required to inhibit ASR ranges from 32 to 81 mL/kg of cement for aggregates tested. This new admixture is formulated to be set neutral, provides water reduction, and significantly improves the compressive strength of concrete in addition to preventing ASR expansion.

The cement and concrete manufacturing contribute significantly to the global anthropogenic carbon emissions, and, for this reason, the goal of the industry is to produce net-zero CO₂ cement and concrete by 2050. In recent years, different solutions have been studied, and great efforts are currently ongoing to find and implement effective decarbonization strategies. At present, the most feasible and noticeable strategy is the reduction of the clinker content in cement. Indeed, the blended cements have become more and more widespread, and their utilization and development are continuously growing. In this framework, it is crucial to study the properties and performances of such cements since they are strongly related to the type of secondary cementitious material (e.g., pozzolanic material, slag, limestone, calcined clay, fly ash) introduced in their composition. The present study aims to investigate the porosity, microstructure, and mechanical performance of mortars produced with low clinker cements, with a particular focus on the modification obtained using chemical additives to improve performance and durability. More precisely, porosity is quantified by an innovative image analysis of sections cut from mortar samples and correlated with macroscopic properties of mortars.

The use of lithium-based admixtures is one of the options to prevent deleterious alkali-silica reaction (ASR) in concrete. In 1992, the Canadian Centre for Mineral and Energy Technology (CANMET) initiated a comparative laboratory-versus-field ASR study. About 700 concrete blocks were cast with various aggregates susceptible to ASR, different binders including supplementary cementitious materials, and lithium-based admixtures. The blocks have been exposed outdoors near Ottawa, Canada, and their expansion has been monitored over time. Out of those, a set of blocks was made with two lithium-based products: lithium hydroxide (LiOH) and lithium nitrate (LiN). They were used with high alkali Portland cement (PC) and in “ternary” concrete systems with PC and Class F fly ashes. The Li-to-(Na+K) molar ratios in the mixtures ranged from 0.37 to 1.10.

This paper presents the expansion of the concrete blocks after 25-30 years of outdoor exposure, for 20 different mixtures. The efficacy of LiOH and LiN to limit the expansion depends on the aggregate type and the dosage. Overall, lithium-based products reduced the expansion caused by ASR. However, there are indications that lithium may only have a retarding effect on ASR expansion - not a prevention effect - at least for some aggregates. Finally, two 30-year blocks with and without Li were cored to better characterize the condition of the concrete. The paper presents the results of in-depth investigations, including the damage rating index and stiffness damage test.

Supplementary cementitious materials (SCM) are promising alternatives for the reduction of CO₂ emissions in the cement industry. Despite extensive research, their adoption, using Germany as an example, remains low due to potential disruption in manufacturing processes and standardization issues. Furthermore, the precursor output rate of e. g., fly ash and ground granulated blast furnace slag (GGBFS) is limited not only due to general cement substitution but for shortages in their respective industries. As a result, SCMs may only serve as a viable alternative binder in niche applications, such as concrete maintenance and repair. Additionally, the use of dry activators such as Na₂CO₃ facilitates conventional production methods.

This paper discusses results of sodium carbonate-activated slag mortars that were applied to concrete reference plates (A3-type as described in “Technische Regel Instandhaltung von Betonbauwerken,” eng. technical guideline for maintenance of concrete structures). The performance of alkali-activated slag (AAS) mortars was evaluated in terms of shrinkage behavior, compressive, flexural, and adhesive strength after dry thermal cycling and thunder-shower cycling. Interestingly, the results showed enhanced strength after exposure to these experimental conditions in comparison to ambient temperature curing. This highlights the critical role of proper curing, as AAS mortars are highly susceptible to increased shrinkage when not exposed to sufficient moisture.

The previous results show that AAS mortars - even without optimization - meet the required properties for structural repair class “R3” as defined in DIN EN 1504-3, promoting SCMs as promising alternative binders for these practical use cases.