Influence of Calcined Clay on Workability of Mortars with Low-carbon Cement

The second-largest industrial global emitter of CO2 (Carbon dioxide) is the cement sector. The technology roadmap of low carbon transition for cement industries includes the introduction of calcined clay (CC) as supplementary cementitious material. A new type of alternative binder, called Limestone Calcined Clay Cement (LC3), was recently proposed. This cement can reduce CO2 emissions of cement production by up to 40% and it is prepared using limestone (LS) and clay which are globally available. Many scientific studies aimed to investigate the hydration of LC3 to understand the contribution of CC to the development of the compressive strength. However, recent studies showed that other cement properties, like workability and water demand, are highly impacted by calcined clay. Despite some papers state that an increase in superplasticizer (SP) dosage compensate this effect, such concrete is usually sticky, and hard to handle and deal with. In this sense, a proper understanding of the mechanisms regulating rheology of LC3 is needed. The objective of this study is to analyze workability of CC-based cement pastes and mortar, specifically investigating the role of free water in particle suspensions. Preliminary results show that CC highly influences workability of mortars and pastes. The flow table test results highlight a need to increase SP dosage to achieve target workability with CC cements. Differential scanning calorimetry (DSC) and 1 H time domain-nuclear magnetic resonance (TD-NMR) results clarify that the capillary unbound water is rapidly consumed by CC, being thus unavailable to fluidify cement pastes. This multi-method approach provides a further step in understanding CC impact on workability of mortars with low-carbon cement and opens new ways to understand paste, mortar, and concrete workability

Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium–sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium–sodium aluminosilicate gel structures than that previously established in the literature

Relation of expansion due to alkali silica reaction to the degree of reaction measured by SEM image analysis

Scanning Electron Microscopy Image Analysis (SEM-IA) was used to quantify the degree of alkali silica reaction in affected microbars, mortar and concrete prisms. It was found that the degree of reaction gave a unique correlation with the macroscopic expansion for three different aggregates, stored at three temperatures and with two levels of alkali. The relationships found for the concretes and the mortars overlap when normalised by the aggregate content. This relationship seems to be linear up to a critical reaction degree which coincides with crack initiation within the reactive aggregates. © 2007 Elsevier Ltd. All rights reserved

Quantitative microstructural characterisation of cement paste submitted to different moderate temperature and loading conditions

The temperature is known to influence the strength and durability of concrete. Temperature has also a great influence on the creep of cementitious materials. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. In this paper, a combination of macro and micro characterisation has been adopted to further a comprehensive and quantitative understanding of the development of the mechanical and microstructural properties under different loading conditions and moderate temperatures (< 100°C). Different approaches including both qualitative and quantitative microstructural analysis with SEM, EDS, XRD and TGA were used. A study on different cement pastes has been carried out, to relate microstructural properties such as degree of hydration, CSH composition, relative density and capillary porosity to macroscopic observations. The chemistry of CSH is examined with temperature, the quantity of bound water for different curing conditions was investigated. © 2009 Taylor & Francis Group