A new performance test to evaluate the sulfate resistance of concrete by tensile strength measurements

Concrete structures without sufficient durability can be damaged by sulfates in groundwater and from surrounding rock layers. To evaluate the performance of a concrete mixture, precise and performance-oriented test methods are a must. Therefore, a new a performance oriented concrete test procedure based on tensile strength measurements was developed considering experiences reported in international literature and recommendations of state-of-the-art reports. A vast parameter study with approx. 3850 tensile tests on ASTM briquets, 1900 flexural tensile tests on standard prisms and 2100 expansion tests on mortar flat prisms of different ages and with different storage conditions was statistically assessed. Based on the results a performance-oriented test method could be defined which considers not only the chemical, but also the physical resistance of a concrete against sulfate attack. The method was verified by 23 concretes with different cements or cement fly ash combinations and additional field tests. It could clearly be demonstrated that the results represent the performance of a practical concrete in case of sulfate attack. Furthermore, it leads much faster to an evaluation of the sulfate resistance compared to the most other practical oriented methods

Nucleation seeding with calcium silicate hydrate – A review

The development of green cements, with the aim of reducing CO2 emissions, often results in reduced hydration activity, especially during the first hours and days. Nucleation seeding with C-S-H has enormous potential to accelerate hydration, which can compensate for the above-mentioned effect without compromising the long-term strength of seeded cements. In this work, the effects of calcium silicate hydrate are reviewed in detail, with a focus on synthesis, as well as their influence on the hydration mechanism and the development of mechanical properties, such as early and long-term compressive strength and porosity

Estimation of standard molar entropy of cement hydrates and clinker minerals

It is not straightforward to experimentally measure the standard molar entropy of cement hydrates or clinker minerals. This is further compounded by the controversies surrounding the entropy values reported in established thermodynamic datasets for cements. The purpose of this study is to assess the reliability of standard entropies compiled in those datasets. To this end, a simple but robust method is used in which the standard entropy of an inorganic solid is correlated to its formula unit volume via a linear equation. The results of this analysis show that the standard entropies and/or molar volumes (and in cases solubility products) of the following phases deserve closer scrutiny: meta-ettringite phases; magnesium/aluminium layered double hydroxide solid solutions; almost all iron-bearing monosulfate and hydrogarnet phases; and several calcium silicate hydrate solid solution end-members. In addition, this study reports the provisional estimates for the standard entropies of minerals ternesite and ye'elimite

Thermodynamic modelling of alkali-activated slag cements

This paper presents a thermodynamic modelling analysis of alkali-activated slag-based cements, which are high performance and potentially low-CO2 binders relative to Portland cement. The thermodynamic database used here contains a calcium (alkali) aluminosilicate hydrate ideal solid solution model (CNASH_ss), alkali carbonate and zeolite phases, and an ideal solid solution model for a hydrotalcite-like Mg-Al layered double hydroxide phase. Simulated phase diagrams for NaOH- and Na2SiO3-activated slag-based cements demonstrate the high stability of zeolites and other solid phases in these materials. Thermodynamic modelling provides a good description of the chemical compositions and types of phases formed in Na2SiO3-activated slag cements over the most relevant bulk chemical composition range for these cements, and the simulated volumetric properties of the cement paste are consistent with previously measured and estimated values. Experimentally determined and simulated solid phase assemblages for Na2CO3-activated slag cements were also found to be in good agreement. These results can be used to design the chemistry of alkali-activated slag-based cements, to further promote the uptake of this technology and valorisation of metallurgical slags

Effect of temperature and aluminium on calcium (alumino)silicate hydrate chemistry under equilibrium conditions

There exists limited information regarding the effect of temperature on the structure and solubility of calcium aluminosilicate hydrate (C–A–S–H). Here, calcium (alumino)silicate hydrate (C–(A–)S–H) is synthesised at Ca/Si = 1, Al/Si ≤ 0.15 and equilibrated at 7–80 °C. These systems increase in phase-purity, long-range order, and degree of polymerisation of C–(A–)S–H chains at higher temperatures; the most highly polymerised, crystalline and cross-linked C–(A–)S–H product is formed at Al/Si = 0.1 and 80 °C. Solubility products for C–(A–)S–H were calculated via determination of the solid-phase compositions and measurements of the concentrations of dissolved species in contact with the solid products, and show that the solubilities of C–(A–)S–H change slightly, within the experimental uncertainty, as a function of Al/Si ratio and temperature between 7 °C and 80 °C. These results are important in the development of thermodynamic models for C–(A–)S–H to enable accurate thermodynamic modelling of cement-based materials

A thermodynamic model for C-(N-)A-S-H gel: CNASH_ss. Derivation and validation

The main reaction product in Ca-rich alkali-activated cements and hybrid Portland cement (PC)-based materials is a calcium (alkali) aluminosilicate hydrate (C-(N-)A-S-H) gel. Thermodynamic models without explicit definitions of structurally-incorporated Al species have been used in numerous past studies to describe this gel, but offer limited ability to simulate the chemistry of blended PC materials and alkali-activated cements. Here, a thermodynamic model for C-(N-)A-S-H gel is derived and parameterised to describe solubility data for the CaO–(Na2O,Al2O3)–SiO2–H2O systems and alkali-activated slag (AAS) cements, and chemical composition data for C-A-S-H gels. Simulated C-(N-)A-S-H gel densities and molar volumes are consistent with the corresponding values reported for AAS cements, meaning that the model can be used to describe chemical shrinkage in these materials. Therefore, this model can provide insight into the chemistry of AAS cements at advanced ages, which is important for understanding the long-term durability of these materials

Thermodynamics of cement hydration

The use of thermodynamic methods in cement hydration was often doubted, as the watercement system was considered to be too complex to model. Furthermore metastable features occur, e.g. C-S-H, which lead to the conclusion cement hydration is a 'non-equilibrium' process. Nevertheless several literature studies prove that cement hydration follows the basic principles of physical chemistry by minimisation of the free energy of an isochemical system. Hence thermodynamic equilibrium models are useful to assess and predict mineralogical changes during cement hydration. However the success and the accuracy of these predictions are strongly linked to a reliable thermodynalU'ic database, including the standard state properties ofthe aqueous species and the cement hydrates. Whereas the thermodynamic properties of the aqueous ions are well described in the literature, the dataset for cement hydrates is incomplete or inconsistent, or both. Thus the main goal of this Thesis was to develop a consistent thermodynamic database, which enables the assessment of the constitution of hydrated Portland cements. Because hydrated concretes are exposed to different service temperatures, data were obtained in the range -1°C to 99°C. The database is developed for commonly-encountered cement substances including C-S-H, Ca(OHh selected AFm, AFt and hydrogarnet compositions as well as solid solutions. Literature data were critically assessed and completed with own experiments. The tabulated thermodynamic properties were derived by a harmonisation ofthe available data. The new database enables the hydrate mineralogy to be calculated from the bulk chemical composition of the system: most solid assemblages, the persistence of C-S-H and failure to nucleate siliceous hydrog~rnet apart, correspond closely to equilibrium. This realisation means that hydrate assemblages can be controlled. The development of a thermodynamic approach also enables a fresh look at how mineralogical changes occur as a function of cement composition as well as in response to environmentally-conditioned reactions. The constitution of the AFm phase in Portland cement is very sensitive with respect to its chemical environment. Carbonate is shown to interact strongly with stabilisation of AFm across a broad range oftemperatures and, at low temperatures, to substitute into AFt. Relative to previous databases, sulfate AFm is shown to have a defmite range of stability at 25°C thus removing long-standing doubts about its stability in normal hydrated cement pastes. Keywords: thermodynamics, thermodynamic data, modelling, cement hydration, AFm, AFt, sulfate, carbonateEThOS - Electronic Theses Online ServiceGBUnited Kingdo