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This paper presents a new method to thermodynamically model the hydration behaviour of fly-ash (FA) blended cements by deriving individual phase descriptions depending on the proportion of FA in the blended cement. The predicted hydrated phase assemblage, pore solution chemistries and pH over 1,000 days of hydration and with increasing FA proportions are presented. The thermodynamic data for the FA phases are derived using oxide proportions and mineral compositions are copied directly into the PHREEQC input file. The FA phases take account of all minerals to give a more accurate description of its behaviour during hydration. The calcium aluminosilicate hydrate (C-A-S-H) gel model consists of several Discrete Solid Phases (DSPs) derived from the quinary solid solution end-members in the cemdata18 database [1]. This method has been used previously by the authors to give reliable and computationally efficient results when modelling OPC hydration and extended here for C-A-S-H, accounting for its strongly incongruent dissolution. A number of blended cements with FA contents ranging from 0-35% (in 5% steps) were simulated. As the amount of FA in the blended cement increases, the results show a destabilization of calcium hydroxide at higher replacement levels, more hydrotalcite than OPC, the formation of strätlingite and AFm & AFt phases like monosulfate/monocarbonate and ettringite respectively. The dissolution of Portland cement is modelled using a well-known empirical approach. FA dissolution is modelled using an approach taken from the literature that gave good correlations with experimental data.

This paper presents a review of fly-ash as a Supplementary Cementitious Material (SCM) in concrete in terms of its effects on hydration and durability. The climate change agenda has focused the cement and concrete industry on using low embodied CO2 materials and much effort has been made on incorporating industrial by-products into cement as SCMs. With worldwide cement production (circa 4 billion tonnes) currently accounting for approximately 8% of global CO2 emissions and 7% of industry energy use, the use of suitable SCMs to partially replace cement in concrete is extremely important. However, while coal-fired power stations are in the decline, due to the need for more sustainable energy generation, there remains stockpiles of fly-ash for potential use as an SCM. This creates opportunities for ashes not previously used in concrete to be studied both in terms of its behaviour during hydration and durability performance in harsh environments. However, these new fly-ash sources need to be studied carefully due to uncertainties about their physical and chemical constituents, reactivity, long term stability and phase relationships and minor elements distribution due to the variability in the source of coal. The work presented includes a review of fly-ash in terms of its effects during cement hydration and contribution to concretes performance in harsh environments from the literature.

Thermodynamics have been successfully applied to the field of cement hydration science to predict the formation of phase assemblages and pore solution chemistry. For any cement hydration model to be accepted, it must provide accurate forecasts of which solids may form and how the cement will dissolve over time. This is particularly important for the ongoing development of new sustainable cements and understanding their hydration behaviour in service. HYDCEM is a cement hydration model that simulates volumetric changes of cement and gypsum dissolution and product growth that, up to now, assumed which solids would form. In order to improve its usefulness, the PHREEQC geochemical software has been coupled with HYDCEM to provide more sophisticated and flexible predictions of which phases may form under equilibrium conditions and generate their change in volume over time for curing temperatures between 5-45°C, variable w/c ratio and cement oxide compositions. To incorporate the coupling of PHREEQC into the model, HYDCEM was re-written in the C# programming language (previously coded in MATLAB) which also improved overall performance and functionality. This paper presents analysis of a cement system with a w/c ratio of 0.5 at a curing temperature of 20°C and provides predictions of the phase assemblage, phase and product changes in volume and heat evolution over a 1,000-day period in one hour time-steps.

The compressive strength of mortar is a significant property that will influence its performance in concrete or masonry. Being able to accurately model and predict the mortar compressive strength would be of great benefit to suppliers and end users alike that could possibly reduce the need for multiple physical testing. A section of the original HYDCEM cement hydration model (amoungst others) has been partitioned to focus on predicting the compressive strength of Portland cement and cement-limestone mortars, entitled HYDCEM_CompressiveStrength. The model uses the cement/binder oxide composition along with other inputs to predict the compressive strength development over time. This paper presents a study into how accurately the HYDCEM_CompressiveStrength model can predict the mortar’s compressive strength over time for European cements. Experimental results of mortar cube’s and bar’s compressive strength in accordance with ASTM C 109 for a CEM I + 10% limestone binder and EN196-1 for a CEM I and CEM II cement are presented along with predictions from the model following a parametric study. Comparisons have shown reasonably good agreement between measured and predicted values over time.

Hydration models can aid in the prediction, understanding and description of hydration behaviour over time as the move towards more sustainable cements continues. HYDCEM is a new model to predict the phase assemblage, degree of hydration and heat release over time for cements undergoing hydration for any w/c ratio and curing temperatures up to 450C. HYDCEM, written in MATLAB, complements more sophisticated thermodynamic models by predicting these properties over time using user-friendly inputs within one code. A number of functions and methods based on up to date cement hydration behaviour from the literature are hard-wired into the code along with user-changeable inputs including w/c ratio, curing temperature, chemical compositions, densities and enthalpies. Predictions of hydration product volumes from the silicate, aluminate and ferrite phases can be determined, including C-S-H, calcium hydroxide, hydrogarnet (if applicable) ettringite and monosulfate. A number of comparisons have been made with published phase assemblages using thermodynamic models and HYDCEM predictions to assess its accuracy and usefulness. This paper presents simulations of cement hydration and microstructure development with and without the additional of ground limestone using the HYDCEM model, both in terms of monocarbonate growth at the expense of monosulfate and ettringite. Comparisons with published phase assemblages show good agreement in terms of volumetric growth and behaviour.