Review of fly-ash as a supplementary cementitious material

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

Modelling the hydrating behaviour of fly-ash in blended cements using thermodynamics

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

Thermodynamic Modelling of Harsh Environments on the Solid Phase Assemblage of Hydrating Cements Using PHREEQC

Poor durability of reinforced concrete structures can lead to serious structural failures. An accurate model to observe the effects of aggressive agents like carbonation, sulfate ingress, and seawater solutions on the solid phase assemblagewill help designers and specifiers better understand howcement behaves in these environments. This paper presents the first steps in developing such a model using the PHREEQC geochemical software by accounting for alkali binding and dissolution. It also presents the use of discrete solid phases (DSPs) to account for the solid-solution behaviour of siliceous hydrogarnet and magnesium silicate hydrate (M-S-H). A new thermodynamic description of the vaterite phase has also been developed for this work using the cemdata18 thermodynamic database. The predicted phase assemblages of cements in these environments here agree with previously published findings using a different thermodynamic model supported with experimental data

Employing Discrete Solid Phases to Represent C-S-H Solid Solutions in the Cemdata07 Thermodynamic Database to Model Cement Hydration Using the PHREEQC Geochemical Software

This paper presents a cement hydration model over time using the cemdata07 thermodynamic database and a series of derived discrete solid phases (DSPs) to represent calcium silicate hydrate (C-S-H) as a binary solid solution with two end-members. C-S-H in cement is amorphous and poorly crystalline with a range of molar Ca/Si ratios from 0.6 to 1.7. It displays strongly incongruent dissolution behaviour, where the release of calcium into solution is several orders of magnitude greater than silicon. It is, therefore, important that any cement hydration model provides a credible account of this behaviour. C-S-H has been described in the cemdata07 thermodynamic database as a number of solid solutions using different end-members with differing levels of complexity. While solid solutions can be included in most modern geochemical software programs, they often lead to a significant increase in computation time. This paper presents how an incongruent solid solution between two C-S-H end-members may be represented as a number of DSPs to model cement hydration over time using the PHREEQC geochemical software. By using DSPs rather than modelling C-S-H as a nonideal solid solution, this gives the user full control of the input for the model, reducing the computational demand and analysis time with no loss in accuracy in predicting stable-phase assemblages and their associated pore chemistry over time

Comparing the Measured and Thermodynamically Predicted AFm Phases in a Hydrating Cement

In hydrating Portland cements, more than one of the AFm family of calcium aluminates may exist. Depending on the amount of carbonate and sulfate present in the cement, the most common phase to precipitate is monosulfate, monocarbonate and/or hemicarbonate. It has been reported in the literature that hemicarbonate often appears in measurements such as XRD but not predicted to form/equilibrate in thermodynamic models. With the ongoing use of commercial cements such as CEM I and CEM II containing more and more limestone, it is important to understand which hydrate solids physically precipitate and numerically predict over time. Using 27 cement samples with three w/c ratios analysed at 1, 3 and 28 days, this paper shows that although hemicarbonate was observed in a hydrating commercial Portland cement, as well as being predicted based on its carbonate (CO2/Al2O3) and sulfate (SO3/Al2O3) ratios, thermodynamic analysis did not predict it to equilibrate and form as a solid hydrate. Regardless of the w/c ratio, thermodynamic analysis did predict hemicarbonate to form for calcite contents \u3c 2 wt.%. It appears that the dominant stability of monocarbonate in thermodynamic models leads to it precipitating and remaining as a persistent phase

Deriving discrete solid phases from CSH-3T and CSHQ end-members to model cement hydration in PHREEQC

This paper presents a cement hydration model over time using the CEMDATA thermodynamic database and a series of discrete solid phases (DSP) to represent calcium silicate hydrate (C-S-H) as a ternary (CSH-3T) and quaternary (CSHQ) solid solution. C-S-H in cement is amorphous and poorly crystalline with a range of molar Ca/Si ratios = 0.6-1.7 and displays strongly incongruent dissolution behaviour where the release of calcium into solution is several orders of magnitude greater than silicon. It is therefore important that any cement hydration model provides a credible account of this behaviour. C-S-H has been described in the CEMDATA thermodynamic database as a number of binary, ternary and quaternary solid solutions using different end-members with differing levels of complexity. While solid solutions can be included in most modern geochemical software programs, it often leads to a significant increase in computation time. This paper presents how the two of the more complex C-S-H solid solutions, CSH-3T and CSHQ, available in the CEMDATA database, can be represented by DSP to model cement hydration over time using the PHREEQC geochemical software. By using DSP in place of solid solutions, analysis time is much improved with no loss in accuracy in producing stable phase assemblages and reasonable predictions of pH over time

Using Photovoltaics to Power Electrochemical Chloride Extraction from Concrete

Corrosion of embedded steel in reinforced concrete (RC) is a world-wide problem, that reduces structural performance and lifespan. Chloride attack may be a result of seawater, de-icing salts or contaminated admixtures, brought on by ingress of chlorides into the concrete. Electrochemical Chloride Extraction (ECE) is a non-destructive treatment for contaminated RC structures, that due to uncertainty of treatment times and applied current densities, is only 50% effective. It is often diesel powered has an environmental impact and often very costly due to the long treatment times. To improve the efficiency of ECE the influences of concrete resistance, cement type and duration of treatment have been investigated in an experimental programme. The use of Photovoltaic (PV) panels to improve the efficiency of ECE is presented which replace fossil fuels as a power source enabling a more environmentally sustainable treatment. These findings will increase the life span of vital infrastructure and reduce expensive ongoing repairs with decreased traffic congestion and inconveniences associated with bridge repairs

Using PHREEQC to model cement hydration

This paper presents the steps involved in undertaking an analysis of hydrating cements with different levels of limestone powder using the PHREEQC geochemical software with the Notepad++ editor. The analysis begins with determining which solid phases are thermodynamically predicted to precipitate and form using the oxide compositions of commercial CEM I and CEM II/A-L cements. When the phases are known, PHREEQC is programmed to provide predictions of the phase dissolution and phase assemblage over time (here, 1000 days of hydration) as well as the pore solution chemistry. Thermodynamics has been successfully applied to the field of cement hydration to predict phase assemblages and pore solution changes. With an appropriate cement-based thermodynamic database, PHREEQC has the potential to be a very powerful tool in the ongoing development of sustainable cements into the future. The paper also discusses the ongoing work to couple PHREEQC with the HYDCEM model to provide users with an all-in-one platform to undertake a complete simulation of cement hydration

Performance Modeling and Analysis of a Thermoelectric Building Envelope for Space Heating

To provide energy-efficient space heating and cooling, a thermoelectric building envelope (TBE) embeds thermoelectric devices in building walls. The thermoelectric device in the building envelope can provide active heating and cooling without requiring refrigerant use and energy transport among subsystems. Thus, the TBE system is energy and environmentally friendly. A few studies experimentally investigated the TBE under limited operating conditions, and only simplified models for the commercial thermoelectric module (TEM) were developed to quantify its performance. A holistic approach to optimum system performance is needed for the optimal system design and operation. The study developed a holistic TBE-building system model in Modelica for system simulation and performance analysis. A theoretical model for a single TEM was first established based on energy conversion and thermoelectric principles. Subsequently, a TBE prototype model combining the TEM model was constructed. The prototype model employing a feedback controller was used in a whole building system simulation for a residential house. The system model computed the overall building energy efficiency and dynamic indoor conditions under varying operating conditions. Simulation results indicate the studied TBE system can meet a heating demand to maintain the desired room temperature at 20 °C when the lowest outdoor temperature is at -26.3 degrees C, with a seasonal heating COP near 1.1, demonstrating a better heating performance than electric heaters. It suggests a potential energy-efficient alternative to the traditional natural gas furnaces and electric heaters for space heating

Characterization and Performance Enhancement of Cement-Based Thermoelectric Materials

Thermoelectric materials enable the direct conversion of thermal to electrical energy. One application of this is ambient heat energy harvesting where relatively stable temperature gradients existing between the inside and outside of a building could be utilized to produce electricity. Buildings can thus change from energy consumers to energy generators. This could ultimately help reduce the surface temperatures and energy consumption of buildings, especially in urban areas. In this paper, research work carried out on developing and characterizing a cement-based thermoelectric material is presented. Cement-based samples are doped with different metal oxides (Bi2O3 and Fe2O3) to enhance their thermoelectric properties, which are defined through their Seebeck coefficient, electrical conductivity and thermal conductivity. The study also discusses the positive impact of moisture content on the electrical conductivit