Showing papers in "Cement and Concrete Research in 2019"
TL;DR: The Cemdata18 database as mentioned in this paper contains thermodynamic data for common cement hydrates such as C-S-H, AFm and AFt phases, hydrogarnet, hydrotalcite, zeolites, and M-S -H that are valid over temperatures ranging from 0 to at least 100°C.
Abstract: Thermodynamic modelling can reliably predict hydrated cement phase assemblages and chemical compositions, including their interactions with prevailing service environments, provided an accurate and complete thermodynamic database is used. Here, we summarise the Cemdata18 database, which has been developed specifically for hydrated Portland, calcium aluminate, calcium sulfoaluminate and blended cements, as well as for alkali-activated materials. It is available in GEMS and PHREEQC computer program formats, and includes thermodynamic properties determined from various experimental data published in recent years. Cemdata18 contains thermodynamic data for common cement hydrates such as C-S-H, AFm and AFt phases, hydrogarnet, hydrotalcite, zeolites, and M-S-H that are valid over temperatures ranging from 0 to at least 100 °C. Solid solution models for AFm, AFt, C-S-H, and M-S-H are also included in the Cemdata18 database.
TL;DR: In this paper, a review of emerging supplementary cementitious materials (SCM) sources is presented, along with new developments in characterizing and qualifying SCMs for use and improved knowledge of SCM on long-term concrete performance and durability.
Abstract: Conventional supplementary cementitious materials (SCMs), such as blast furnace slags or fly ashes, have been used for many decades, and a large body of knowledge has been collected regarding their compositional make-up and their impacts on cement hydration and concrete properties. This accumulated empirical experience can provide a solid, confident base to go beyond the status quo and develop a new generation of low-clinker cements composed of new types and combinations of SCMs. The need for new sources of SCMs has never been greater, as supplies of traditional SCMs are becoming restricted, and the demand for SCMs to reduce CO2 emissions from concrete production is increasing. In this paper, recent research on emerging SCM sources is reviewed, along with new developments in characterizing and qualifying SCMs for use and improved knowledge of SCMs on long-term concrete performance and durability.
TL;DR: In this paper, the authors review recent progress in the description and understanding of the reactivity of SCMs and their impact on Portland clinker hydration, as well as recent work studying the impact of common SCMs on hydration and microstructure of blended cements.
Abstract: Supplementary cementitious materials (SCMs) are key components of sustainable, low carbon cements. To maximize their use in blended cements, the impact of SCMs on cement hydration needs to be understood and accurately captured by models. A central element in such models is the reactivity of the SCM, which is tedious to measure. Establishing relationships between SCM properties and their intrinsic reactivity is therefore highly important. Moreover, mechanisms enhancing or limiting SCM reactivity in blended cements need to be well-understood. This work reviews recent progress in the description and understanding of the reactivity of SCMs and their impact on Portland clinker hydration. Insights derived from fundamental work using synthetic SCMs, dissolution experiments and model systems are discussed as well as recent work studying the impact of common SCMs on hydration and microstructure of blended cements. Particular attention is paid to recent work on calcined clays, which are currently receiving substantial interest.
TL;DR: In this article, an experimental study on the relation between the 3DCP process parameters and the bond strength of 3D printed concrete is presented, in which the effect of 3 process parameters (interlayer interval time, nozzle height, and surface dehydration) on two mechanical properties (compressive strength and tensile strength, determined through flexural and splitting tests), has been established, in three perpendicular directions.
Abstract: The technology of 3D Concrete Printing (3DCP) has progressed rapidly over the last years. With the aim to realize both buildings and civil works, the need for reliable mechanical properties of printed concrete grows. As a consequence of the additive manufacturing technique, 3D printed structures may consist of several layers that should exhibit bond to guarantee a safe structural design. This paper presents the results of an experimental study on the relation between the 3DCP process parameters and the bond strength of 3D printed concrete. The effect of 3 process parameters (interlayer interval time, nozzle height, and surface dehydration) on two mechanical properties (compressive strength and tensile strength, determined through flexural and splitting tests), has been established, in three perpendicular directions. A very limited influence of layer orientation was found for the given process-material combination, given a sufficiently short interlayer interval time. However, the bond strength between the layers reduced for increasing interlayer interval times. This was also reflected by the failure mode of the samples. The reduction in strength became more pronounced for the samples that were left uncovered during the interval time, exposed to drying. No clear relation was found between the height of the nozzle, and the bond strength between layers. The results of this study, in comparison to various other works on 3DCP, emphasize the need for standardization of test methods and characterization of 3D printed concrete, as individual process parameters clearly must be considered in relation to the applied material and other process parameters.
TL;DR: The development of low-carbon binders has been recognized as a means of reducing the carbon footprint of the Portland cement industry, in response to growing global concerns over CO2 emissions from the construction sector as mentioned in this paper.
Abstract: The development of low-carbon binders has been recognized as a means of reducing the carbon footprint of the Portland cement industry, in response to growing global concerns over CO2 emissions from the construction sector. This paper reviews recent progress in the three most attractive low-carbon binders: alkali-activated, carbonate, and belite-ye'elimite-based binders. Alkali-activated binders/materials were reviewed at the past two ICCC congresses, so this paper focuses on some key developments of alkali-activated binders/materials since the last keynote paper was published in 2015. Recent progress on carbonate and belite-ye'elimite-based binders are also reviewed and discussed, as they are attracting more and more attention as essential alternative low-carbon cementitious materials. These classes of binders have a clear role to play in providing a sustainable future for global construction, as part of the available toolkit of cements.
TL;DR: A review of the state of the art in the newly forming field of digital fabrication with concrete, and aims to provide some direction in terms of the research challenges encountered thus far is provided in this article.
Abstract: Digital fabrication techniques with concrete and cementitious materials have seen a large amount of research and industrial activity recently, with industrialization of techniques such as 3D printing becoming more of a reality. The potential to revolutionize construction is real, not only through reducing costs, but also bringing more sustainability and increased functionality. Material challenges are significant, chief among them understanding and controlling early age hydration and the link to rheology, incorporation of reinforcement, and overall, the link between processing, material, and performance, both from a structural and durability point of view. Interdisciplinarity is crucial, as the field brings together many disparate fields and has been driven by fields such as architecture so far. This article is a review of the state of the art in the newly forming field of digital fabrication with concrete, and aims to provide some direction in terms of the research challenges encountered thus far.
TL;DR: In this article, the authors reviewed the hydration mechanisms of alite and Portland cement and found that the main heat evolution peak hydration is dominated by the growth of outer C-S-H with a spiky or needle-like morphology.
Abstract: Progress in understanding hydration mechanisms of alite and Portland cement is reviewed. Up to the end of the induction period, dissolution rates determined by the undersaturation of the solution dominate the reaction, but, better understanding is needed about the alite solution interface. The main heat evolution peak hydration is dominated by the growth of outer C-S-H with a spiky or “needle” like morphology. Growth is rapid over several hours (acceleration period) and then slows (deceleration period). At later ages the consumption of water and lack of water filled pores above about 10 nm, along with the consumption of anhydrous material are major factors leading to the continual reduction in the rate of reaction. There is no evidence that diffusion becomes the rate controlling mechanism even at this stage. The microstructure of cement differs significantly from that of alite, largely due to the influence of alumina on C-S-H growth and distribution.
TL;DR: In this paper, a large amount of studies are devoted to seeking synergies between FA, HCFA, granulated blast furnace slag (GBFS), and limestone, focusing on durability characteristics of composites cement containing FA and HCFA.
Abstract: Low-calcium (FA) and high-calcium (HCFA) fly ash and granulated blast furnace slag (GBFS) are the most widely known, standardized and used SCMs in the composition of cement and concrete. In the last 4 years, scientific work has focused on improving binder properties (e.g. long setting time, low early strength etc.) containing large quantities of FA, HCFA and GBFS. The main directions of activity are the introduction of high-level additives to concrete composition, such as nano-materials, chemical and mechanical activation. Due to the limited access to FA and GBFS, a large amount of studies is devoted to seeking synergies between FA, HCFA, GBFS and limestone. The research works focused on durability characteristics of composites cement containing FA, HCFA and GBFS. Moreover, attention was given to prospects of future application of other types of fly ashes and slags in cement and concrete.
TL;DR: In this article, the authors presented the first analysis of a large data set (>10,000 observations) of measured compressive strengths from actual (job-site) mixtures and their corresponding actual mixture proportions.
Abstract: The use of statistical and machine learning approaches to predict the compressive strength of concrete based on mixture proportions, on account of its industrial importance, has received significant attention. However, previous studies have been limited to small, laboratory-produced data sets. This study presents the first analysis of a large data set (>10,000 observations) of measured compressive strengths from actual (job-site) mixtures and their corresponding actual mixture proportions. Predictive models are applied to examine relationships between the mixture design variables and strength, and to thereby develop an estimate of the (28-day) strength. These models are also applied to a laboratory-based data set of strength measurements published by Yeh et al. (1998) and the performance of the models across both data sets is compared. Furthermore, to illustrate the value of such models beyond simply strength prediction, they are used to design optimal concrete mixtures that minimize cost and embodied CO2 impact while satisfying imposed target strengths.
TL;DR: In this article, a rice husk ash (RHA)-based reactive filler was used in ultra-high performance concrete (UHPC) to improve mechanical properties without heat-treatment.
Abstract: In this study, rice husk ash (RHA)-based reactive filler was used in ultra-high performance concrete (UHPC) to improve mechanical properties without heat-treatment. This strategy replaces inert quartz filler with the reactive RHA filler, to increase the amorphous silica content while maintaining the physical role of the micron-sized quartz filler. Due to the high porosity of RHA, internal curing is effective, which promotes the hydration reaction over a long period of time. Experimental results show an outstanding strength around 200 MPa after 91 days, under ambient conditions (20 °C and 60% relative humidity). This was possible due to the promotion of pozzolanic reaction by additional water and amorphous silica provided by the porous (i.e., internal curing effect) and reactive filler, respectively; hence, the volume of capillary pores was reduced. The result reported herein will further promote the utilization of agricultural byproduct for the development of reactive RHA-based construction materials.
TL;DR: In this paper, an overview of the rheological properties of UHPC, applicable flow models, measurement techniques and errors associated with the interpretation of Rheological measurements are discussed.
Abstract: Ultra-high-performance concrete (UHPC) is attracting increasing interests worldwide due to its superior mechanical properties and durability. Securing proper rheological properties can affect fiber dispersion and alignment with marked effect on UHPC performance. Tailoring the rheological properties of UHPC to secure enhanced performance is not widely considered in the mixture design stage. In this paper, an overview of the rheological properties of UHPC, applicable flow models, measurement techniques and errors associated with the interpretation of rheological measurements are discussed. The effect of various constituent materials on rheological properties of UHPC is presented. This includes the cementitious materials, sand, chemical admixtures, fibers, nanomaterials, and internal curing agents. Most importantly, the paper discusses the rheological properties requirements of UHPC and strategies to control rheology of UHPC targeted for different applications, such as repair and rehabilitation, bridge deck panel connections, construction of structural and architectural elements, and digital fabrication.
TL;DR: In this paper, a review of service life modelling and prediction of reinforced concrete structures is presented, and a critical review on common assumptions in service life modeling and on the application and limitations of various approaches is presented.
Abstract: Ever increasing attention is being paid to deterioration prediction and service life modelling of reinforced concrete structures. Research has progressed to a stage where service life models and design philosophies are, to varying degrees, included in some codes and standards, such as the fib Model Codes and ISO 13823. This has helped to base practical durability design on sound engineering approaches. This paper reviews service life modelling and prediction, and service life design, covering limit state design philosophies and deterioration models. An overview on recent developments, and a critical review on common assumptions in service life modelling and on the application and limitations of the various approaches, are presented. It is emphasised that design approaches and models need to be validated with field observations. It is argued that a performance-based approach is the most suitable engineering tool for durability design.
TL;DR: In this article, the effects of RFP on the hydration, microstructure, shrinkage and mechanical properties of ultra-high performance engineered cementitious composites (UHP-ECC) with different replacement ratios up to 50% were investigated.
Abstract: The recycled fine-powder (RFP), produced during the recycling process, will induce a serious impact on the environment with improper disposition. A potential green way to reuse RFP is to add it as supplementary cementitious material in concrete. The effects of RFP on the hydration, microstructure, shrinkage and mechanical properties of ultra-high performance engineered cementitious composites (UHP-ECC) with different replacement ratios up to 50% were investigated. The hydration kinetics were compared among the different replacement ratios using the isothermal calorimetry, which demonstrated an accelerating effect of RFP to the hydration of UHP-ECC matrix. The phase development was quantified by the thermal gravimetric analysis and proved the pozzolanic effect of RFP. The compressive and tensile properties of UHP-ECCs were obtained at 3, 7 and 28 days, respectively, to trace their development along the curing ages. The addition of RFP significantly reduced the autogenous shrinkage of UHP-ECC. Besides, the single fiber pullout test was investigated to quantify the influence of RFP at the fiber level. The environmental scanned electron microscope analysis was conducted to study the morphology of PE fiber at the fracture surface.
TL;DR: In this paper, a simple process to produce sodium silicate powder from glass cullet has been developed, and a mixture of glass powder, sodium hydroxide powder, and water was heated at temperatures of 150 to 330°C.
Abstract: A simple process to produce sodium silicate powder from glass cullet has been developed. A mixture of glass powder, sodium hydroxide powder, and water was heated at temperatures of 150 to 330 °C. The effects of glass to NaOH ratio, temperature and duration, inclusion of water and fineness of NaOH were investigated. Fly ash and fly ash/GGBS blends were the precursors for alkali activated binder (AAB) mortars produced with this sodium silicate. Compressive strengths were similar to or better than those obtained with commercially available sodium silicate and sodium hydroxide solutions. FT-IR tests suggested that the reactivity of the glass derived sodium silicate powder was related to the number of non-bridging oxygen atoms in the silicate structure. Cost comparison between AAB and Portland cement concretes gave similar results for normal strength concretes (35 MPa). AAB concretes with higher strengths (50 and 70 MPa) can be cheaper than equivalent traditional concrete.
TL;DR: In this article, the effect of silica fume content, ranging from 0 to 25%, by mass of cementitious materials, on rheological, fiber-matrix bond, and mechanical properties of non-fibrous UHPC matrix and uHPC made with 2% micro-steel fibers was investigated.
Abstract: High silica fume content used in ultra-high performance concrete (UHPC) can increase viscosity and render agglomeration issue, leading to reduction in mechanical properties, including bond to fibers. This study investigates the effect of silica fume content, ranging from 0 to 25%, by mass of cementitious materials, on rheological, fiber-matrix bond, and mechanical properties of non-fibrous UHPC matrix and UHPC made with 2% micro-steel fibers. Binary mixtures consisting of cement and silica fume with targeted mini-slump flow were specifically prepared. The involved mechanical properties include compressive, flexural, and tensile behavior. Rheology, fiber-matrix bond, flexural, and tensile strengths of UHPC were linked to each other using fiber dispersion and orientation analyses or the Composite Theory. Test results showed that UHPC made with 10% to 15% silica fume obtained the highest fiber-matrix bond, flexural, and tensile properties. Such silica fume content was found to result in lower viscosity and more uniformly distributed fibers as determined by image analysis. The flexural and tensile strengths of UHPC made with 5%–20% silica fume can be effectively predicted using the Composite Theory considering obtained fundamental inputs, such as flexural or tensile strengths of matrix, fiber characteristics, and fiber-matrix bond strength. The experimental-to-predicted tensile and flexural strength ratios were in the range from 0.9 to 1.1. However, the predicted strengths of UHPC mixtures with 0 and 25% silica fume were greatly lower than the experimental values due to high viscosity and low packing density.
TL;DR: Wang et al. as mentioned in this paper presented an optimized design method in developing ultra-high performance concrete with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC packing mode are considered.
Abstract: This study presents an optimized design method in developing ultra-high performance concrete (UHPC) with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC packing mode are considered. Specifically, a model based on D-optimal method is firstly established to assess the influence of solid granular particles and superplasticizer (SP) on the UHPC packing density. Based on the theoretical analysis, the mixture proportions can be optimized, and UHPC with high wet packing density could be produced. Then, to evaluate the reliability of the math-physical method in developing UHPC, its compressive strength and pore structure are tested. The obtained experimental results show that the designed UHPC contributes high packing density, leading to optimized pore structure and extraordinary compressive strength. The experimental verification further highlights that D-optimal method is a promising and effective approach to design UHPC, and eventually for the development of sustainable UHPC.
TL;DR: In this article, a silane polymer emulsion treatment method was used to promote the strength of pervious concrete while maintaining its permeability, and the results revealed that silane treatment significantly improved the strength while maintaining acceptable permeability due to the redistribution of the cement paste.
Abstract: The applications of pervious concrete made with recycled aggregate (RA) have been hindered by its significantly reduced strength. In this study, a silane polymer emulsion treatment method was used to promote the strength of pervious concrete while maintaining its permeability. The mechanical and physical properties of RA pervious concrete were experimentally investigated. Membrane-forming ability tests and X-ray micro-tomography were conducted to determine the influence of the silane polymer emulsion on the membrane-forming behaviour and thickness distribution of cement paste. The results revealed that silane treatment significantly improved the strength of RA pervious concrete while maintaining acceptable permeability due to the redistribution of the cement paste. Meso-structure analyses confirmed that more cement pastes gathered around the bonding regions between adjacent RA particles. The findings suggest that this method can be used to enhance the strength of RA pervious concrete through effective utilisation of cement paste.
TL;DR: In this paper, the pore solutions of a series of hardened alkali-activated slag/fly ash pastes were extracted by the steel-die method, and analyzed using ICP-OES analysis technique.
Abstract: The pore solutions of a series of hardened alkali-activated slag/fly ash pastes were extracted by the steel-die method, and analyzed using ICP-OES analysis technique. According to the saturation index from thermodynamic calculations, the pore solutions of alkali-activated slag pastes kept oversaturated with respect to solid reaction products with time. In the pore solutions of alkali-activated fly ash pastes, an increase of temperature (from 40 °C to 60 °C) led to decreases of the concentrations of Si, Al, Ca, Na, OH−, K, Fe and Mg, while the soluble silicate in the alkaline activator resulted in increases of the concentrations of these elements. Compared to the alkali-activated slag paste with the same alkaline activator, 50% replacement of slag by fly ash did not result in a substantial change of the pore solution composition. Based on the experimental results, conceptual models were proposed to describe the elemental concentrations in the pore solutions.
TL;DR: In this article, it was shown that the drop in interface strength is due to the water evaporation from the free surface occurring during the short time interval between two successive layers.
Abstract: Requirements on material properties for extrusion-based additive manufacturing mostly focus on the rheological behavior of the cementitious material being printed. The layer interface strength is therefore often considered to result from a proper mixing or remixing of two consecutive layers induced by the deposition process itself and therefore from the material thixotropic behavior. We show however here that, in the case of smooth interfaces, the drop in interface strength finds its origin in the water evaporation from the free surface occurring during the short time interval between two successive layers. Our results and their analysis within the framework of drying physics suggest that the water loss is localized in a dry region at the free surface leading to an incomplete cement hydration and high local porosity. We moreover compare here various experimental protocols allowing for the assessment of a drop in bond strength.
TL;DR: In this article, the authors presented data on the chloride diffusion coefficient (Dcl), ageing coefficient (m) and chloride threshold (Clth) related to seven concrete mixes (four M35 and three M50) with OPC, OPC+PFA (pulverised fuel ash) and limestone-calcined clay cement (LC3).
Abstract: This paper presents data on the chloride diffusion coefficient (Dcl), ageing coefficient (m) and chloride threshold (Clth) related to seven concrete mixes (four M35 and three M50) with OPC, OPC + PFA (pulverised fuel ash) and limestone-calcined clay cement (LC3). Using these, the service lives of a typical bridge pier and girder with the PFA and LC3 concrete were found to be much higher than those with OPC concrete of similar strength. From life-cycle assessment, the CO2 footprint of PFA and LC3 concrete were found to be significantly lower than those of OPC concrete of similar strength. Further, the CO2 emissions per unit of concrete per year of estimated service life, as a combined indicator of service life and carbon footprint, are similar for concrete with PFA and LC3, which are much lower than that with OPC.
TL;DR: In this paper, the C-A-S-H composition, morphology and density are characterized for Limestone Calcined Clay Cement (LC3-50) blends containing clays with various kaolinite contents.
Abstract: In this study, the C-A-S-H composition, morphology and density are characterized for Limestone Calcined Clay Cement (LC3-50) blends containing clays with various kaolinite contents. Significant incorporation of aluminium is observed for LC3-50 blends compared with reference Portland cement (PC), and this incorporation increases with the kaolinite content of the calcined clay. No change of C-A-S-H morphology was observed by Transmission Electron Microscopy (TEM) between reference PC and LC3-50 blends, with a “fibrillar” morphology for all samples. The determination of the density of C-A-S-H measured by 1H NMR also shows very similar results with a bulk density close to 2.0 g·cm−3 and a solid density of around 2.8 g·cm−3 for all systems.
TL;DR: In this article, the critical chloride content (Ccrit) of reinforcement corrosion in concrete has been reviewed and the authors conclude that the state-of-the-art on Ccrit has advanced negligibly over the last decades and the time is now to change the way corrosion initiation in concrete is scientifically addressed.
Abstract: The critical chloride content (Ccrit) of reinforcement corrosion in concrete is of great importance for the condition assessment of existing structures and for the service life design of new structures in chloride exposure environments. Here, we review the extensive body of literature on Ccrit published in Chinese since the 1960s. In agreement with earlier reviews considering European and North American literature, we find that Ccrit scatters widely and cannot be predicted on the basis of parameters such as w/b ratio, binder type, steel surface condition, etc. It appears that the stress state of the reinforcing steel may play a more important role than generally assumed. On the basis of this review, complementing earlier reviews, we conclude that the state-of-the-art on Ccrit has advanced negligibly over the last decades. The time is now to change the way corrosion initiation in concrete is scientifically addressed. Recommendations for further work are made.
TL;DR: In this paper, the chemical deformation of metakaolin-based geopolymer (MKG) was investigated and correlations between the chemical deformations and the reaction processes during geopolymers were found.
Abstract: Chemical deformation (chemical shrinkage/expansion), the absolute volume change during reactions, is a key parameter influencing the volume stability, especially the autogenous deformation of a binder material. This work, for the first time, reports an in-depth investigation on the chemical deformation of metakaolin based geopolymer (MKG). Unlike ordinary Portland cement-based binders with monotonic chemical shrinkage, MKG experiences three stages of chemical deformations: chemical shrinkage in the first stage, chemical expansion afterward and chemical shrinkage again in the final stage. Various experimental techniques (XRD, FTIR and NMR) plus theoretical calculations are applied to explore the mechanisms behind the chemical deformation of MKG. Clear correlations are found between the chemical deformations and the reaction processes during geopolymerization. A conceptual chemical deformation model for geopolymer is summarised. The insights into the chemical deformation provided by this study will play a fundamental role in further understanding, controlling and even utilizing the deformation behaviours of geopolymers.
TL;DR: In this paper, the authors used CEM I with two w/c ratios (0.45 and 0.60) to characterize the possible altered zone and to identify the mechanisms of degradation under such conditions.
Abstract: Sulfate ions in seawater or underground can attack the cement paste leading to expansion and strength loss. This expansion is usually related to ettringite and gypsum precipitation. This study aims to characterize the possible altered zone and to identify the mechanisms of degradation under such conditions. For this study, cement pastes manufactured using CEM I with two w/c ratios (0.45 and 0.60) were exposed to sodium sulfate solutions (semi-immersion at a controlled pH = 8.0 ± 0.1) after one year of curing in water. The analysis of the physical aspect of this phenomenon shows that two modes of transfer occur: transfer of sulfate ions to the cement matrix and leaching of calcium ions to the external solution. The analysis of the chemical composition of the affected material highlights the progressive consumption of portlandite at the surface, the decalcification of C-S-H and the formation of AFt from both Afm and aluminium incorporated in C-S-H.
TL;DR: In this paper, the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC) has been investigated and the results demonstrate that mechanical properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressivestrength, which is opposite to normal concrete.
Abstract: This study addresses the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC). The results demonstrate that the mechanical properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressive strength, which is opposite to normal concrete. The nano-mechanical properties are also enhanced by heat treatment. The ultra-high density phase is dominated hydrates. Microstructure observation indicates that heat treatment promotes the formation of additional hydrates with high-packing density and stiffness such as tobermorite and xonotlite, enhancement of transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2. The evolution of hydrates and microstructure due to curing regimes and the presence of quartz play key roles in controlling the unusual behavior of the strength ratio and improvement of mechanical properties.
TL;DR: In this paper, the authors investigated whether chloride ingress testing can be done using NaCl solution instead of seawater when assessing the performance of concrete in marine conditions and found that the outer 1 1/5mm was enriched in sulfur and magnesium.
Abstract: This study investigates whether chloride ingress testing can be done using NaCl solution instead of seawater when assessing the performance of concrete in marine conditions. Seawater contains besides sodium and chlorine additional elements such as sulfur and magnesium, which can change the phase assemblage in the concrete and thereby affect chloride ingress. Mortar samples prepared with Portland cement were exposed to seawater or NaCl solution with a similar chloride concentration. After 180 days of exposure to seawater, only the outer 1 mm was enriched in sulfur and magnesium, which had only a limited impact on the chloride ingress. Leaching, observed in the outer 10–20 mm both for NaCl and for seawater exposure had a much stronger influence on the chloride ingress. Hence, chloride ingress in marine exposed concrete can be assessed using NaCl solutions. To mirror the leaching in field exposure, the volume of exposure solution needs to be high.
TL;DR: In this article, the effect of binder composition on hydrate assemblage in modern Portland composite cements and its effect on durability is investigated using thermodynamic models and modelling approaches.
Abstract: Thermodynamic modelling is essential to understand the effect of binder composition on hydrate assemblage in modern Portland composite cements and its effect on durability. Binary and ternary plots of the hydrates' volumes can visualize the effect of different SCMs at different hydration times. The pore solution data analysis and the derived saturation indices can be used to gain further insights into the processes governing the dissolution and precipitation of hydrates. The modelling of the hydrated cements exposed to different environments has contributed significantly to the understanding of the determining factors governing sulfate, chloride and carbonate attack. Thermodynamics models and modelling approaches have achieved a mature level. In the last years, more thermodynamic data have become available making thermodynamic calculations more reliable. The main gaps identified include the refinement of models describing the uptake of ions and water by C-S-H, as well as data for ASR products and zeolites.
TL;DR: In this paper, the authors investigated the effect of metakaolin and limestone on the sulfate balance of blended cements, and found that the filler effect of the SCM, which accelerates the reaction of alite, is the main factor impacting the sulfates balance, while no significant impact of aluminum content of these SCMs was observed.
Abstract: In some blended cements, the optimum amount of sulfate addition differs from that observed in OPC. This study aims to understand the mechanism behind the impact of two SCMs, namely metakaolin and limestone, on the sulfate balance of blended cements. No significant impact of the aluminum content of these SCMs was observed. Instead, it is observed that the filler effect of the SCM, which accelerates the reaction of alite, is the main factor impacting the sulfate balance. As the rate of precipitation of C-S-H is increased, more sulfate is adsorbed by the C-S-H and consequently, the depletion of gypsum is reached earlier in time during the hydration process. A relationship between heat release at the onset of the aluminate peak and the gypsum content of the system was established.
TL;DR: In this article, a three-stage evolution model was observed in the property evolution of concrete samples under the field-like exposure condition, and a distinct coupling physicochemical damage on concrete samples was detected when subjected to drying-wetting cycle exposure.
Abstract: Accelerated laboratory tests on resistance of concrete exposed to sulfate attack have generally been conducted in high-concentration sulfate solutions or under drastic drying-wetting cycle conditions. However, these accelerated regimes radically alter the nature of sulfate attack mechanisms on concrete under real field exposure situation. To obtain reliable information on the long-term behaviors of concrete under real field conditions, the behaviors of concrete samples under three different exposure regimes, i.e., continuous full immersion, full immersion with general use drying-wetting cycles and full immersion with natural drying-wetting cycles, were investigated in this research. Three different concentrations of sulfate sodium solutions, i.e., 0% water for control, 2.1% for field-like condition and 15% for high-concentration condition, were considered. Physical and mechanical properties, such as mass, expansion, permeability and compressive strength, were tested at regular time intervals during the whole exposure period to describe the associated evolution laws. Microanalysis was also carried out to identify the underlying mechanisms. Results from this study showed that the exposure regime of full immersion in 2.1% sulfate sodium solutions subjected to natural drying-wetting cycles can well reproduce the field exposure condition of concrete under certain sulfate-rich environments. Both concentration and exposure type affect the nature of sulfate attack mechanism on concrete, along with the evolution of physical and mechanical properties. A three-stage evolution model, consisting of the enhancement stage, the incubation stage and the degeneration stage, was observed in the property evolution of concrete samples under the field-like exposure condition. Meanwhile, a distinct coupling physicochemical damage on concrete samples was detected when subjected to drying-wetting cycle exposure. In addition, the effects of water-to-binder ratio and binder type were duly studied.