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Stabilising admixtures are commonly used additives in repair mortars and grouts. Beyond this, such type of admixture is increasingly used in concrete and other cementitious materials. In particular when fresh mortar or concrete properties have to be adjusted reliably, stabilising agents can be beneficially used to improve workability and robustness of the mixture. The mode of operation of these admixtures varies, rather affecting either the liquid phase or the solid particles in the dispersion, both causing strong interactions with the mortar or concrete system, and significant changes in their rheological behaviour. Furthermore, these are strongly affected by the environmental temperature during the casting process. In the paper the effect of temperature on the performance of stabilising agents in cementitious systems is presented and how performance changes affect fresh and hardening mortar or concrete properties. Particular attention is placed on interactions between stabilising agents and superplasticizers. Results are discussed with special focus on self-compacting concrete.
Im Hinblick auf die Konformitätsbewertung der Druckfestigkeit von Beton wird es in der für 2014 geplanten Fassung der EN 206 signifikante Änderungen geben. Neben den bisher bekannten Verfahren werden auch sogenannte Kontrollkartensysteme als Konformitätsnachweis zugelassen. Dabei können beliebige Kontrollkartensysteme verwendet werden, sofern nachgewiesen werden kann, dass der maximale Durchschlupf (Average Outgoing Quality Limit, AOQL) 5 % beträgt. Im informativen Normenanhang werden zwei grundlegende Verfahren, das sogenannte Shewhart-Verfahren sowie das CUSUM-Verfahren (Cumulative Sum) beschrieben, welche beide die Forderung an das AOQL erfüllen.
Kontrollkartensysteme sind eigentlich Werkzeuge zur Prozesssteuerung. Sie können effektiv dabei helfen, die Produktion technisch, ökonomisch und ökologisch zu optimieren. Allerdings sind Kontrollkarten deutlich komplexer in der Anwendung als die bisher normativ erfassten Methoden. Der vorliegende Beitrag stellt die neuen Verfahren und deren Besonderheiten anhand von Beispielen vor. Darüber hinaus werden der Zusatznutzen bei sachgerechter Anwendung im Vergleich und Vor- und Nachteile zu den bisherigen Verfahren dargestellt und diskutiert.--------------------------------------------------------------------------------------
With regard to the conformity assessment of the compressive strength of concrete, there will be significant changes in the EN 206, which is scheduled for publication in 2014. So called control chart systems will be approved supplementary to the existing methods. Any control chart can be used as long as the system can grant an average outgoing quality limit (AOQL) of 5 %. The informative appendix of the standard presents two fundamental control chart methods, the so called Shewhart method and the CUSUM method (cumulative sum), both of which fulfill the required AOQL.
Control card systems are actually tools for process control. They can effectively help to optimise production processes technically, economically, and ecologically. However, control charts are much more complex to use than the methods known from the recently applied standard. This paper presents these new methods and their characteristics using examples. Furthermore, the additional benefit of these systems is illustrated and advantages and disadvantages are discussed in comparison to existing methods.
This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage.
Rice husk as (RHA) is an eco-friendly material, which can be used as a supplementary cementitious material (SCM) in cement and concrete. Due to the high water demand for the material, superplasticizers (SPs) are essential to improve the performance. However, the interaction between the SPs and RHA systems is limited. This paper investigates the interaction of the binders with three SPs, i.e. two polycarboxylate ethers (PCEs) and one lignosulphonate (LS). The investigations are performed on blended systems of mortar containing various percentages of RHA and limestone powder (LSP). LSP is used in this research to improve the workability of the mortar. The results from the zeta potential (ZP) shows that the SPs are extremely dependent on the pH of the suspension. At higher pH values such as in a cementitious system, the ZP becomes less negative indicating that the ions in the suspension interact with the carboxyl groups in the backbone of the polymers thus reducing the surface charges of the SP. The mini-slump flow shows that the workability of the blended mortar systems is significantly improved with the addition of SPs. LS systems with increasing RHA is observed to have similar workability as the control mix over time. The compressive strength test results show increased strength for all mortar specimens with added RHA and LSP at later ages.
The fresh behaviour of self-compacting concrete (SCC) at varying temperatures differs from that of normal vibrated concrete. This is because the rheology of SCC depends not only on degree of cement hydration, but also on the adsorption of superplasticizers – mostly polycarboxylate based polymers (PCE) -, which is affected by the time and hydration progress. Due to the variety of PCEs and mixture compositions for SCC a prediction of the rheology at varying temperatures is complicated. The charge densities of PCEs as well as the water to solid ratio in the paste are identified to be the main decisive parameters for robust fresh concrete properties.
Rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the PCE were conducted. SCC which is rich in powder components showed robust performance at low temperatures while SCC with low powder content was favourable at high temperatures. High charge density PCE pointed out to be very robust at low temperatures but at high temperatures it significantly reduced the flow retention. Low charge density PCE could not generate self-compacting properties at low temperatures but retained the flow performance over sufficiently long time. Based on considerations about particle interactions and adsorption mechanisms of PCEs, the relevant processes are explained and options for the development of robust mixture compositions for individual temperature ranges are itemised.
To design robust self-consolidating concrete (SCC) for various environmental conditions, it is essential to understand the relevant mechanisms that control the flow performance. This paper depicts how high range water reducing agents (HRWRAs) interact with clinker and hydration phases, and it discusses the important role of the charge density of polycarboxylic HRWRAs in the way the rheology is affected. Based on the rheometric investigations on SCC mixture compositions with different water to powder ratios (w/p) and observations of their pastes' Vicat setting times, the study shows that increasing charge densities of the HRWRA and decreasing w/p reduce the flow retention and have lesser retarding effect on the setting. Based on the test results and discussions optimization procedures for the mixture composition and the HRWRA modification are suggested to achieve optimized performance for varying environmental situations and highest robustness for specific conditions.
Polysaccharides provide high potential to be used as rheology modifying admixtures in mineral binder systems for the construction industry such as concrete or mortar. Since superplasticizers have become state of technology, today, concrete is more and more adjusted to flowable consistencies. This often goes along with the risk of segregation, which can be effectively avoided by adding stabilising agents supplementary to superplasticizers. Stabilising agents are typically based on polysaccharides such as cellulose, sphingan gum, or starch. Starch clearly distinguishes in its effect on rheology from other polysaccharides, mainly due to the strong influence of amylopectin on the dispersion and stabilisation of particles. Based on rheometric investigations on cementitious and limestone based dispersions with different volumetric water to solid ratios, the mode of operation of modified potato starch is explained in comparison to a sphingan gum. It is shown that the stabilising effect of starch in a coarsely dispersed system is mainly depending upon the water to solid ratio and that above a certain particle volume threshold starch mainly affects the dynamic yield stress of dispersions, while plastic viscosity is affected only to a minor degree. Sphingans operate more independent of the particle volume in a coarsely dispersed system and show significantly higher effect on the plastic viscosity than on the yield stress. In systems incorporating superplasticizers, influences of both stabilising agents on yield stress retreat into the background, while both observed polysaccharides maintain their effect on the plastic viscosity.
Repair is an indispensable part of the maintenance of structures over their lifetimes. Structural grouting is a widely used remediation technique for concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. A structural grout system should be injectable in narrow spaces and hence include ingredients with finer particles. Ultrafine cements are ideal for these type of demanding grouts due to their superior properties compared to that of the less expensive, but coarser ordinary Portland cement (OPC). Supplementary cementitious materials (SCMs) are often used to replace OPC clinker based binder in order to modify certain properties and to reduce costs. The most commonly used SCMs are fly ash (FA), and ground granulated blast furnace slag (GGBS). For various special applications microsilica (MS), and metakaolin (MK) are also used. Identifying the optimum replacement contents of OPC by SCMs are a challenge during the design of such grouts. The aim of this experimental study is to investigate the effect of the selected SCMs (FA, MS and MK) on the slump flow, time of efflux, viscosity, shrinkage, and compressive and flexural strength of ultrafine cement based grouts with constant water-binder ratio and superplasticizer content. The test program was formulated using Box-Behnken design principles. Maximum percentages of replacement with ultrafine cement was 6% by volume of cement for MS and 16% for FA, and MK. The results suggest that most investigated grouts have the potential to be used for structural applications. The appropriate quadratic models are then formulated through statistical tools and presented as response surfaces. The trends indicate that fly ash improves the rheological properties, whereas microsilica and metakaolin positively affect shrinkage and mechanical properties to some extent. Based on the influence of SCMs and priorities among the properties, Decision Matrix Analysis (DMA) is carried out to select the most suitable ones among the SCMs. The analysis suggests that microsilica and fly ash are more suitable as SCMs than metakaolin without affecting the properties.
Structural design and application have always been linked to the compressive strength of concrete as the main relevant criterion. This was justifiable in the past, where concrete consisted of water, ordinary Portland cement and aggregates, but this concept is no longer relevant for modern and more sustainable cement and concrete.
Despite these new developments, existing standards, guidelines and academic curricula have not been much updated and are still used worldwide. There is a need to change this situation by proper education of the users.
This overview describes the challenges that arise at a user Level from the higher complexity of modern concrete, and defines needs and requirements for enhanced applicability of sustainable concrete concepts.
Furthermore, recommendations are given on how better concrete practice can be communicated to all the involved parties, from civil and design engineers to constructors and site-appliers on the construction site.
Superplasticizer and Shrinkage Reducing Admixture Dosages for Microfine Cement in Grout Systems
(2019)
Grouts have numerous applications including crack repair as maintenance in construction industries. Microfine cements are intensively used for high strength mortar and grout products. They are ideal for injection grouting in structural repair. Such grouts should have suitable rheological properties to be injectable, especially those used in repair and rehabilitation. The use of superplasticizers (SP) in these products is thus becoming increasingly crucial to achieve favorable workability and viscosity properties. A difficulty in such grouts is the plastic shrinkage due to finer particles used. It is thus necessary to determine optimum SP and shrinkage reducing admixture (SRA) dosages for a microfine cement based grout. In this study, a saturation dosage was decided from two Polycarboxylate ether (PCE) based SPs in relation to neat cement using slump flow and rheological parameters. A range of grout mixtures was formulated containing micro silica (MS) and fly ash (FA), and tested for suitable rheological and mechanical parameters. Based on the results, a grout mixture with MS and FA was selected to determine optimum SRA content. According to the results, a SP dosage of 3% by weight of neat cement is sufficient to achieve saturation. The grout material including MS and FA can produce comparable properties to neat cement grout. MS is found to improve compressive strength within the range considered, whereas a higher FA content provides favourable rheological properties. Finally, a SRA dosage of 4%, which could reduce the shrinkage by about 43% after 28d days, is determined for the grout system.
Time-resolved in situ investigation of Portland cement hydration influenced by chemical admixtures
(2016)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
Sub-Saharan Africa is populated by more than half a billion people,
and rapid urbanization is creating challenges in the areas of housing,
roads, railways, power supply, dams and water pipelines —
aspects of infrastructure that are critical to the well being of the
population. This situation indicates that cement and concrete will
play a major role in the future construction technology in Africa, despite
the fact that it does not have a long-lasting tradition over most
of the continent. Although the continent is rich in natural resources
for the production of mineral binder systems, many sub-Saharan
African countries exhibit challenging boundary conditions, that have
to be well understood in order to use cement and concrete technologies
in a sustainable and reasonable way.
Selbstverdichtender Beton verhält sich unter Temperatureinfluss anders als Normalbeton, da die Rheologie neben der fortschreitenden Hydratation zusätzlich durch die von der Zeit und dem Hydratationsfortschritt abhängige Adsorption von Fließmitteln beeinflusst wird. Anhand rheometrischer Betonversuche an SVB unterschiedlicher Entwurfskonzepte mit variierter anionischer Ladungsdichte im Fließmittel wird verdeutlicht, dass mehlkornreiche SVB bei niedrigen Temperaturen sehr robust sind, während bei hohen Temperaturen mehlkornärmere Entwürfe zu bevorzugen sind. Darüber hinaus wird gezeigt, wie sich unterschiedliche Fließmittelmodifikationen in bestimmten Temperaturbereichen verhalten. Anhand des Wasser-Feststoff-Verhältnisses und des Adsorptionsverhaltens von Fließmitteln werden die maßgeblichen Prozesse erläutert und Möglichkeiten für die Entwicklung robuster Mischungen für individuelle Temperaturbereiche aufgeführt.--------------------------------------------------------------------------------------------------------------------------------------------------------------
Regarding the temperature dependent performance, self-compacting concrete (SCC) distinguishes from normal concrete, since its rheology does not only depend upon the hydration itself but supplementary upon the adsorption of superplasticizers, which is affected by the time and the hydration progress. Based on rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the superplasticizers, it is shown that SCC, which is rich in powder components, shows robust performance at low temperatures, while compositions with lower powder contents are favourable at high temperatures. Furthermore, the performance of different superplasticizer modifications at different temperature ranges is demonstrated. The relevant processes are explained by means of the water to powder ratio as well as the adsorption behaviour of superplasticizers, and options for the development of robust mixture compositions for individual temperature ranges are itemised.
Since its invention in the early 1990’s, self-consolidating concrete has never become well established in the ready-mix sectors worldwide. The reason for this can be mainly found in the fact that the sophisticated compositions are sensitive against changing environments. This lack of robustness can be attributed to the interaction between cement hydration reaction and high range water reducing agent (HRWRA). Understanding the relevant mechanisms that control the initial flow performance as well as the flow retention helps optimizing SCC mixtures that perform either steadily in one specific environment or that perform largely similarly at steadily changing environments.
This paper depicts how HRWRAs interact with clinker and hydration phases and discusses the important role of the charge density of a polycarboxylic HRWRA in the way the rheology is affected. Based on rheometric results and observations of the Vicat setting times, it is shown that increasing charge densities of the HRWRA and decreasing water to powder ratios (w/p) reduce the flow retention and have lesser retarding effect on the setting. Based on the discussion, optimization procedures for the mixture composition and the HRWRA modification are suggested to achieve optimized performance for varying environmental situations or highest robustness for specific conditions.
Polysaccharides are incorporated into cement based Systems in order to modify the rheological properties. Typically, cellulose ethers, sphingan gums, guar gum or starch ethers are applied. Depending upon their chemistry, molecular architecture, and adsorption tendency, polysaccharides interact differently with the entire cementitious system. Some stabilising agents like diutan gum mainly affect the cementitious paste; other stabilising agents like starch tend to interact with the sand fraction and even with the coarse aggregates. Cellulose and guar gum shows more diverse performances.
Typically stabilising admixtures like polysaccharides are used, when sophisticated rheological properties are adjusted. Therefore, polysaccharides are often used in combination with superplasticisers, which are added to reduce the yield stress of concrete. This can cause interactions, particularly when the stabilising Agent shows a strong tendency to adsorb on particle surfaces. Adsorptive stabilising agents may reduce the amount of adsorbed superplasticisers, thus affecting both viscosity and yield stress, while non-adsorptive stabilising agents mainly affect the plastic viscosity independently of the superplasticiser. Due to the strong influence of superplasticisers on the yield stress, influences of the stabilising agent on the yield stress retreat into the background, so that their major effect is an increase of the plastic viscosity.
The paper provides a comprehensive overview of how different polysaccharide superplasticisers affect cementitious flowable systems and points out the challenges of the combined use of polysaccharides and superplasticisers. Based on rheometric experiments and observations of the hydration process, time dependent effects on the workability as well as of the hydration of cement are presented and discussed.
Superplasticizers (SPs) have been employed in concrete technology for decades to improve the workability of concrete in its fresh state. The addition of SPs in cement-based systems affects the early properties. Although the interaction of the cement particles with various SPs has been extensively researched, there still exists limited research on the interaction of SPs with supplementary cementitious materials such as rice husk ash (RHA). This paper investigates the rheological properties and early hydration kinetics of RHA-blended systems with three types of SPs, a polycarboxylate ether (PCE) and two lignosulphonates (LS-acc and LS-ret). In rheological properties, the addition of SP causes an initial improvement of workability as the yield stress is significantly reduced. The pastes with PCE and LS-acc show a slight increase of yield stress over time whereas pastes with LS-ret tend to lower the yield stress slightly over time, further improving the workability. Without SP, pastes with RHA show a lower yield stress but an increase in plastic viscosity as cement is further replaced with RHA. The addition of the LS SPs is observed to lower the plastic viscosity but remains constant with further replacement of cement with RHA. This indicates that LS SPs further adsorbs on RHA particles and hydration products produced causing dispersion of the particles
within the system. In early hydration kinetics, pastes with PCE retard hydration and the degree of retardation is further increased with LS SPs. In the presence of RHA, the retardation of LS SP systems is significantly reduced. The pastes with PCE show more ettringite in the SEM micrographs, but is observed to be shorter needles. This indicates an initial good workability for PCE. However, C-S-H and CH were observed to be low in quantity, whereby the pastes with LS show more nucleation sites for C-S-H and CH. The ettringite needles in the LS systems were similar in quantity and more elongated in some cases but not abundant as in the PCE systems.
Rising temperatures worldwide pose an increasing challenge for safe and healthy living conditions. Particularly inner cities have been affected by these environmental changes because of the materials used to build houses, streets and infrastructure. The most common building material is concrete. It shows a specific heat capacity, while the heat conductivity for Standard concrete is low. Thus, the use of concrete generates a high capacity of heat storage. In addition, extensive soil sealing also contributes to the temperature rise of inner city areas compared to
their surroundings. To mitigate this so-called urban heat island effect, a self-cooling concrete paver was developed. This paver is able to store water. The evaporation of the water at elevated temperatures provides a cooling effect. This paper focuses on determination of this new paver’s capability to cool the surface and the surrounding. The new paver’s cooling Qualities were analysed in a series of laboratory tests. To prove the results outside of laboratory conditions, two fields (12 m × 8 m) with self-cooling and reference pavers were installed in Spain.
This paper presents and discusses the results of the tests. Correlation between reduced surface temperature of the self-cooling concrete pavers and the air temperature is examined.
Self-compacting concrete typically contains a higher content of powder materials than normal concrete. Furthermore it is assumed that the use of superplasticizers yields a more homogenous microstructure in the hardened paste. Both aspects generate a very dense microstructure, which can be assumed to cause material behaviour that differs from than normal concrete when it is exposed to elevated temperatures. However, the paste volumes in SCC can vary significantly based on the mixture composition. At a low paste volumes SCC can be very similar to normal concrete at hardened state while it can vary greatly at higher paste volumes. Since the high temperature behaviour of concrete is strongly affected by the different physical behaviour of the paste and the aggregates, it is likely that the high temperature behaviour is consequently strongly affected by the ratio of these mixture components. In the present study different SCC mixtures were observed with similar mechanical properties, but with significantly differing paste to aggregate ratios. Based on observations of the heat evolution at the fire exposed surface and at different depths inside the specimens as well as based on photogrammetric observations of the spalled dimensions, the results indicate that with increasing paste volumes the heat conductivity is reduced and as a result concrete with higher paste to aggregate ratios shows less spalling.
Control chart systems are commonly used in quality management systems to keep production processes stable. The most efficient control chart systems are cumulated sums (cusum), which look at the deviations from a target value. Cusum charts react more rapidly on systematic changes in processes than other charts such as Shewhart charts, which look at the process values. The cusum method is often applied for compressive strength but it is particularly suitable if counteractions be taken immediately, like in the case of fresh concrete properties.
The present study shows that regardless of the manipulation, a steady slump flow can be achieved only by adding supplementary superplasticizer in case of loss of flow and adding stabilising agent (ST) in case of increased flow if the V-mask indicates a systematic change. A reference SCC was artificially manipulated in order to achieve either increased or reduced flowability. Arithmetic mean values and standard deviations were determined experimentally without and after taking counteractions. These parameters were used for case studies of steady SCC productions based on normally distributed random values. For the indication of a systematic change, a standard V-mask was used.
The results point out that productions with applied cusum methods were capable of keeping the slump flow deviation from the target in the order of magnitude of about 1% despite drastic manipulations, while productions without applied cusum method deviated up to 14% and more from the target slump flow value.
Numerical simulations of concrete castings are complex and time consuming. In order to decrease simulation time and to simplify simulation procedure, an innovative modelling approach, which treats reinforced sections in a formwork as porous media, was proposed. In the previous studies, this numerical model was proved suitable to simulate casting of model yield-stress fluids through reinforced elements. This article focuses on the experimental validation of the proposed model at the concrete scale. For this purpose, a large-scale laboratory casting of a highly reinforced beam is performed. The casting process is numerically simulated and the numerical results are compared to the experimental measurements.
Fünf verschiedene Messverfahren wurden zur Partikelgrößenanalyse von nanodispersen ZrO₂-Pulver verglichen. Mit der Laserstreulichtanalyse, der dynamischen Lichtstreuung (heterodyne DLS und homodyne DLS-PCS), der Sedimentationsanalyse im Zentrifugalfeld und der Ultraschallspektrometrie wurden wässrige ZrO₂-Suspensionen mit verschiedenen Feststoffkonzentrationen hinsichtlich ihrer Partikelgrößenverteilung analysiert. Als Referenz diente die REM-Analyse zur Ermittlung der Primärpartikelgröße (ca. 40 nm). Mit den hier vorgestellten Messverfahren konnten in den entsprechenden Suspensionen nur Sekundärpartikel im Bereich von 105 nm bis 224 nm detektiert werden, die somit auf das Vorhandensein von Aggregaten bzw. harten Agglomeraten hinweisen.
Viscosity-modifying agent (VMA) are used in several applications of concrete, such as underwater concrete, Self-Compacting Concrete (SCC) or Self-Levelling Underlayments (SLU) in order to improve the washout resistance and the stability. The study focuses on the modifications of cement pastes properties implied when a VMA (hydroxypropyl guars or HPG) and a superplasticizer are introduced together. For reaching this objective, two chemically different polycarboxylate-based superplasticizers (PCE) and two HPGs exhibiting different molar substitution ratios (MSHP), were studied. A method, combining total organic carbon and size exclusion chromatography, was developed in order to quantify the adsorption of the both admixtures. The adsorption of HPGs appears being significantly lowered by the presence of PCE, while only the adsorption of the less charged PCE is slightly affected by the HPG. As consequence, strong modifications of the rheological properties of cement pastes were noticed when HPG and PCE are combined. The desorption of HPG leads to higher yield stress and residual viscosity than with PCE alone.
This paper presents the results of an interlaboratory study of the rheological properties of cement paste and ultrasound gel as reference substance. The goal was to quantify the comparability and reproducibility of measurements of the Bingham parameters yield stress and plastic viscosity when measured on one specific paste composition and one particular ultrasound gel in different laboratories using different rheometers and measurement geometries.
The procedures for both in preparing the cement paste and carrying out the rheological measurements on cement paste and ultrasound gel were carefully defined for all of the study’s participants. Different conversion schemes for comparing the results obtained with the different measurement setups are presented here and critically discussed. The procedure proposed in this paper ensured a reasonable comparability of the results with a coefficient of variation for the yield stress of 27% and for the plastic viscosity of 24%, despite the individual measurement series’ having been performed in different labs with different rheometers and measurement geometries.
Tricalcium aluminate (C3A) is found with less than 10% wt. of the total composition; however, during hydration, C3A plays an important role in the early hydration of cement in the presence of gypsum as a set retarder.
The aim of this investigation is to assess the suitability of optical spectroscopy and a dye-based optical probe to monitor early hydration of C3A in the presence of gypsum and hemihydrate. Optical evaluation was performed using steady-state fluorescence and diffuses reflectance spectroscopy (UV-VisDR). Phase characterization during hydration was done with in-situ X-ray diffraction. UV-VisDR with a cyanine dye probe was used to monitor the formation of metastable phases and was employed together with fluorescence spectroscopy, to follow the Aggregation and disaggregation of the dye during hydration. In conclusion, for the first time, a cyanine dye was identified as a feasible and stable probe to monitor C3A hydration changes in the presence of calcium sulfate.
The interaction of different charged polymers, namely anionic polycarboxylate
superplasticizer (PCE) and neutral polyethylene glycol (PEG) with potassium ions, and their effect on the yield stress of highly concentrated glass bead suspension (GBS), were studied under different concentrations of potassium ions ([K+]). It was found that, compared to the neutral PEG, the negatively charged PCE can be adsorbed on glass beads (GB), and then decreases the yield stress of GBS. The increasing concentration of free polymer in the interstitial liquid phase with the increased polymer dosage leads to the higher yield stress of GBS, which may be caused by the higher Depletion force. In addition, this effect is also related to the charge density of the polymer and the [K+] in the solution. Along with the increase in [K+], the yield stress of GBS increases significantly with the addition of PCE, but this cannot be observed with PEG, which indicates that potassium ions can interact with negatively charged PCE instead of the neutral PEG. At last, the interparticle Forces between two single GB with adsorbed PCE in solutions containing [K+] and PCE were measured by colloidal probe atomic force microscopy to better understand the interaction of the charged polymer with counterions.
A thorough characterization of starting materials is the precondition for further research, especially for cement, which contains various phases and presents quite a complex material for fundamental scientific investigation. In the paper at hand, the characterization data of the reference cement CEM I 42.5 R used within the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented from the aspects of chemical and mineralogical compositions as well as physical and chemical properties. The data were collected based on tests conducted by nine research groups involved in this cooperative program. For all data received, the mean values and the corresponding errors were calculated. The results shall be used for the ongoing research within the priority program.
Supplementary cementitious materials (SCM) can reduce the total amount of Portland cement clinker in concrete production. Rice husk ashes (RHA) can be converted from an agricultural by-product to a high-performance concrete constituent due to a high amount of reactive silica with pozzolanic properties if they are burnt under controlled conditions. The way and duration of combustion, the cooling process as well as the temperature have an effect on the silica form and thus, the chemical and physical performance of the RHA. Various studies on the best combustion technique have been published to investigate the ideal combustion techniques. Yet, the process mostly took place under laboratory conditions. Investigating the difference between the performance of RHA produced in a rural environment and laboratory conditions is useful for the assessment and future enhancement of RHA production, and its application both as building material, for example in rural areas where it is sourced in large quantities, and as additive for high performance concrete. Thus, the paper presents a comparison between RHA produced under rudimentary conditions in a self-made furnace in the rural Bagamoyo, Tanzania and under controlled laboratory conditions at the Technical University of Munich, Germany, with different combustion methods and temperatures. In a second step, RHA was ground to reach particle size distributions comparable to cement. In a third step, cement pastes were prepared with 10%, 20% and 40% of cement replacement, and compared to the performance of plain and fly ash blended cement pastes. The results show that controlled burning conditions around 650 °C lead to high reactivity of silica and, therefore, to good performance as SCM. However, also the RHA burnt under less controlled conditions in the field provided reasonably good properties, if the process took place with proper burning parameters and adequate grinding. The knowledge can be implemented in the field to improve the final RHA performance as SCM in concrete.
Two types of cements were selected as the reference cement in the priority program 2005 of the German Research Foundation (DFG SPP 2005). A thorough characterization of CEM I 42.5 R has been made in a recent publication. In this paper, the characterization data of the other reference cement CEM III/A 42.5 N are presented from the aspects of chemical and mineralogical compositions as well as physical and chemical properties. The characterization data of the slag, which is the second main constituent of this specific cement besides the clinker, are presented independently. For all data received, the mean values and the corresponding errors were calculated. The data shall be used for the ongoing research within the priority program. Also, researchers from outside this priority program can benefit from these data if the same materials are used.
NMR is becoming increasingly popular for the investigation of building materials as it is a non-invasive technology that does not require any sample preparation nor causes damage to the material. Depending on the specific application it can offer insights into properties like porosity and spatial saturation degree as well as pore structure. Moreover it enables the determination of moisture transport properties and the (re-)distribution of internal moisture into different reservoirs or chemical phases upon damage and curing. However, as yet most investigations were carried out using devices originally either designed for geophysical applications or the analysis of rather homogeneous small scale (< 10 mL) samples. This paper describes the capabilities of an NMR tomograph, which has been specifically optimized for the investigation of larger, heterogeneous building material samples (diameters of up to 72 mm, length of up to 700 mm) with a high flexibility due to interchangeable coils allowing for a high SNR and short echo times (50 - 80 m s).
The effect of resting time on the rheological properties of cement suspensions is generally explained by early formed structure and overconsumption of polycarboxylate superplasticizers (PCEs). In this paper, we propose that the influence of resting time on the rheological properties is closely related to size variation of non-absorbed PCE. To identify this, glass bead suspensions were prepared with various amounts of PCE and ionic solution, and their rheological properties were evaluated at various times. We found that the yield stress increases with time at higher PCE concentrations and higher ionic strength solutions. Adsorbed PCE during resting tends to bridge the particles rather than disperse them. In addition, it was found that hydrodynamic radius of PCE increased with resting time, and depletion forces resulting from non-absorbed PCE size changes correlate well with the increased yield stress.
A thorough characterization of base materials is the prereq- uisite for further research. In this paper, the characterization data of the reference materials (CEM I 42.5 R, limestone pow- der, calcined clay and a mixture of these three components) used in the second funding phase of the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented under the aspects of chemical and min- eralogical composition as well as physical and chemical properties. The data were collected based on tests performed by up to eleven research groups involved in this cooperative program.
Effect of Polycarboxylate Superplasticizer in Ordinary Portland Cement and Sulfate Resistant Cement
(2023)
Polycarboxylate superplasticizers (PCE) are the most widely used admixtures in today’s cementitious construction materials. The molecular structure has a decisive influence on the effectiveness of PCE, which also can be tailored to serve in a different cementitious system.
The current study investigates the influence of the backbone charge density of PCE on the rheology, hydration kinetics, and adsorption behavior of ordinary Portland cement (OPC) and sulfate-resistant cement (SRC). The results indicate that regardless of the PCE type, OPC requires a higher amount of PCE to be adsorbed to induce changing of rheological parameters and hydration kinetics. Regardless of the cement type, the PCE with a higher charge density exhibits higher adsorption behavior, corresponding to lower viscosity measurement.
Compared to OPC, SRC is more sensitive to the introduction of both PCEs and has a lower saturation dosage, indicating that SRC generally provides better workability properties regardless of the PCE.
The structural build-up of fresh cement paste is often considered as a purely thixotropic phenomenon in literature even though cementitious materials undergo a non-reversible hydration process that can have an influence on the structuration process. In the current paper a method is proposed to validate the impact of the non-reversible structural build-up. It is shown that fresh cement paste samples lose their structural gain almost completely due to thixotropy while the structural build-up due to hydration can be observed but occurs in a significantly lower order of magnitude over the course of the first hours of hydration. In addition, it is shown, that the chemical component of the structural build-up accelerates with the onset of the acceleration period of hydration, while its contribution in the entire structural build-up remains constant.
In this work, engineered recycled mineral admixtures were obtained from four different industrial residues: basic oxygen furnace slag, iron ore tailings, quartz mining tailings and quartzite mining tailings.
The grinding performance was evaluated in two different programs and the characterization included chemical and mineralogical composition, particle morphology, and physical properties. Performance evaluations were carried out in blended pastes and mortars, including flow properties, hydration kinetics, soundness, pozzolanic activity, and compressive strength. Coarser-than-cement admixtures allowed better flow performance, greater dimensional stability and more economical production, while finer-thancementn admixtures improved mechanical performance by both filler effect and cementing activity.
The existing cement and concrete standards are not capable of making full use of the current technology capacity due to strong focus on conventional concrete and thus they are not fit for the current and future challenges of construction industry. The paper highlights shortcomings with regard to the implementation of the existing standards. It can be seen that future-oriented standards are generally required to contribute to a lower-carbon footprint of the industry. These changes are significantly more relevant in sub-Saharan Africa, due to the rapidly increasing urbanisation challenge and the enormous potentials to develop lower-carbon technologies than elsewhere in the world.
Abstract: Increasing demand for sustainable, resilient, and low-carbon construction materials has highlighted the potential of Compacted Mineral Mixtures (CMMs), which are formulated from various soil types (sand, silt, clay) and recycled mineral waste. This paper presents a comprehensive inter- and transdisciplinary research concept that aims to industrialise and scale up the adoption of CMM-based construction materials and methods, thereby accelerating the construction industry’s systemic transition towards carbon neutrality. By drawing upon the latest advances in soil mechanics, rheology, and automation, we propose the development of a robust material properties database to inform the design and application of CMM-based materials, taking into account their complex, time-dependent behaviour. Advanced soil mechanical tests would be utilised to ensure optimal performance under various loading and ageing conditions. This research has also recognised the importance of context-specific strategies for CMM adoption. We have explored the implications and limitations of implementing the proposed framework in developing countries, particularly where resources may be constrained. We aim to shed light on socio-economic and regulatory aspects that could influence the adoption of these sustainable construction methods. The proposed concept explores how the automated production of CMM-based wall elements can become a fast, competitive, emission-free, and recyclable alternative to traditional masonry and concrete construction techniques.
We advocate for the integration of open-source digital platform technologies to enhance data accessibility, processing, and knowledge acquisition; to boost confidence in CMM-based technologies; and to catalyse their widespread adoption. We believe that the transformative potential of this research necessitates a blend of basic and applied investigation using a comprehensive, holistic, and transfer-oriented methodology. Thus, this paper serves to highlight the viability and multiple benefits of CMMs in construction, emphasising their pivotal role in advancing sustainable development and resilience in the built environment.
Non-conventional densely packed concrete mixtures are proposed and evaluated in this paper using engineered recycled mineral admixtures and recycled aggregates obtained from steel slag, quartz mining tailings, and quartzite mining tailings. High fines content sand-concretes containing coarser- and finer-than-cement recycled powders were designed to obtain blends with broader particle-size ranges and improved packing density. As a result, compressive strength up to 99 MPa, cement intensity up to 2.33 kg/m³/MPa, and consumption of recycled material up to 95 vol% were obtained. Compressive strengths up to 66 MPa and cement intensity up to 2.34 kg/m³/MPa were also obtained with the addition of coarse aggregates to such sand-concrete mixtures, with consumption of recycled material up to 96.5%. The results launch new insights on the role of recycled admixtures and aggregates on the mixture design of cement-based composites regarding efficiency improvement and technological performance.
The built environment significantly impacts the health of individuals and populations in various ways. The health and durability of the built environment are intertwined with availability ofnaturally occurring and man-made resources and their supply chains. Therefore, resource con servation is a key to ensure sustainability of built environments. Many industrial wastes can be turned into valuable resources for reuse in construction of the built environment. For instance, biowaste (woody/ leafy biomass and animal waste) have been used to make construction adhesives (Fini et al., 2011), some urban wastes have been used in road construction (Poulikakos et al., 2017; Schmidt et al., 2021), end of life plastics and polymers have made their way to roadway construction, bio-oils and algae harvested from wastewater treatment plants has been used to make antiaging for outdoor building elements to mitigate UV aging (Kabir et al., 2021); sulfur has been used as an extender in asphalt and recycled mineral powders such as silica and alumina have been used to increase strength and durability against acidic compounds (Fini et al., 2019). These are just a few examples of recycling venues with beneficial uses in the built environment.
The construction and operation of the built environment and the traffic that it attracts or facilitates significantly contribute to the emis sion of greenhouse gasses (GHG) and cause air pollution. The direct and indirect impacts of GHG and air pollutants on the environment and so cieties have been weil established. The built environment can be used as a powerful platform not only for recycling and resource conservation but also to remove near-ground gaseous contaminants. This can be done via tailored design and engineering of adsorptive construction materials via recycling of waste materials. For instance, advanced sorbent systems can be made for removing C02, H2S, and formaldehyde from air. This re quires tailored sorbent design, topology optimization, and catalytic conversion of collected gaseous compounds to name a few.
This special issue covers innovative materials, methods, and man agement practices which aim to simultaneously address durability ofthe built environment, air quality, resource conservation, and supply chain resilience. Such innovative materials, methods, and management prac tices will transform the built environment into not only an active contributor to no waste, no pollution for healthy environment, but also a medium that converts the waste and pollution into beneficial products for use in the built environment, thereby promoting resource conser vation. Followings are examples of topics that the special issue is interested in:
• Venuses to advance resource conservation specifically via novel ap proaches in the built environment.
• Innovative construction materials for passive or active adsorption of harmful gaseous emissions to conserve air, energy, and water.
• Advanced materials, methods, and management practices to pro mote well-being of the built environment.
• Smart buildings to promote resource conservation.
• Research convergence in the built environment for zero waste, zero pollution economy.
Polymers that help tailoring rheological properties during the casting process have become inevitable constituents for all kinds of high-performance concrete technologies. Due to lacking industries, these typically crude-oil based admixtures are not readily available in many parts of the world, which limits the implementation of more sustainable high-performance construction technologies in these regions. Alternative polymers, which often demand for less processing, can be derived from local plant-based resources. The paper provides experimental data of flow tests of cement pastes with polysaccharides from Triumfetta pendrata A. Rich, acacia gum and cassava without and in the presence of polycarboxylate ether superplasticizer. The flow tests are amended by observations of the zeta potentials and the hydrodynamic diameters in the presence of and without calcium ions in the dispersion medium. The results show that in the presence of and without calcium ions all polysaccharides provide negative zeta potentials, yet, they affect flowability and thixotropy in different ways. Cassava starch, acacia gum, and the gum of Triumfetta pendrata A. Rich qualified well for robustness improvement, strong stiffening, and additive manufacturing, respectively. The reason for the different effects can be found in their average sizes and size distribution. Due to the promising results, a flow chart for local value chains is derived on the example of yet unused cassava wastes, which can be converted in parallel.
Design : and use of engineered recycled mineral admixtures obtained from industrial and mineral waste are promising strategies to increase the range of materials suitable for use in cement-based composites. In this work, Portland cement-blended pastes containing mineral admixtures designed for improving particle packing were evaluated in the presence of low- and high-charge polycarboxylate-based superplasticizers. The powders were obtained from basic oxygen furnace slag, iron ore tailings, quartz mining tailings, and quartzite mining tailings. The zeta-potentials of the particles were obtained via electrophoretic mobility. The flow properties were evaluated by rheological tests performed in a Couette type rheometer. The hydration kinetics was evaluated by isothermal calorimetry and an adapted method based on the Vicat needle test. The high-charge PCE and the finer mineral admixtures produced more stable blends. Coarser mineral admixtures led to increased flowability and delayed hydration compared to finer ones. Steel slag powders presented the most significant plasticizer effects, but also the largest setting delays and segregation tendency. Quartz-rich superfines reduced the setting delays caused by the superplasticizers. In summary, both superplasticizers were effective in improving flow properties, but the high-charge PCE was effective in preventing segregation in pastes containing mineral admixtures coarser and heavier than cement.