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Eingeladener Vortrag
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Reaction products and strength development of wastepaper sludge ash and the influence of alkalis
(2014)
Wastepaper sludge ash (WSA) from a newsprint paper mill was investigated for its mineralogical composition and its reaction products and strength development after activation with water and sodium and potassium hydroxide solutions. The results showed the WSA to consist of calcite, free lime, gehlenite, tricalcium aluminate, belite, talc, quartz and probably a glassy phase. The principle reaction product was monocarboaluminate (CO3–AFm) for the water- as well as for the alkali-activated WSA. Formation of monocarboaluminate and strength gain was more rapid for the alkali-activated WSA until 1 day of curing. However, afterwards reactions proceeded much slower when alkali solutions were used, leading to an about twice as high compressive strength for water-activated WSA mortars after 28 days of curing. The observed behavior is tentatively ascribed to a less uniform microstructure of the alkali-activated WSA. Significant differences between NaOH- and the KOH-activated WSA were not observed.
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.
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.
In paper recycling several types of waste sludges are produced, which contain organics as well as inorganic fillers of the recycled paper, e.g. kaolinite, calcite, talc, or titanium dioxide [1,2]. Part of these sludges is incinerated to recover energy and to reduce the volume of waste to be handled. The ashes generated in this process are commonly referred to as wastepaper sludge ash (WSA). The quantities of WSA produced are large and at present much of it is still landfilled or otherwise handled entailing costs.
Depending on the composition of the recycled paper and the conditions during processing and incineration, WSA can have cementitious properties. In the present investigation a CaC03-rich WSA was analyzed for reaction products and strength development after hydration with water, 2 M NaOH solution and 8 M NaOH solution. The rationale for activating the WSA with NaOH was to assess the behaviour of the WSA when alkalis are introduced to the System or when the WSA is used as addition in alkali-activated binders.
In paper recycling large amounts of waste sludge, containing organics as well as inorganic fillers of the recycled paper, are produced. A large fraction of these sludges is incinerated; the ashes generated in this process, often referred to as wastepaper sludge ash (WSA), have the potential to be used as binder material, either alone or as supplementary cementitious material. However, ashes from different paper mills differ in their Chemical and mineralogical composition, thus, the reaction products and the properties of the hardened binders may vary s i gn i fi cantley.
In the present study, a calcite-rich WSA was analyzed for reaction products and strength development. The WSA contained, besides calcite, hydraulic phases such as tricalcium aluminate (C3A) and belite (C2S), lime as well as inert phases. When activated with water, the WSA yielded a compressive strength of up to 12.3 MPa and the major hydration product was monocarboaluminate (C4A CO2 IIH2O or C4ACH11). Activation with 2M NaOH or 2M KOH solution caused a more rapid strength gain until one day of curing, but afterwards the strength development slowed down and the strength afiter 28 days was significantly lower than for the water-activated pastes. From TG analyses it is apparent that the higher strength of the alkaliactivated pastes after one day of curing was caused by a more rapid consumption of calcite and associated formation of C4ACH11. At later curing times the amount of calcite did not change substantially and consequently strength development almost ceased.
The results demonstrate the complex influence of alkaline conditions on the hydration of WSA and show that the reaction products, which determine the durability properties of the hardened pastes (and materials made from it), of the calcite-rich WSA differ from the products of other WS As, as discussed in the article.
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.
Design of a cementitious coating system for corrosion protection: Phase 1 binder materials selection
(2014)
This paper presents the first results of an ongoing research project on the development of a cementitious thin layer coating system with high resistance against chloride penetration and carbonation. It is intended to display the decision process used for selecting raw materials as well as the conclusions based on test results and theoretical deliberations. In this phase, only the influence of the binder materials was analyzed concerning mechanical properties. The system should enhance the durability of new and old concrete structures exposed to aggressive environments, be based on market available materials and fulfill the requirements of the German guideline for concrete repair (Rili-SIB).