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Even after autoclaving for 2 weeks in the temperature range of 120 to 200 °C, CEM I paste with a water-to-cement ratio of 0.5 still contains some low-crystalline C-S-H. The conversion of low-crystalline C-S-H to crystalline is faster with increasing temperature. The observed low-crystalline and crystalline C-S-H phases (reinhardbraunsite, jaffeite) indicate that the sampleshave not reached equilibrium yet under these experimental conditions. However, there is a good agreement between experiment and thermodynamic modelling for the other solids indicating that the used datasets are suitable for this application.
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.
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.
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.
A silica residue from waste treatment of chlorosilane production was used together with solid sodium aluminate to test its applicability for the production of one-part geopolymers. The blend was activated with water and cured at 70 °C. The degree of reaction and strength were determined after 1, 3, and 7 days. The reaction products were analyzed by XRD and SEM/EDX. Until the third day of curing the degree of reaction of the residue reached 51 % and the strength was 8.9 MPa. The reaction product was identified as geopolymer containing zeolite A. Thus, the results confirmed that the residue may be used in the production of geopolymers. However, after 3 days of curing no further progress of reaction was observed and the strength slightly decreased, which was attributed to changes in the structure of the geopolymeric gel. It was further observed that even harsh vacuum drying left some water (presumably zeolitic water and surface hydroxyl groups) in the geopolymer.
In the past decade, nanoindentation has become commonly used experimental technique for obtaining quantitative data on elastic and inelastic material properties in small volumes, typically in submicron length scale. The methodology was developed mainly for homogeneous materials or, at least, the homogeneity is assumed far enough from the tested volume in evaluation procedures1. Measurement on a heterogeneous system, in which the material phases are intermixed in small volume, is still a challenging task due to several circumstances.
First, the preparation of a flat surface on the heterogeneous sample with high differences in stiffness needs Special care. Second, the mechanical interaction between the material phases during surface loading is often unavoidable and therefore the extraction of intrinsic properties of individual phases from the heterogeneous system may not be possible. If differences between the stiffnesses of the interacting phases are low, finding of intrinsic properties can be successful using conventional Evaluation. In other cases, the error in estimating intrinsic properties can reach tens of percent.