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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.
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.
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.