International Concrete Abstracts Portal

This paper reports an investigation on the mechanical properties and durability of high strength concrete containing rice husk ash (RHA). Mixtures containing 10% RHA by weight of cement and w/cm of 0.27 were cast. RHA was incorporated either as an admixture or as cement replacement material. A superplasticizing admixture was used to provide flowing characteristics. Results on the workability, mechanical properties and initial surface absorption (ISA) test are reported. The specimens were subjected to water and air-drying conditions and tests on the specimens were carried out up to 180 days. By applying a superplasticizer based on polycarboxylic ether, workability of RHA concrete in the range of 200—250 mm slump can be attained. This type of concrete can achieve high strength of 80 N/mm2 at 28 days, irrespective of method of inclusion of rice husk ash or curing conditions. Compared to condensed silica fume (CSF) concrete at similar w/cm and workability, the strength of RHA concrete is about 6% lower. In general, the mechanical properties of RHA concrete are higher than the control super-plasticized concrete but marginally lower than the CSF concrete. Durability of RHA concrete with regards to ISA is similar or better than CSF concrete.

Plain cement and silica fume cement paste and mortar specimens prepared using Type I cement, 8% silica fume, and potable and treated waters were tested for setting time and compressive strength. Pore solutions extracted from the mortar specimens were analyzed for alkalinity and chloride content. The results showed that the treated water tested in this study qualifies to be used in making concrete on the basis that the chemical composition of treated water and variations in the setting time and compressive strength were within the tolerable limits.

The influence of ground granulated blast-furnace (GGBF) slag on autogenous shrinkage in concrete with a water/cementitious material (w/cm) of 0.3 and 91-day strength in excess of 80 MPa was investigated under tropical conditions. Cement re-placement percentages of 30, 50, 65 and 80% by GGBF slag were examined as well as the finenesses of 4200, 6000 and 7900 cm2/g for the 65% replacement percentage. The GGBF slag increased significantly the cumulative autogenous shrinkage for all replacement percentages and fineness levels tested. The increased autogenous strain occurred within the first 14 days when hydration would have been dominated by the portland cement component, well before significant additional hydration or pore refinement would have been possible due to hydration of the GGBF slag component. This suggests that the driving force for autogenous shrinkage in GGBF slag concrete may differ fundamentally from that for portland cement and silica fume concrete. Possible mechanisms for the increased early autogenous shrinkage in GGBF slag concrete are discussed.

The utilization of waste waters from zeolite production, for the activation of mixtures containing high content of granulated blast-furnace slag with moderate amounts of portland cement clinker and gypsum, was examined. The evaluation of properties of the product, depending on the composition of the starting blend, showed that good compressive strength and satisfactory resistance to carbonation, sulfate attack and freezing and thawing cycling can be obtained with some compositions.

In most Dutch power stations secondary fuels are co-combusted. It is important that the quality of the generated fly ash does not degrade because of co-combustion, as this will hinder utilization. Therefore, a project was started to assess the consequences of co-combustion of selected fuels (demolition wood, chicken manure and refuse de-rived fuel) on the properties of the fly ashes. The co-combustion experiments were carried out in the KEMA Test boiler (pilot-scale, 1 MW1h). The properties of the generated fly ashes were analyzed and the behavior of the fly ashes when applied as pozzolanic filler was assessed. It was concluded that the cystalline components as identified by X-ray diffraction are the same as normally found in Dutch fly ashes (ASTM class F), with exception of fly ash from chicken manure. The glass content is reduced by co-combustion depending on the co-fired fuel. If chicken manure is co-combusted, the chemical composition changes due to the increase of Ca, P, K and S. Co-combustion of demolition wood and refuse derived fuel (RDF) causes an increase of Ca, Na and K. The pozzolanic behavior of fly ash appears not to be influenced by co-combustion of demolition wood. However, co-combustion of RDF causes a decrease in pozzolanic behavior, but relatively high percentages are possible up to the point where it no longer meets the limits. Co-combustion of chicken manure appears to have a positive influence, but this is related to other reaction mechanisms than pozzolanity. It can be stated that fly ashes from high percentages of co-combustion are able to meet the basic requirements of the European standard (EN 450). It all depends on the nature of the co-fired fuel and especially on its inorganic matter and ash content.