Kwesi Sagoe-Crentsil

Performance of concrete made with commercially produced coarse recycled concrete aggregate

Performance tests have been carried out for fresh and hardened properties of concrete made with commercially produced coarse recycled concrete aggregate and natural fine sand. Test results indicate that the difference between the characteristics of fresh and hardened recycled aggregate concrete and natural aggregate concrete is perhaps relatively narrower than reported for laboratory-crushed recycled aggregate concrete mixtures. For concrete without blast furnace slag having similar volumetric mixture proportions and workability, there was no difference at the 5% significance level in concrete compressive and tensile strengths of recycled concrete and control normal concrete made from natural basalt aggregate and fine sand. Water absorption rates and carbonation of recycled concrete and reference concrete were comparable for most applications. However, the abrasion loss of recycled aggregate concrete made with ordinary portland cement increased by about 12% compared to normal concrete, while the corresponding drying shrinkage was about 25% higher at 1 year. The ratio of splitting tensile strength to compressive strength was found to be in good agreement with established values derived for equivalent grade concretes made with normal-weight natural aggregates. One-year test results indicate that incremental improvements in durability characteristics can further be achieved with the use of blast furnace slag cement. Enhanced fresh and hardened concrete properties of the investigated recycled concrete aggregate as compared to aggregate derived from laboratory-crushed concrete arise primarily from improved aggregate grading and quality achievable in plant crushing operations.

Reuse of de-inking sludge from wastepaper recycling in cement mortar products.

This paper presents results of an investigation into the use of de-inking sludge from a paper recycling mill as feedstock material in the manufacture of cement mortar products, including masonry blocks and mortar renders. Both physical and mechanical properties of mortar specimens containing various amounts of de-inking sludge were investigated. It was observed that the addition of de-inking sludge to cement mortar at a fixed water-to-cement ratio significantly reduced flow properties and increased setting time. Water absorption and volume of permeable voids of cement mortar increased with increased dosage of de-inking sludge, with a corresponding reduction of bulk density. The 91-day compressive strength of mortar samples with 2.5 wt% and 20 wt% de-inking sludge loadings retained 83% and 62% respectively of the reference mortar strength. The corresponding drying shrinkage increased by up to 160% compared to reference samples. However, a de-inking sludge loading of up to 2.5 wt% did not significantly alter measured physical and mechanical properties. The results demonstrate that despite the high moisture absorbance of de-inking sludge due to its organic matter and residual cellulose fibre content, it serves as a potential supplementary additive and its cellulosic content proving to be an active set retardant to cementitious masonry products.

Effect of acetylacetone on the preparation of PZT materials in sol-gel processing

Abstract Sol–gel processing is an advantageous route to produce PZT thin film materials with high quality. PZT powders and thin films with good properties were fabricated by sol–gel processing, using acetylacetone as modifier, in the study. The morphologies of PZT powders and thin films were observed by field emission microscopy, and the dielectric and properties of the PZT thin films were measured. Thermal analysis, XRD and FTIR were used to investigate the influence of ligands on the phase transformation in PZT materials, and it is found that the difficulty of decomposing acetylacetone ligands completely resulted in a higher pyrolysis temperature to form PZT with pure phase.

Properties of wastepaper sludge in geopolymer mortars for masonry applications

This paper presents the results of an investigation into the use of wastepaper sludge in geopolymer mortar systems for manufacturing construction products. The investigation was driven by the increasing demand for reuse options in paper-recycling industry. Both fresh and hardened geopolymer mortar properties are evaluated for samples incorporating dry wastepaper sludge, and the results indicate potential end-use benefits in building product manufacture. Addition of wastepaper sludge to geopolymer mortar reduces flow properties, primarily due to dry sludge absorbing water from the binder mix. The average 91-day compressive strength of mortar samples incorporating 2.5 wt% and 10 wt% wastepaper sludge respectively retained 92% and 52% of the reference mortar strength. However, contrary to the normal trend of increasing drying shrinkage with increasing paper sludge addition to Portland cement matrices, the corresponding geopolymer drying shrinkage decreased by 34% and 64%. Equally important, the water absorption of hardened geopolymer mortar decreased with increasing paper sludge content at ambient temperatures, providing good prospects of overall potential for wastepaper sludge incorporation in the production of building and masonry elements. The results indicate that, despite its high moisture absorbance due to the organic matter and residual cellulose fibre content, wastepaper sludge appears compatible with geopolymer chemistry, and hence serves as a potential supplementary additive to geopolymer cementitious masonry products.

Surface Charge Modification of Nano-Sized Silica Colloid

The surface of commercial 30-nm colloidal silica particles were modified by reaction with functional silanes. The high specific surface area and reactivity of the particles, due to their small size, makes the process susceptible to irreversible aggregation not found previously with larger particles. The present study compares surface charge results from amino silanes with one or three alkoxy groups. Measurements of the zeta potential as a function of pH, and gelation kinetics shed light on the mechanism of surface charge changes from the modification. Instability in suspensions before and after the surface modification is also studied using a new data analysis technique from simple light-scattering equipment. Experimental results show very stable particles are obtained by amino silane surface modification. Factors affecting susceptibility of small particles to irreversible aggregation caused by a non-aqueous solvent or high concentration of a trialkoxy silane, including the large number of reactive silanol groups in the surface gel layer of the particles, are discussed.

Distribution of carbon nanotubes in fresh ordinary Portland cement pastes: understanding from a two-phase perspective

Significant research advances have been made in the field of carbon nanotube (CNT) reinforced ordinary Portland cement (OPC) paste composites in recent years. However, the distribution of CNTs in fresh OPC paste is yet to be fully researched and quantified, thereby creating a technical barrier to CNT utilization in concrete construction. In this study, fresh OPC paste was treated as a two-phase material containing solid particles (cement grains) and liquid solutions (pore solutions). A centrifugation-based technique was proposed to separate these two phases and the presence of CNTs in each phase was quantified. UV-Vis spectrometry showed that the degree of dispersion can achieve above 90 wt% using polycarboxylate superplasticizer. The results suggested an upper limit of 0.26 wt% for CNT addition into water before mixing with OPC, and the dispersion was found to be stable for at least 4 hours. Based on scanning electron imaging, the adsorption phenomenon of CNTs on OPC grains with size less than 4 μm was discovered. Energy-dispersive X-ray spectroscopy indicated these adsorptive particles have lower Ca to Si ratio. It was observed that about 0.5 mg of CNTs per gram of OPC grains was adsorbed in solid OPC grains in typical fresh OPC pastes. On the basis of these results, a conceptual model was proposed for the distribution of CNTs in fresh OPC paste where about 33 wt% of the CNTs stay in pore solution and 65 wt% of CNTs are adsorbed on OPC grains.

Influence and role of feedstock Si and Al content in Geopolymer synthesis

This study evaluates the role and relationship between SiO2 and Al2O3 contents of aluminosilicate material on the properties of Si-rich geopolymer systems using Australian fly ashes and existing published data. The selected fly ashes, with varying amounts of amorphous and total SiO2 and Al2O3 were assessed for their overall reactivity. XRD analyses yielded traces of sodalite in addition to conventional amorphous geopolymer phase for the fly ash mixtures with amorphous SiO2/Al2O3 = 3.31. The other two fly ashes with higher amorphous SiO2/Al2O3 ratio tended to produce sodium aluminosilicate zeolitic phases comprising mainly of Na-P1 zeolite and faujasite. These latter phases originate from Na2O–Al2O3–SiO2–H2O gel assemblages but the pathway appears to be governed by the available reactive SiO2 and Al2O3 in the reaction environment. However, indices based on the amorphous content or more precisely amorphous SiO2/Al2O3 ratio provided a relatively good correlation with the compressive strength of the final product.

Evaluation of fly ash geopolymer mortar incorporating calcined wastepaper sludge

This study evaluates fresh and hardened properties of fly ash geopolymer mortars incorporating calcined wastepaper sludge cured under ambient conditions. The calcined sludge geopolymer mixtures were characterized by XRD, XRF, and SEM–EDS techniques. It was observed that addition of up to 40 wt% calcined sludge in geopolymer mortars at a fixed liquid/solid mass ratio of 0.2 reduced flow of mortars from 112 to 61%, while the corresponding setting time was shortened from 113 h to just under 1 h. At the maximum calcined sludge loading of 40 wt%, the compressive strength at 7 days was 29.0 MPa, a significant increase from 3.8 MPa for reference mortar samples with no addition of sludge. Correspondingly, a nearly fourfold reduction in 56-day drying shrinkage measured at 3600 microstrains was observed, compared to reference mortars. The plausible reaction mechanisms, in particular, apparent concurrent formation of C-A-S-H/N-(C)-A-S-H phases alongside geopolymer gel networks are discussed in detail.

Retraction notice to Surface modification for stability of nano-sized silica colloids [Journal of Colloid and Interface Science 315 (2007) 123–127]

This article has been retracted at the request of the Editorin-Chief. Reason: This article has been retracted, please see Elsevier Policy on Article Withdrawal: http://www.elsevier.com/locate/ withdrawalpolicy. This article has been retracted at the request of the editor as it duplicates significant parts of a paper that had already appeared in the Australian Journal of Chemistry 60 (2007) 662–666,