Chi Sun Poon

On-site sorting of construction and demolition waste in Hong Kong

Abstract The construction industry is the major solid waste generator in Hong Kong. In 1998, it generated about 32 710 t per day of construction and demolition (C&D) waste. In the management of such a huge quantity of C&D waste, Hong Kong has adopted a strategy of depositing the inert portion (e.g. sand, bricks and concrete) of the waste at public filling areas for land reclamation and the non-inert portion (e.g. plastics, paper, wood) at municipal solid waste landfills. However, the C&D waste arisen is usually in the form of a mixture of both inert and non-inert materials. As a result, the waste has to be disposed of at landfills, aggravating the landfill shortage problem. There is a paramount need to separate the C&D waste into its constituent parts before it is delivered to either the landfills or the public filling areas for disposal. In order to study the feasibility of carrying out on-site waste sorting and the current situation of the building relating C&D waste generated in Hong Kong, a survey was conducted. This paper presents the results of the survey undertaken to evaluate three alternative waste sorting methods on building construction sites and to compare them with the use of an off-site central waste sorting facility. The results indicate source separation has the advantages of requiring less effort and resulting in better segregation of inert and non-inert wastes as compared with waste sorting centrally carried out at a designated area on- or off-site. In addition, the views of the building industry participants were also obtained through a questionnaire survey to give a better understanding of their attitude on on-site waste sorting. The results indicate that the building construction participants are reluctant to carry out on-site waste sorting. Even when high a tipping fee is imposed, they have little incentive to perform on-site waste sorting which is considered to be time and labour demanding. Only through contractual requirements or legislation can on-site waste sorting be fully implemented and becomes a long-term solution to the landfill shortage problem in Hong Kong.

USE OF RECYCLED AGGREGATES IN MOLDED CONCRETE BRICKS AND BLOCKS

Abstract This study aimed to develop a technique for producing concrete bricks and paving blocks using recycled aggregates obtained from construction and demolition waste. Laboratory trials were conducted to investigate the possibility of using recycled aggregates from different sources in Hong Kong, as the replacement of both coarse and fine natural aggregates in molded bricks and blocks. A series of tests were carried out to determine the properties of the bricks and blocks prepared with and without recycled aggregates. The test results showed that the replacement of coarse and fine natural aggregates by recycled aggregates at the levels of 25 and 50% had little effect on the compressive strength of the brick and block specimens, but higher levels of replacement reduced the compressive strength. However, the transverse strength of the specimens increased as the percentage of replacement increased. Using recycled aggregates as the replacement of natural aggregates at the level of up to 100%, concrete paving blocks with a 28-day compressive strength of not less than 49 MPa can be produced without the incorporation of fly ash, while paving blocks for footway uses with a lower compressive strength of 30 MPa and masonry bricks can be produced with the incorporation of fly ash.

COMPARISON OF THE STRENGTH AND DURABILITY PERFORMANCE OF NORMAL-AND HIGH STRENGTH POZZOLANIC CONCRETES AT ELEVATED TEMPERATURES

The strength and durability performance of normal- and high-strength pozzolanic concretes incorporating silica fume, fly ash, and blast furnace slag was compared at elevated temperatures up to 800°C. The strength properties were determined using an unstressed residual compressive strength test, while durability was investigated by rapid chloride diffusion test, mercury intrusion porosimetry (MIP), and crack pattern observations. It was found that pozzolanic concretes containing fly ash and blast furnace slag give the best performance particularly at temperatures below 600°C as compared to the pure cement concretes. Explosive spalling occurred in most high-strength concretes (HSCs) containing silica fume. A distributed network of fine cracks was observed in all fly ash and blast furnace slag concretes, but no spalling or splitting occurred. The high-strength pozzolanic concretes showed a severe loss in permeability-related durability than the compressive strength loss. Thirty percent replacement of cement by fly ash in HSC and 40% replacement of cement by blast furnace slag in normal-strength concrete (NSC) was found to be optimal to retain maximum strength and durability after high temperatures.

Strength and durability recovery of fire-damaged concrete after post-fire-curing

The effect of post-fire-curing on the strength and durability recovery of fire-damaged concrete was investigated. Twenty normal- (NSC) and high-strength concrete (HSC) mixes incorporating different pozzolans were prepared and exposed to elevated temperatures till 800°C. After natural cooling, the specimens were subjected to post-fire-curing in water and in a controlled environment for a total duration of 56 days. Unstressed compressive strength, rapid chloride diffusion, and mercury intrusion porosimetry (MIP) tests were conducted to examine the changes in the macro- and microstructure of the concrete. The test results indicated that the post-fire-curing results in substantial strength and durability recovery and its extent depend upon the types of concrete, exposure temperature, method, and duration of recuring. In one case, the recovered strength was 93% of the original unfired strength. Scanning electron microscopy (SEM) investigations indicated that the recovery was due to a number of rehydration processes that regenerate the calcium-silicate-hydrate (C-S-H). The new rehydration products were smaller in size than the original hydration products and filled the internal cracks, honey combs, and capillaries created during the fire. The surface crack widths were also reduced during the recuring process, and in most cases, they were found within the maximum limits specified by the American Concrete Institute (ACI) building code.

Properties of concrete blocks prepared with low grade recycled aggregates

Low grade recycled aggregates obtained from a construction waste sorting facility were tested to assess the feasibility of using these in the production of concrete blocks. The characteristics of the sorted construction waste are significantly different from that of crushed concrete rubbles that are mostly derived from demolition waste streams. This is due to the presence of higher percentages of non-concrete components (e.g. >10% soil, brick, tiles etc.) in the sorted construction waste. In the study reported in this paper, three series of concrete block mixtures were prepared by using the low grade recycled aggregates to replace (i) natural coarse granite (10mm), and (ii) 0, 25, 50, 75 and 100% replacement levels of crushed stone fine (crushed natural granite <5mm) in the concrete blocks. Test results on properties such as density, compressive strength, transverse strength and drying shrinkage as well as strength reduction after exposure to 800 degrees C are presented below. The results show that the soil content in the recycled fine aggregate was an important factor in affecting the properties of the blocks produced and the mechanical strength deceased with increasing low grade recycled fine aggregate content. But the higher soil content in the recycled aggregates reduced the reduction of compressive strength of the blocks after exposure to high temperature due probably to the formation of a new crystalline phase. The results show that the low grade recycled aggregates obtained from the construction waste sorting facility has potential to be used as aggregates for making non-structural pre-cast concrete blocks.

The influence of different curing conditions onthe pore structure and related properties of fly-ash cement pastes and mortars

Abstract The influence of two different curing conditions (in water at 27°C, and in air at 15°C and 60% relative humidity) on the mechanical and durability properties of fly-ash cement pastes and mortars are studied. Cement pastes and mortars at two water/cement or binder ratios were prepared in the laboratory and tested for compressive strength, chloride and water penetration. The mercury intrusion porosity of the samples is monitored to provide mechanistic explanations for the measured results. The results show that fly ash has significantly different influence on the strength, porosity and durability parameters of cement pastes and mortars when the cementitious materials are subjected to different curing conditions.

Degradation kinetics of cuprophenyl yellow RL by UV/H2O2/ultrasonication (US) process in aqueous solution

Abstract Experiments were conducted to study the kinetic aspects of the UV/H 2 O 2 /US treatment process for Cuprophenyl Yellow RL. It was found that the reduction in absorbance of the dye solution followed the pseudo-first-order kinetic model at different pH and hydrogen peroxide dosages. The rate constants at different pH as well as hydrogen peroxide dosages were obtained. The TOC removal for the treatment process with different hydrogen peroxide dosages followed different kinetic patterns and it was inferred that they would follow different degradation pathways. When lower H 2 O 2 dosage was used, there were intermediates produced, while the dye might be directly mineralized when higher H 2 O 2 dosage was adopted.

Performance Enhancement of Recycled Concrete Aggregates through Carbonation

AbstractThe cement paste attached to natural aggregates has a significant effect on the quality of recycled concrete aggregates (RCAs) because it usually has higher porosity and lower strength than natural aggregates. This work attempted to improve the quality of RCAs through carbonation of the attached cement paste. During carbonation reactions, CO2 reacted with Ca(OH)2 and calcium silicate hydrate (C─S─H) to form CaCO3 and silica gel, which filled the pores in the attached cement paste. Thus, carbonation increased the density, and decreased the water absorption and crushing values of the RCAs. It, thus, increased the flowability and compressive strength and decreased drying shrinkage of the recycled aggregate mortars.

The limitation of the toxicity characteristic leaching procedure for evaluating cement-based stabilised/solidified waste forms

Abstract The United States Environmental Protection Agencys Toxicity Characteristic Leaching Procedure (TCLP) is commonly used as a regulatory tool to determine whether or not a waste can be classified as a hazardous waste. The validity of the test procedure for assessing cement-based stabilized/solidified heavy metal wastes is examined in this paper. Synthetic cement-based heavy metal waste forms with different ANC were prepared and subjected to TCLP to study the effect of waste acid neutralizing capacity (ANC) on metal leaching. A “real” waste was also obtained from a local commercial treatment facility and tested to verify the findings. The results showed that as long as the stabilized/solidified waste forms have sufficient ANC to neutralize the acidity of the leachant, the leaching of metals will be small and the performance of the different waste forms cannot be differentiated. The test therefore has limited use in comparing the performance of different cement-based waste forms. A modified test procedure is proposed.

The use of ferric chloride and anionic polymer in the chemically assisted primary sedimentation process

Abstract Chemically Enhanced Primary Treatment (CEPT) or Chemically Assisted Primary Sedimentation Process (CAPS) involves the use of chemical coagulants to enhance the coagulation or flocculation of wastewater particles. The effect of a metal salt, ferric chloride (FeCl 3 ) and an anionic polymer on the removal of suspended solids (SS) of wastewater collected from two sewage treatment plants was studied by jar test experiments. The results showed that the optimum dosage for the removal of 60% of SS was 30 ppm of FeCl 3 with 0.5 ppm polymer. A larger scale test further revealed that the addition of 30 ppm of FeCl 3 and 0.5 ppm. polymer could provide a reduction of SS, total nitrogen (N) and total phosphorous (P) higher than 80%, 70% and 40%, respectively. The experimental results confirmed that CAPS can be used as an alternative for the treatment of sewage to traditional biological processes.