S.C. Kou

Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete

The influence of moisture states of natural and recycled aggregates on the properties of fresh and hardened concretes was investigated. Concrete mixes were prepared with natural and recycled aggregates at different proportions. The moisture states of the aggregates were controlled at air-dried (AD), oven-dried (OD) and saturated surface-dried (SSD) states prior to use. The ratio of cement to free water was kept constant for all of the mixes. At the fresh state, the slump loss for various concrete mixtures was determined, while the compressive strength was determined after curing for 3, 7 and 28 days. The test results showed that the initial slump values of the concrete mixtures were dependent on the initial free water contents, and the slump loss values of the mixtures were related to the moisture states of the aggregates. Slump loss was significant when 100% AD or OD recycled aggregate was used. The effect of the moisture states of the aggregates on the strength of the concretes prepared with OD and SSD state aggregates at early age (i.e., 3 and 7 days) was noticeable. The concrete prepared with the AD aggregates achieved the highest average strength values at 3, 7 and 28 days. However, at 28 days, the concrete strengths prepared with different types of aggregates were similar. The results suggested that an AD aggregate that contains not more than 50% recycled aggregate is optimum for producing normal strength recycled aggregate concrete.

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.

A study on the hydration rate of natural zeolite blended cement pastes

Abstract Natural zeolite is a type of mineralogical material containing large quantities of reactive SiO 2 and Al 2 O 3. It is widely used in the cement industry in China as a cement blending material. Like other pozzolanic materials such as silica fume and fly ash, zeolite contributes to concrete strength mainly through the pozzolanic reaction with Ca(OH) 2 , Thus, the pozzolanic reactivity of this type of material in comparison with other pozzolans is of much interest. This paper presents experimental results on the compressive strength, degree of pozzolanic reaction, and porosity of zeolite modified cement pastes. These results are compared with those obtained from similar blended cement pastes prepared with silica fume and fly ash replacements. Based on the experimental results, it can be concluded that natural zeolite is a pozzolanic material, with a reactivity between that of silica fume and fly ash. Generally, in blended cement pastes with a lower water-to-cementitious materials ratio, the natural zeolite contributes more to the strength of the pastes. But in the pastes with a higher water to cementitious ratio and a lower cement replacement level it undergoes a higher degree of reaction.

Activation of fly ash/cement systems using calcium sulfate anhydrite (CaSO4)

A number of studies had been conducted on the activation of fly ash using gypsum and sodium sulfate. Anhydrite, another form of calcium sulfate, has not been used for this purpose. This paper presents an exploratory study on the effectiveness of anhydrite in activating fly ash cement systems. Anhydrite (10%) was added into cement mortars with up to 55% fly ash replacement. The prepared mortars were allowed to cure in steam at 65°C for 6 h before normal room temperature water curing. Significant strength increases (up to 70%) compared to the control mortars were observed as early as after 3 days curing. Improvements in the pore size distribution of the mortars were also observed due to the activation. The results of scanning electron microscopy (SEM) examination and quantitative X-ray diffraction (XRD) analysis show that, with accelerated curing, a large quantity of ettringite (AFt) was formed during the early stage of hydration of the anhydrite-activated fly ash cement pastes. This might be the main cause of the high early strength of the activated fly ash cement systems. A comparison was made using anhydrite and gypsum as activators. For an equivalent SO3 content, anhydrite is more effective in increasing the early-age strength of the cement/fly ash mortars, but less effective in increasing the later-age strength than gypsum.

Properties of lightweight aggregate concrete prepared with PVC granules derived from scraped PVC pipes

This paper aims to investigate the fresh and hardened properties of lightweight aggregate concretes that are prepared with the use of recycled plastic waste sourced from scraped PVC pipes to replace river sand as fine aggregates. A number of laboratory prepared concrete mixes were tested, in which river sand was partially replaced by PVC plastic waste granules in percentages of 0%, 5%, 15%, 30% and 45% by volume. Two major findings are identified. The positive side shows that the concrete prepared with a partial replacement by PVC was lighter (lower density), was more ductile (greater Poissons ratios and reduced modulus of elasticity), and had lower drying shrinkage and higher resistance to chloride ion penetration. The negative side reveals that the workability, compressive strength and tensile splitting strength of the concretes were reduced. The results gathered would form a part of useful information for recycling PVC plastic waste in lightweight concrete mixes.

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.

Use of wastes derived from earthquakes for the production of concrete masonry partition wall blocks.

Utilization of construction and demolition (C&D) wastes as recycled aggregates in the production of concrete and concrete products have attracted much attention in recent years. However, the presence of large quantities of crushed clay brick in some the C&D waste streams (e.g. waste derived collapsed masonry buildings after an earthquake) renders the recycled aggregates unsuitable for high grade use. One possibility is to make use of the low grade recycled aggregates for concrete block production. In this paper, we report the results of a comprehensive study to assess the feasibility of using crushed clay brick as coarse and fine aggregates in concrete masonry block production. The effects of the content of crushed coarse and fine clay brick aggregates (CBA) on the mechanical properties of non-structural concrete block were quantified. From the experimental test results, it was observed that incorporating the crushed clay brick aggregates had a significant influence on the properties of blocks. The hardened density and drying shrinkage of the block specimens decreased with an increase in CBA content. The use of CBA increased the water absorption of block specimens. The results suggested that the amount of crushed clay brick to be used in concrete masonry blocks should be controlled at less than 25% (coarse aggregate) and within 50-75% for fine aggregates.

Pore size distribution of high performance metakaolin concrete

The Compressive strength, porosity and pore size distribution of high performance metakaolin (MK) concrete were investigated. Concretes containing 0.5%, 10% and 20% metakaolin were prepared at a water/cementitious material ratio (W/C) of 0.30. In parallel, concrete mixtures with the replacement of cement by 20% fly ash or 5 and 10% silica fume were prepared for comparison. The specimens were cured in water at 27°C for 3 to 90 days. The results show that at the early age of curing (3 days and 7 days), metakaolin replacements increase the compressive strength but silica fume replacement slightly reduces the compressive strength. At the age of and after 28 days, the compressive strength of the concrete with metakaolin and silica fume replacement increases. A strong reduction in the total porosity and average pore diameter were observed in the concrete with MK 20% and 10% in the first 7 days.