C.S. Poon

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.

Degree of hydration and gel/space ratio of high-volume fly ash/cement systems

Although fly ash has been widely used in concrete as a cement replacement, little work has been done on determining the degree of hydration of high-volume fly ash/cement (FC) systems. In the present study, the degree of hydration of the cement in Portland cement (PC) paste was obtained by determining the non-evaporable water (Wn) content. The degree of reaction of the fly ash in FC pastes was determined using a selective dissolution method. Based on the relation between the degree of cement hydration and effective water-to-cement (w/c) ratio, the degree of hydration of the cement in FC pastes was also estimated. It was found that high-volume fly ash pastes underwent a lower degree of fly ash reaction, and in the pastes with 45% to 55% fly ash, more than 80% of the fly ash still remained unreacted after 90 days of curing while the hydration of the cement in high-volume fly ash pastes was enhanced because of the higher effective w/c ratio for the paste. This effect was more significant for the pastes with lower water-to-binder (w/b) ratios. Thus, preparing high-volume fly ash concrete at lower w/b ratios can result in less strength losses. This paper also introduces a model to describe the relationship between the w/c ratio and the degree of cement hydration and gel/space ratio. The gel/space ratios of the FC pastes, evaluated based on the proposed model, were found to be consistent with the gel/space ratio of PC pastes in terms of the relationship with compressive strength. The gel/space ratio data correlated (inversely) linearly with mercury intruded porosity, but the former correlated more with compressive strength than the latter.

A study on high strength concrete prepared with large volumes of low calcium fly ash

Abstract This paper presents the results of a laboratory study on high strength concrete prepared with large volumes of low calcium fly ash. The parameters studied included compressive strength, heat of hydration, chloride diffusivity, degree of hydration, and pore structures of fly ash/cement concrete and corresponding pastes. The experimental results showed that concrete with a 28-day compressive strength of 80 MPa could be obtained with a water-to-binder (w/b) ratio of 0.24, with a fly ash content of 45%. Such concrete has lower heat of hydration and chloride diffusivity than the equivalent plain cement concrete or concrete prepared with lower fly ash contents. The test results showed that at lower w/b ratios, the contribution to strength by the fly ash was higher than in the mixes prepared with higher w/b ratios. The study also quantified the reaction rates of cement and fly ash in the cementitious materials. The results demonstrated the dual effects of fly ash in concrete: (i) act as a micro-aggregate and (ii) being a pozzolana. It was also noted that the strength contribution of fly ash in concrete was better than in the equivalent cement/fly ash pastes suggesting the fly ash had improved the interfacial bond between the paste and the aggregates in the concrete. Such an improvement was also reflected in the results of the mercury intrusion porosimetry (MIP) test.

Rate of pozzolanic reaction of metakaolin in high-performance cement pastes

This paper assesses the hydration progress in metakaolin (MK)-blended high-performance cement pastes with age from the measurements of compressive strength, porosity, and pore size distribution, the degree of pozzolanic reaction, and the Ca(OH)2(CH) content of the MK-blended cement pastes at a water-to-binder ratio (w/b) of 0.3. Comparisons are also made with pastes containing silica fume (SF), fly ash (FA), and control Portland cement (PC). It is found that at early ages, the rates of pozzolanic reaction and CH consumption in the MK-blended cement pastes are higher than in the SF- or FA-blended cement pastes. The higher pozzolanic activity of MK results in a higher rate of strength development and pore structure refinement for the cement pastes at early ages. Although the rate of pozzolanic reaction of MK becomes slower after 28 days of curing, the pozzolanic reaction in the blended cement pastes with a w/b of 0.3 still continues at the age of 90 days. At this age, about half of the MK still are unreacted.

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.

Effect of Fly Ash and Silica Fume on Compressive and Fracture Behaviors of Concrete

Abstract The effects of replacing cement by fly ash and silica fume on strength, compressive stress-strain relationship, and fracture behavior of concrete were investigated. The investigation covered concrete mixes at different water-cementitious material ratios, which contained low and high volumes of fly ash, and with or without the addition of small amount of silica fume. It was found that fly ash substantially improved the post-peak compressive behavior of concrete, with a relatively smaller gradient in the descending part of the stress-strain curve. Low volumes of fly ash improved the tensile strength of concrete. High volume fly ash concrete showed slightly lower tensile strength, but higher values of crack tip opening displacement and final mid-span deflection in the fracture tests, with the corresponding K IC and G F values similar to or higher than the plain cement concrete. A small amount of silica fume had a large positive effect on the cylinder compressive strength and tensile strength but less on the cube compressive strength, while the fracture behavior of the resulting concrete was brittle. Improving interfacial bond between the paste and the aggregates in concrete had positive effects on K IC , but did not necessarily produce higher G F values.

Impact of high temperature on PFA concrete

After being subjected to high temperatures, the residual properties of pulverized fly ash (PFA) concrete have been investigated. Both mechanical and durability properties of concrete were tested on concretes made with different water to binder ratios and PFA contents. Microscopic techniques were then employed and the pore structure and microhardness values of hardened cement paste (hcp) were determined. The results of rapid chloride diffusion tests revealed that concrete durability deterioration commences after exposure to temperatures which are lower than those at which compressive strength deterioration commences. The rise in compressive strength, which occurs after exposure to 250°C, may be largely due to the hardening of cement paste caused by drying and the further hydration of cementitious materials. The simultaneous loss of durability, however, can be explained by a weakened transition zone between hcp and aggregate, and by the concurrent coarsening of the hcp pore structure. When PFA is included, an improvement of fire resistance as characterized by the residual compressive strength was observed, and this relative improvement over non-PFA concrete was the most pronounced for maximum exposure temperatures of 450°C and 650°C.

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 fly ash-modified cement mortar-aggregate interfaces

This paper investigates the effect of fly ash on strength and fracture properties of the interfaces between the cement mortar and aggregates. The mortars were prepared at a water-to-binder ratio of 0.3, with fly ash replacements from 15 to 55%. Notched mortar beams were tested to determine the flexural strength, fracture toughness, and fracture energy of the plain cement and fly-ash modified cement mortars. Another set of notched beams with mortar-aggregate interface above the notch was tested to determine the flexural strength, fracture toughness, and fracture energy of the interface. Mortar-aggregate interface cubes were tested to determine the splitting strength of the interface. It was found that a 15% fly ash replacement increased the interfacial bond strength and fracture toughness. Fly ash replacements at the levels of 45 and 55% reduced the interfacial bond strength and fracture toughness at 28 days, but recovered almost all the reduction at 90 days. Fly ash replacement at all levels studied increased the interfacial fracture energy. Fly ash contributed to the interfacial properties mainly through the pozzolanic effect. For higher percentages of replacement, the development of interfacial bond strength initially fell behind the development of compressive strength. But at later ages, the former surpassed the latter. Strengthening of the interfaces leads to higher long-term strength increases and excellent durability for high-volume fly ash concrete.