Y.L. Wong

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.

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.

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.

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.

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 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.

Influence of PFA on cracking of concrete and cement paste after exposure to high temperatures

Abstract Cracking is a visible type of damage to concrete that has significant adverse effects on the mechanical and durability properties of concrete. An experimental study on the identification and quantification of cracking in postheated concrete was conducted to provide a better understanding of the mechanisms of damages to concrete after exposure to high temperatures. In addition to the quantification of the residual compressive and tensile strengths of concrete after high temperature exposure, both macroscale and microscopic cracks were observed and measured. The crack patterns in different concretes, including concrete made with different water to binder (w/b) ratios and PFA dosages, were classified. Also examined was the cracking in the corresponding hardened cement pastes (hcps) prepared without adding aggregates. The relation of cracking with deterioration of the durability properties of concrete, with respect to the chloride diffusion test results, was discussed. Crack density, a quantitative term, which had been introduced to study the microcrack properties in concrete, was adopted for measuring the severity of cracking. Severe cracking of concrete was observed after exposure to 450 °C and higher temperatures. The presence of PFA reduced the extent of these thermal cracks.

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.