Chun-Tao Chen

Influence of Cyclic Humidity on Carbonation of Concrete

AbstractCarbonation of concrete under various humidity conditions was examined in this study. Cylindrical specimens were subjected to accelerated carbonation using 50% carbon dioxide by volume at 25°C. Two relative humidities (RHs) (70 and 90% RH) and two humidity cycles (70–90% and 50–90% RH) were applied during the process. The results showed that carbonation of concrete was affected by the ambient humidity, humidity cycles, and surface geometry. The extent of carbonation was quantified using the coefficient of carbonation, which is the ratio of the carbonation depth to the square root of time. The maximum coefficient of carbonation occurred at 70% RH, and humidity cycles induced coefficients of carbonation between those induced at 70 and 90% RH. Specimens with curved surfaces had higher coefficients of carbonation than specimens with plane surfaces. The carbonation coefficients measured in the accelerated tests were also related to the carbonation of on-site concrete, which indicates that taking humidi...

Influence of Mixing Sequence on Cement-Admixture Interaction

Dynamic rheology was used to investigate cement pastes prepared using various mixing sequences and containing a high-range water-reducing admixture. Delayed admixture addition, delayed water addition, double mixing, and vigorous mixing techniques were used to prepare samples. Mixing sequence effects varied with cement composition, admixture dosage, and addition time. Of these mixing sequences, the most effective at dispersion enhancement was delayed admixture addition. A 2.5 minute delay time was sufficient to disperse all cement-admixture combinations. An optimum amount of delayed water exists with delayed water addition. Pastes with longer between mixing time intervals were dispersed more with double mixing. In general, pastes with vigorous mixing were dispersed by the admixture. At low admixture dosages, however, the dispersing effect was not apparent. Study results demonstrate that mixing procedures and batching sequences must be taken into account when comparing different concrete batches, even through they have the same raw materials and mixture proportions. Study results suggest, moreover, a direction for optimizing the batching sequence and mixing procedure for both paste and concrete.

Correlation Between Paste and Concrete Flow Behavior

Initially a poor correlation was seen between concrete workability and cement paste rheology in polycarboxylated acrylate ester-containing mixtures, in that, in order to produce full cement paste dispersion, the required admixture dosage, as determined using dynamic rheology, was approximately four times that required for full concrete dispersion production, as determined using slump and slump loss. Using a high-speed blender for shear history difference reduction between concrete and cement paste, a preshear protocol provided a much improved concrete workability and paste rheology correlation. That both pastes and concretes, when fully dispersed, were at full admixture saturation, was verified by adsorption isotherms.

Hydration Process and Compressive Strength of Slag-CFBC Fly Ash Materials without Portland Cement

AbstractThis study mixed ground granulated blast-furnace slag (S) and circulating fluidized bed combustion (CFBC) fly ash (CA) without any portland cement or alkaline activator to produce an eco-binder, abbreviated as SCA binder. The hydration process, microstructure, and compressive strength of hydrated SCA materials were investigated. Although both the slag and CA had poor hydration with water, the SCA binder produced satisfactory hydration products with sufficient cementitious properties. These hydration products detected by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were ettringite (AFt), calcium silicate hydrate (C-S-H), and calcium aluminosilicate hydrate (C-A-S-H). The scanning electron microscope (SEM) micrograph showed these hydration products formed dense microstructure for SCA pastes. As a result, the SCA materials had sufficient compressive strength for practical applications in building materials and civil engineering structures. The compressive strengths of th...

Circulating Fluidized Bed Combustion Fly-Ash-Activated Slag Concrete as Novel Construction Material

The performance of a novel concrete made with an eco-binder, referred to as SCA binder, that only contains raw slag (S) and raw circulating fluidized bed combustion (CFBC) fly ash (CA) without ordinary portland cement was evaluated. The major hydration products of SCA binders are ettringite, C-S-H, and C-A-S-H, which lead to SCA pastes with proper setting times, dense microstructures, and high compressive strengths up to approximately 80 MPa (11,600 psi). The SCA concrete, which is suitable for practical applications, has compressive and tensile strengths at 91 days of approximately 50 MPa (7250 psi) and nearly 5 MPa (725 psi), respectively. In addition, the SCA concrete shows moderate expansion at early ages and a low rate of shrinkage after 91 days of exposure. The regression equations that relate the splitting tensile strength, modulus of elasticity, and ultrasonic pulse velocity to the compressive strength are presented with a satisfactory coefficient of determination.

Sustainable Performance of Limestone Cement

This study explores the effects of the limestone addition on the mechanical properties and durability of the cement. Portions of the Portland cements were replaced by the limestone powders. Results showed that the compressive strengths of the mortar were reduced by the limestone addition. In general, the addition of limestone helps to reduce the shrinkage and improve the sulfate attack resistance. The lowest sulfate-expansion occurred at the specimens with a limestone addition of 10%. Moreover, with additions of 10% or less, the reductions in strength were likely recovered by the addition of fly ash.

Mix Proportion and Engineering Behavior of San-Ho-Tu Building Material for Temples and Ancestral Clan Houses

The San-Ho-Tu building material is manufactured by adequately mixing a ternary mixture of sintered oyster shell ash, laterite and sand with water. It has been broadly used for construction and restoration of the ancestral temples and clan houses in China and Taiwan for hundreds of years due to its adequate engineering properties and easy availability of raw constituents. The main purpose of this study is aimed at understanding its engineering properties and proper mix proportioning. Cylindrical specimens of ϕ50 × 100 mm for nine sets of mixtures were cast, including three sets of traditional San-Ho-Tu building material (L-group), three sets of L-group mixtures with Portland cement (C-group) and three sets of single oyster ash paste an laterite paste (S-group). They were tested for compressive and tensile strengths at six ages of 7, 14, 21, 28, 56 and 90 days, respectively. Experimental results showed that at 90 days, the average compressive strengths of three specimens for S-group, L-group and M-group were 296.2, 2144.1 and 4915.4 kPa respectively. The ratios of the splitting tensile strengths to the corresponding compressive strength of San-Ho-Tu cylindrical specimens were between 16.1% and 19.9% which were higher than those of 10.0% and 14.0% for concrete cylinder made from the pure Portland cement paste.

Stiffening of the Cement Paste Monitored Using Vibrating Fork Technique

This study explored the stiffening behavior of the cement pastes using a new vibration folk technique. The viscosity evolutions of the cement pastes were evaluated using either a rotational viscometer or a tuning fork vibration viscometer. Results showed that the viscosities were increased with hydration time. The viscosity increases of the cement pastes measured by the vibro-viscometer were more stable than those measured by the rotational viscometer. In addition, the subtle changes in the paste viscosities during the very early hydration were well detected by the vibro-viscometer. The new vibration folk technique has been shown a useful tool to characterize the microstructure changes in the early cement pastes.

Effects of Activating Solution and Liquid/Solid Ratio on Engineering Properties of Metakaolin-Based Geopolymer

In this paper, the metakaolin is used as the raw material with aluminosilicate compounds to produce the geopolymer. The effects of three levels of two major controlling factors, the degree of polymerization of the activating solution (weight ratio of SiO2 to Na2O) of 0.4, 0.7 and 1.0 and the weight ratio of liquid to solid (L/S) of 0.7, 0.85 and 1.00 on the engineering properties of geopolymer are investigated. The experimental results show that, at age of 28 days, the compressive strength increases from the lowest 37.33 MPa (SiO2/Na2O = 0.4 and L/S = 0.7) to the highest 71.21 MPa (SiO2/Na2O = 0.7 and L/S = 0.7). While, the thermal conductivity increases from the lowest 0.39 w/mk (SiO2/Na2O = 0.4 and L/S = 1.0) to the highest 0.761 w/mk (SiO2/Na2O = 1.0 and L/S = 0.7).