Effect of Fine Steel Slag Powder on the Early Hydration Process of Portland Cement

Hydration heat evolution, non-evaporative water, setting time and SEM tests were peorormed to investigate the effect of fine steel slag powder on the hydration process of Portland cement and its mechanism. The results show that the effect of fine steel slag powder on the hydration process of Portland cement is closely related to its chemical composition, mineral phases, fineness, etc. Fine steel slag powder retards the hydration of portland cement at early age. The major reason for this phenomenon is the relative high content of MgO , MnO2, P2O5 in steel slag, and MgO solid solved in C3 S contained in steel slag.

Early Age Hydration of Cement Paste Monitored with Ultrasonic Velocity and Numerical Simulation

Early age hydration of cement paste was investigated by monitoring the ultrasonic propagation velocity and simulated with the numerical model CEMHYD3D. The ultrasonic velocity of the P-wave was recorded during the first 24 hours of the hydration process and its evolution was analyzed. It is shown that the UPV method is an effective, accurate and non-destructive method for monitoring the early age hydration process of cement paste. The early age hydration process can be classified as four periods, which are the dormant period, acceleration period, deceleration period and stabilization period. Higher w/c ratios result in lower UPV, and delay initial setting time due to the decreased solid volume. The change of UPV is clearly related to the hydration degree of cement particles.

Effect of Superplasticizers on the Early Age Hydration of Sulfoaluminate Cement

The effects of two types of superplasticizers on the properties of CSA cement pastes during early hydration were studied. The influences of two types of superplasticizers on the properties of cement pastes, including the normal consistency, setting time, fl uidity, and compressive strength, were investigated by using various methods. The hydration products of the cement pastes cured for 1 day and 3 days were studied by X-ray diffraction（XRD） and scanning electron microscopy（SEM）. The results show that the PCE type superplasticizer retards the early age hydration while the FDN type superplasticizer accelerates the early age hydration of the CSA cement. Both types of superplasticizers have no infl uence on the further hydration of CSA cement, confi rmed by the calorimeter tests as well. The ultrasonic pulse velocity measurements were used to probe the influence of two types of superplasticizers on the hydration of CSA cement pastes at a high water-cement ratio（0.45）. The results show that the PCE type superplasticizer retards the early age hydration of the CSA cement while the FDN type superplasticizer has little infl uence on the early age hydration of the CSA cement.

Hydration Heat Evolution of Cement and Its Relation With Setting Time

In order to veritably measure the first peak of hydration heat evolution that has been illustrated important in indicating cement behavior in early hydration, an improved way of water addition into cement in isothermally calorimetric experiment is put forward. The experimental results indicated that: the magnitude of first peak of heat evolution varies from sample to sample, correlation between heat evolution during first peak of heat evolution and initial (as well as final) setting time is unsatisfactory when samples are not classified; while groups of sample classified based on strength grade represent satisfactory correlations, which indicating the existence of close relation between hydration heat evolution in much earlier hydration age and setting property of cement in rather later age. Importance of first peak in hydration heat evolution for understanding cement setting property and reasons for sample classification are also discussed in this paper.

Hydration Mechanism of Cement-based Materials with Different Si-rich Mineral Admixtures

Early hydration mechanism of cement-based materials with silica fume, nano-SiO2 and silica sol of different contents was investigated, and the detailed effect of these Si-rich mineral admixtures in three stages of early hydration（NG, I, D） using kinetics model was focused. The results showed that silica fume, nano-SiO2, and silica sol have significant effect on kinetic parameters n, k1, k2 and k3, the fineness and existing form of SiO2 particles in these Si-rich mineral admixtures are two important factors to affect the hydration process and on the parameters. Through integrated use of methods of hydration heat-Krstulovic-Dabic Modelsynthetical thermal analysis, data of hydration heat were collected, hydration degree was characterized, as well as the resulting crystallization behavior of early hydration, to build a numerical relationship between parameter n and CH contents that n decreases with increasing CH, and thus, a direct connection between hydration heat release behavior and crystallization behavior has been established.

Thermal and Stress Analysis of Early Age Concrete for Spread Footing

The early age performance of spread footing, especially the growth of cracks, is deeply influenced by the heat of hydration of cement. In this paper, 3D finite element method(FEM)models are set up to analyze the temperature distribution and thermal stresses of the spread footing during the first seven days after concrete placement. The mechanical properties of early age concrete are calculated, which are further used in the FEM models. The possibilities of crack growth are estimated by the method of crack index. The crack indexes of quite a number of points are very close to the allowable limit of 1.0 during the last three days. It is also indicated that the influence of foundation ring on the thermal stresses of concrete can be neglected.

Effect of CO_(2)-mixing dose and prolonged mixing time on fresh and hardened properties of cement pastes

This study aims to investigate the influence of CO_(2)-mixing dose(mass fractions of 0.3%,0.6%,and 0.9%)and prolonged mixing time on the fresh and hardened properties of cement pastes.The CO_(2)-mixing can act as coagulant in fresh cement mixtures,resulting in a significant reduction in workability associated with the formation of a rich calcium carbonate network on the surface of cement particles.The CO_(2)-mixing cement pastes were found to be much stiffer and more difficult to handle,place,and compact than the control mixture,which had a negative effect on the mechanical strength performance of the hardened pastes.However,prolonging the mixing time for 1 min(immediately after CO_(2)-mixing)can effectively improve the workability(by~53%–85%)by breaking up the flocculation network of deposited calcium carbonates.As a result,the presence of detached calcium carbonate accelerated early cement hydration and densified the microstructure;this improved early-age compressive strength by~6%–32%,depending on the CO_(2)-mixing dose used.Therefore,it seems that the CO_(2)-mixing dose should be controlled at≤0.6%with the mixing time prolonged in order to attain satisfactory workability and mechanical strength.