Xiangming Kong

Study on the rheological properties of Portland cement pastes with polycarboxylate superplasticizers

The coupling effects of temperature and time on the fluidity of fresh cement mixtures were investigated. Mini-cone tests on cement mortars and rheological tests on cement pastes under different temperatures (0 to 60 °C) were conducted to characterize the development of the fluidity of fresh cement mixtures over time. In addition, total organic carbon tests were performed to quantify the adsorption amount of superplasticizers on the cement surface. The amount of free water in cement pastes was determined via centrifugation. Isothermal calorimetry was employed to characterize the hydration kinetics of cement under different temperatures. Results show that the spread diameter of mortars decreases in a roughly linear fashion over elapsed time. Higher temperature facilitates a sharper decrease in fluidity with time, although the initial fluidity of fresh mortars is not significantly affected by temperature. Higher temperature results in a greater amount of adsorbed polycarboxylate ester/ether on the cement surface and a lower amount of free water in fresh cement pastes, which is believed to result from the higher hydration rate of cement. The evolution of rheological properties over time can be attributed to the development of hydration degree. Relative hydration degree is introduced to indicate the development of rheological properties with time. Two models to describe the evolution of yield stress and plastic viscosity for fresh cement pastes were developed.

Rheological Behaviors of Fresh Cement Pastes with Polycarboxylate Superplasticizer

The rheological behaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated. Influential factors including superplasticizer to cement ratio (Sp/C), water to cement ratio (w/c), temperature, and time were discussed. Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0, 20 and 40 °C, respectively. Flowability and rheological tests on cement pastes were conducted to characterize the development of the rheological behavior of fresh cement pastes over time. The exprimental results indicate that the initial flowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer. Higher temperature usually leads to a sharper drop in initial flowability and flowability retention. However, for the cement paste with high Sp/C or w/c, the flowability is slightly affected by temperature. Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influential factors at low Sp/C. In the case of high Sp/C, yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature. Moreover, two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C, w/c, temperature and time are developed on the basis of the existing models, in which experimental constants can be extracted from a database created by the rheological test results.

Mechanical properties of silica aerogels prepared from a mixture of TEOS and organo-alkoxysilanes of type R1SiX3

Silica aerogels were prepared from a mixture of tetraethylorthosilicate and organoalkoxysilanes. The effects of organo-alkoxysilanes on the mechanical properties of the silica aerogels were studied. The flexibility of silica aerogels was significantly improved by incorporation of organo-alkoxysilanes. When MTES and TEOS were combined as precursors of silica areogels, with the increased amount of MTES, the apparent elastic modulus and apparent compressive strength monotonously rose. At the same organoalkoxysilanes to TEOS ratio, the size of alkyl groups of the organo-alkoxysilanes had little effect on the mechanical properties. In series of MTES and TEOS, the lowest elastic modulus of silica skeleton and the highest compressive strength of silica skeleton were observed at MTES to TEOS ratio of around 50:50. At a certain organo-alkoxysilanes to TEOS ratio, the elastic modulus of silica skeleton increased and the compressive strength of silica skeleton decreased with the size increase of the alkyl groups.