ICE - Institution of Civil Engineers - Virtual Library

The durability of cement-based materials with respect to exterior aggressions is one of the current priorities in civil engineering. Depending on their use, the cement-based materials can be exposed to different types of aggressive environments. For instance, damage to concrete structures in contact with a sulfate and saline environment are of most importance. Belite cements have excellent durability against determined expansive attacks and lower heat of hydration than that of Portland cements. These properties convert belite cements into good candidates for the immobilisation of low-level liquid waste and medium-level liquid waste. In this work, the authors present a long-term durability study at 40°C of a kind of belite cement from coal fly ash class C as alternative raw material. The aggressive was a 0·5 mol/l sodium sulfate (Na₂SO₄) solution, which is common in some evaporator concentrate solutions of medium-level liquid waste and low-level liquid waste. The attack was evaluated by means of flexural strength changes of mortars over 180 days. Furthermore, the changes in microstructure were followed by mercury intrusion porosimetry and mineralogy by X-ray diffraction.

To examine the feasibility of a possible application of a steel slag that is locally available in large quantities, for the preparation of steel slag composite cement (SSCC), the effects of the granularity of the steel slag and of varying the amounts of clinker, steel slag, and activators on the properties of SSCC were investigated. An optimal composition, which generated a cement that was comparable with 42·5R cement, was obtained. The hydration products of the SSCC and micro-structure of hardened paste were analysed by X-ray diffraction and scanning electron microscopy. The results show that the 3 day compressive strength of SSCC was increased by 119·7% by the addition of a compound activator. The large amount of hydration products generated in the early hydration period contributed to the significant increase in the early strength of the prepared SSCC.

The relationship between stress and strain for polymer concrete under compression was analysed, and the effects of resin content, silane treatment, curing age, and temperature were investigated. These properties are necessary to gain a better understanding of the mechanical behaviour of polymer concrete elements. The polymer concrete cured at an elevated temperature showed a higher rate of compressive strength and elastic modulus gain than that cured at room temperature (20°C). For cylinders that were cured at an elevated temperature (70°C) and silane-treated, both the ascending and descending branches of the compressive stress-strain1 curve became steeper and more linear. A stress-strain model was developed to represent the stress-strain relationship of polymer concrete under compression. This model fitted the experimental data remarkably well and accurately reflected the descending branch of the stress-strain curve and the declining trend in ductility.

A non-contacting electrical resistivity measurement device was used for measuring the electrical resistivity of early hydration samples of sulfoaluminate cement, a type of rapid-hardening cement. The early hydration process was monitored and the results show that the main early hydration of sulfoaluminate cement was accomplished within the initial 6 h. With the addition of the accelerator to improve the early strength, the hydration process was accelerated as demonstrated by the resistivity—time curve. The agent performed its role in the initial 15 min and reduced the electrical resistivity. The experimental results obtained by means of X-ray diffraction and the hydrated heat instrument were assessed to elucidate the relationship between the hydration process and the electrical resistivity of sulfoaluminate cement.

The transition of the Friedel phase to the chromate-AFm phase in chromate solution and the thermal stability of the products was investigated using thermal analysis combined with mass spectrometry, X-ray diffraction and Fourier transform infrared spectrometer. The results indicated that Cr O 4 2− easily substituted for Cl in the Friedel phase forming chromate-AFm. The chemical formula of chromate-AFm was characterised as 3CaO·Al₂O₃·CaCrO₄·12H₂O. Chromate-AFm gave two steps of dehydration, two steps of dehydroxylation and one step of redox reaction of Cr(VI) in the whole heating process from 40 to 1000°C. The Fourier transform infrared spectra showed different OH-metal binding ways for the Freidel phase and chromate-AFm.

This paper deals mainly with the application of a microwave heat curing method to ultra-high strength and performance concrete specimens made up of combinations of micro and nanoscale pozzolans. The application of this curing method enables very high early strength, as high as 420 MPa to be obtained after 1 day by a total microwave curing duration of 2 h. In addition it was observed that the achieved strength depended on the kind of nanoscale pozzolan (oxide) used and on the microwave energy, time of treatment and treatment duration. In comparison with the oven curing method this method has the advantages of much shorter curing times and better curing efficiency. The microwave curing of ultra-high performance concrete seems to be very advantageous for applications where extreme concrete strength in very short time is required.