N.R Short

Diffusion of chloride ions in hardened cement pastes

Abstract Kinetics of diffusion of chloride ions in hardened cement pastes have been investigated. For Portland cement pastes of fixed type, made with various water-cement ratios (w/c), activation energies for the diffusion process have been measured and the results have been related to the pore structures of the materials. The influence of certain other factors, including the curing conditions, the presence in the samples of interfacial zones of segregation and the use of several other types of cement, have also been examined.

The influence of different cements on chloride-induced corrosion of reinforcing steel

Abstract Corrosion of embedded steel in concrete may occur as a result of the depassivating effects of chloride ions. Two important parameters governing the risk of chloride-induced corrosion in cement matrices of varied compositions are believed to be: (i) the relative concentrations of chloride and hydroxyl ions in the pore electrolyte and (ii) the diffusivities of chloride ions. Measurements of these parameters for cement pastes of constant water/cement ratio and fixed total chloride content have been used to rank a series of Portland cements, slag blended cement and fly-ash blended cement in terms of their expected levels of corrosion protection. The validity of the predicted rank orders has been independently assessed by electrochemical monitoring of the corrosion rates of embedded steel electrodes by means of the method of linear polarisation.

A static fatigue model for the durability of glass fibre reinforced cement

This paper presents a model for predicting service lives for glass-fibre reinforced cement (grc) components using hot-water accelerated ageing. It improves on previous models, being derived from consideration of a specific proposed micro-mechanical strength loss mechanism based on static fatigue principles and can be applied from time = 0. The model fitted well to all available strength vs. time data pertaining to various grc formulations. The activation energies thus derived for the strength loss process (80–90 kJ mol−1) were consistent with those derived previously and those proposed for general glass dissolution mechanisms. Updated acceleration factors for predicting service lives for grc are advanced. The model was also applied to grc made with modified cement matrices. For metakaolin modified matrices, the activation energy appeared similar to that for OPC-grc, thus the use of similar acceleration factors appears justified. There is some evidence that calcium sulphoaluminate modified grc degrades according to a different activation energy. More data are required for modified matrix grcs if the model is to be applied thereto with confidence.

Pore solution chemistry of the hydrated system tricalcium silicate/sodium chloride/water

The relative tendency of different Portland cements to remove chloride ions from concrete mix water by forming insoluble complexes is an important determinant of the corrosion behaviourod steel in concrete. Whilst the C3A phase plays a dominant role in binding chloride ions, other cement minerals may be of secondary importance but their effects are not well established. The reported investigation is an attempt to elucidate the extent to which chloride binding occurs within the hydration products of the C3S (alite) phase of Portland cement when sodium chloride is present in the mix water.

Preliminary investigations into the supercritical carbonation of cement pastes

Interactions between supercritical carbon dioxide (scCO2) and hydrated cement pastes, of various water/cement ratio, have been investigated. The carbonation process was greatly accelerated in the scCO2 compared to that in natural or CO2 enriched environments. The nature of the reactions was dependent on the amount of water present in the paste. Thus carbonation of samples dried prior to treatment resulted in the reaction of all the unhydrated C3S and C2S, but little conversion of calcium hydroxide to calcium carbonate. In contrast, carbonation of samples containing moisture resulted in the conversion of most of the calcium hydroxide whilst the amounts of C3S and C2S reacted increased as the water/cement ratio increased. During the carbonation treatment, the pore structure of the cement pastes was altered and substantial reductions in porosity were achieved. The process may be used to improve the durability of glass fibre reinforced cement by lowering the alkalinity and calcium hydroxide content of the matrix.

Accelerated ageing characteristics of glass-fibre reinforced cement made with new cementitious matrices

Glass-fibre reinforced cement composites have been made using new types of cement matrices and subjected to hot water immersion ageing regimes. The matrices were characterised by pore solution expression and analysis, XRD and DTA/TGA. Composite specimens were subjected to direct tensile tests. The new matrices displayed substantially reduced pore solution alkalinity and calcium hydroxide precipitation when compared with ordinary Portland cement (OPC), thus providing a less aggressive environment for the fibres. Mechanical tests showed that the rate of degradation of composites made from the new matrices, when aged at 65°C in water, was an order of magnitude lower than that for OPC matrix composites. The validity of this type of accelerated test is discussed.

Microstructural observations in new matrix glass fibre reinforced cement

Glass fibre reinforced cement (GRC) durability is generally thought to be governed by weakening of the reinforcement by the alkaline matrix and/or precipitation of hydration products, particularly calcium hydroxide (CH), within and around the strands. Previous microstructural investigations of GRC have exclusively used scanning electron microscopy (SEM). This paper is concerned with the application of thin-section petrography (TSP) to GRC. TSP allows identification of CH and other phases associated with the strands and unlike SEM, sample preparation causes minimal interfacial disturbance. Three matrices were studied; OPC, OPC plus metakaolin and OPC plus calcium sulphoaluminate cement. It was found that the degree of composite degradation was unrelated to the amount of CH or other hydration products precipitated within the strands or at the interface. No significant loss of fibre section was observed in degraded composites. It is postulated that gradual enlargement of pre-existing fibre flaws causes strength loss in GRC.

Super-critical carbonation of glass-fibre reinforced cement. Part 1: mechanical testing and chemical analysis

Abstract The deterioration of glass-fibre reinforced cement (GRC) arises to a substantial extent from the alkalinity and calcium hydroxide content of the matrix. Carbonation of the matrix significantly lowers both factors, but under normal circumstances the reaction proceeds too slowly to be of practical use in improving durability. If carbonation is effected using supercritical carbon dioxide the reaction can be completed within hours rather than years, rendering it potentially attractive as a treatment for enhancing the durability of GRC. The efficacy of such treatment is dependent on the moisture content of the samples prior to treatment. GRC samples thus treated, with various moisture contents, were mechanically tested before and after a period of accelerated ageing. The supercritical carbonation treatment significantly increased the design strength and toughness of the GRC and greatly increased the fibre–matrix bond. Retention of toughness and degree of carbonation were both correlated with pre-treatment moisture content while initial property enhancements were not.

Determination of bond strength in glass fibre reinforced cement using petrography and image analysis

The reinforcement in glass fibre reinforced cement (grc) is not present as discrete fibres, but as ‘strands’ of about 200 filaments each. This configuration greatly complicates determination of the ‘perimeter’ of the reinforcing elements, a crucial parameter in bond strength determination. Previous investigators have attempted to quantify strand perimeters but their methods have always involved at least one subjective step and are prone to operator bias. This paper describes an objective method of determining strand perimeters using digital analysis of images captured from petrological thin sections. The measured perimeters were found to be sensitive to the “threshold value” chosen for the analysis, and consideration of perimeter vs. threshold value curves eliminated subjectivity involved in the analysis. The perimeter value obtained, together with microscopic analysis of the crack patterns produced during tensile testing, were used for calculating bond parameters for different cement matrices. The development of bond with both ageing time and temperature were also studied. The method uses fundamental image analysis concepts and as such is readily adaptable to solve conceptually similar problems in a wide range of materials.

Super-critical carbonation of glass-fibre reinforced cement. Part 2: Microstructural observations

The deterioration of glass-fibre reinforced cement/concrete (grc) arises to a substantial extent from the alkalinity and calcium hydroxide content of the matrix. Carbonation of the matrix significantly lowers both factors, but under normal circumstances the reaction proceeds too slowly to be of practical use in improving durability. If carbonation is effected using super-critical carbon dioxide the reaction can be completed within hours rather than years, rendering it potentially attractive as a treatment for enhancing the durability of grc. The efficacy of such treatment is dependent on the moisture content of the samples prior to treatment expressed in terms of a degree of drying (DOD), as established in Part 1 of this paper. Grc samples thus treated, were examined using petrography, scanning electron microscopy and mercury intrusion porosimetry before and after a period of accelerated ageing. Treatment transforms the matrix into a groundmass of amorphous material intimately mixed with microcrystalline calcium carbonate. Dark inclusions appear to be remnants of unhydrated cement particles. The mechanical property enhancements detailed in Part 1 are attributed to the precipitation of calcium carbonate reducing porosity. The nature of the carbonation reaction was found to be dependent on DOD. In samples indicated by differential thermal analysis to be completely carbonated, some uncarbonated unhydrated cement and uncarbonated amorphous groundmass remained in isolated patches.