C.L. Page

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.

EFFECTS OF CARBONATION ON PORE STRUCTURE AND DIFFUSIONAL PROPERTIES OF HYDRATED CEMENT PASTES

Steady-state diffusion kinetics of dissolved oxygen and chloride ions were studied in well-cured, partially dried, non-carbonated and fully carbonated specimens of OPC, OPC/30% PFA and OPC/65% BFS pastes. Pore size distribution, total porosity and coarse capillary porosity data for the specimens were also determined. The diffusion resistances of all the materials were adversely affected by pre-drying and carbonation, but the effects were considerably more severe for the blended cements than for the OPC. This was associated with coarsening of the pore structures of the various pastes to differing extents as a result of pre-drying and carbonation. Diffusion rates of both chloride and oxygen in all three carbonated materials appeared to be controlled by a common, purely physical mechanism.

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.

Aspects of the pore solution chemistry of hydrated cement pastes containing metakaolin

The hydroxide ion concentrations of expressed pore solutions of Ordinary Portland cement pastes containing 0, 10 and 20% of metakaolin by weight of cement were determined at various stages of hydration. Effects produced by two types of metakaolin, differing in degree of purity, were compared and, in each case, incorporation of metakaolin into the pastes was found to cause a long-term reduction in pore solution hydroxide ion concentration. The presence of metakaolin was also found to result in an increase in the extent to which chloride ions, introduced via the mix water, were excluded from the pore solution phase. The significance of these results in relation to the ability of hydrated cement matrices containing metakaolin to provide corrosion protection to embedded steel is considered.

Diffusion in cementitious materials: II, further investigations of chloride and oxygen diffusion in well-cured OPC and OPC/30%PFA pastes

Abstract Steady-state diffusion of dissolved oxygen and chloride ions in hydrated OPC and OPC/30%PFA pastes, hydrated for 2 weeks at 20 °C and 10 weeks at 38 °C, was studied at water/binder ( w s ) ratios 0.4, 0.5, 0.6 and 0.7. Total porosity and a simple measure of capillary porosity, the volume fractions of the water lost in specimens from a saturated surface dry condition to a near-constant weight at 90.7% relative humidity, were also determined. The diffusion rate of chloride ions diminished markedly, to very low values, as the capillary porosity approached zero. For a given w s ratio or capillary porosity the chloride ion diffusion coefficient for OPC/30%PFA pastes was about one order of magnitude smaller than that for OPC pastes. The rate of diffusion of dissolved oxygen also diminished as the capillary porosity reduced but it was still significant as the capillary porosity approached zero. For a given capillary porosity the oxygen diffusion coefficient for OPC/30%PFA pastes was about 30% smaller than that for OPC pastes. The results support the view that chloride ion diffusion in pastes of low capillary porosity is retarded by the surface charge of the hydrated cement gel. In contrast, the hydrated cement gel is much more permeable to the similarly-sized, neutral oxygen molecule.

Steel/mortar interfaces: Microstructural features and mode of failure

Abstract The interfacial zone of segregation formed between mild steel and Portland cement pastes or mortars has been studied by scanning electron microscopy. It consists of a discontinous layer of polycrystalline portlandite which varies in thickness and contains inclusions of CSH gel. Tensile fracture in specimens of this sort occurs largely at the interface but, for similar test pieces produced from other types of metal, different failure characteristics and bond strengths are observed.

The resistivity of mortars immersed in sea-water

Abstract The resistivities of specimens of five different mortars were monitored during 18 weeks of exposure to sea-water. All specimens exhibited an increase in resistivity, 25mm thick specimens of the two more permeable mixes showing an increase equivalent to over 20mm of additional thickness after only 10 weeks of sea-water exposure. Measurement of pore solution resistivity, mercury intrusion porosimetry and surface examination and analysis were used to determine the mechanisms producing an increase in resistivity. Two distinct mechanisms were isolated, viz the formation of a discrete aragonite/brucite layer on the surface of the specimens and a more widespread bulk effect associated with a modification of the cement paste pore structure.

Surface microstructure and abrasion resistance of concrete

Abstract The influence of several surface finishing techniques on the abrasion resistance of concrete specimens of various mix compositions has been measured by means of a rolling-wheel apparatus. Microstructural features of the cement matrix components of the exposed surfaces have been studied by mercury intrusion porosimetry and microhardness determinations. The results indicate that different methods of surface finishing produce substantially different microstructural characteristics within a surface zone of a few mm maximum thickness. For the systems investigated, it was found that the abrasion resistance was determined largely by the pore structure of this surface zone.

Diffusion in cementitious materials: 1. Comparative study of chloride and oxygen diffusion in hydrated cement pastes

Abstract Steady state diffusion of dissolved oxygen and chloride ions in hydrated OPC and OPC/20%PFA pastes was studied. The effective diffusion coefficients of oxygen were determined by an electrochemical method involving cathodic consumption of the diffused oxygen, while those for chloride ions were determined from conventional steady state diffusion experiments. It was found that, in comparison with oxygen diffusion, chloride diffusion is relatively retarded and that the retardation increases with decrease of water/binder (w/s) ratio. The experimental findings support the view that ionic diffusion kinetics are influence by the surface charge of the micropore walls.

Aspects of the pore solution chemistry of blended cements related to the control of alkali silica reaction

Abstract The effects of four pulverised fuel ashes (PFA) and three ground blastfurnace slags (GBFS) on the alkalinity of the pore solution phase of hardened cement pastes has been studied. It has been found that the total alkali content of a PFA is an important factor, but not the only one, determining its effectiveness in reducing the hydroxyl ion concentration of the pore solution. For GBFS, however, there was found to be no direct correlation between the total alkali content of the slag and the composition of the pore solution. Implications regarding the roles of PFA and GBFS in reducing expansion associated with alkali silica reaction (ASR) are discussed.