The control of alkali silica reaction using blended cements

It has been previously established that alkali silica reaction (ASR) in concrete may be controlled by blending Portland cement with suitable hydraulic or pozzolanic materials. The controlling mechanism has been attributed to the dilution of the cement's alkali content and reduced mobility of ions in concrete's pore solution. In this project an attempt has been made to identify the factors which influence the relative importance of each mechanism in the overall suppression of the reaction by the use of blended cements. The relationship between the pore solution alkalinity and ASR was explored by the use of expansive mortar bars submerged in alkaline solutions of varying concentration. This technique enabled the blended cement's control over expansion to be assessed at given `pore solution' alkali concentrations. It was established that the cement blend, the concentration and quantity of alkali present in the pore solution were the factors which determined the rate and extent of ASR. The release of alkalis into solution by Portland cements of various alkali content was studied by analysis of pore solution samples expressed from mature specimens. The specification for avoiding ASR by alkali limitation, both by alkali content of cement and the total quantity of alkali were considered. The effect on the pore solution alkalinity when a range of Portland cements were blended with various replacement materials was measured. It was found that the relationship between the type of replacement material, its alkali content and that of the cement were the factors which primarily determined the extent of the pore solution alkali dilution effect. It was confirmed that salts of alkali metals of the kinds found as common concrete contaminants were able to increase the pore solution hydroxyl ion concentration significantly. The increase was limited by the finite anion complexing ability of the cement.

Cathodic protection of reinforced concrete:anodic processes in cements and related electrolytes

The nature and kinetics of electrode reactions and processes occurring for four lightweight anode systems which have been utilised in reinforced concrete cathodic protection systems have been studied. The anodes investigated were flame sprayed zinc, conductive paint and two activated titanium meshes. The electrochemical properties of each material were investigated in rapidly stirred de-oxygenated electrolytes using anodic potentiodynamic polarisation. Conductive coating electrodes were formed on glass microscope slides, whilst mesh strands were immersed directly. Oxygen evolution occurred preferentially for both mesh anodes in saturated Ca (OH)2/CaC12 solutions but was severely inhibited in less alkaline solutions and significant current only passed in chloride solutions. The main reactions for conductive paint was based on oxygen evolution in all electrolytes, although chlorides increased the electrical activity. Self-corrosion of zinc was controlled by electrolyte composition and the experimental set-up, chlorides increasing the electrical activity. Impressed current cathodic protection was applied to 25 externally exposed concrete slabs over a period of 18 months to investigate anode degradation mechanisms at normal and high current densities. Specimen chloride content, curing and reinforcement depth were also variables. Several destructive and non-destructive methods for assessing the performance of anodes were evaluated including a site instrument for quantitative "instant-off- potential measurements. The impact of cathodic protection on the concrete substrate was determined for a number of specimens using appropriate methods. Anodic degradation rates were primarily influenced by current density, followed by cemendtious alkalinity, chloride levels and by current distribution. Degradation of cementitious overlays and conductive paint substrates proceeded by sequential neutralisation of cement phases, with some evidence of paint binder oxidation. Sprayed zinc progressively formed an insulating layer of hydroxide complexes, which underwent pitting_ attack in the presence of sufficient chlorides, whilst substrate degradation was minimal. Adhesion of all anode systems decreased with increasing current density. The influence of anode material on the ionic gradients which can develop during cathodic protection was investigated. A constant current was passed through saturated cement paste prisms containing calcium chloride to central cathodes via anodes applied or embedded at each end. Pore solution was obtained from successive cut paste slices for anion and cation analyses. Various experimental errors reduced the value of the results. Characteristic S-shaped profiles were not observed and chloride ion profiles were ambiguous. Mesh anode specimens were significantly more durable than the conductive coatings in the high humidity environment. Limited results suggested zinc ion migration to the cathode region. Electrical data from each investigation clearly indicated a decreasing order of anode efficiency by specific anode material.

Increasing the electrolyte capacity of alkaline Zn-air fuel cells by scavenging zincate with Ca(OH)2

The use of calcium hydroxide for scavenging zincate species is demonstrated to be a highly effective approach for increasing the electrolyte capacity and improving the performance of the zinc-air fuel cell system. A fundamental approach is established in this study to quantify the formation of calcium zincate as the product of scavenging and the amount of water compensation necessary for optimal performance. The good agreement between predicted and experimental results proves the validity of the proposed theoretical approach. By applying the results of theoretical predictions, both the electrolyte capacity and the cell longevity have been increased by more than 40%. It is also found that, using Ca(OH)2 to scavenge zincate species in concentrated KOH solutions, affects mostly the removal of zincate, rather than ZnO, from the electrolyte, whereas the presence of excess, free, mobile H2O plays a key role in dissolving ZnO and forming zincate. The results obtained in this study demonstrate that the proposed approach can widely and effectively be applied to all zinc-air cell systems during their discharge cycle.