Multi-functional aggregates for enhanced concrete performance and its application

Concrete materials are ubiquitous in the developed world due to their versatility and cost-effectiveness as a construction material, but their great potential for increased functionality remains underdeveloped. The prospect of 'multifunctional' concrete is not new; examples of reported research include photocatalytic and electrically conductive concretes and concretes with enhanced thermal and acoustic properties. The importance of the multifunctionality theme has driven some significant commercial interests, and some projects have even developed through to scaled demonstrations and commercial structures.
Concretes comprise a binder (the cement), aggregate (stone in different size fractions) and water. There may be additional additives to provide setting control. Conventional approaches to the incorporation of functional materials to concrete have typically focused on modification of or addition to the binder phase (the cement powder). Performance can be strongly influenced by poor dispersibility or occlusion of the active additive by cement hydration products. The novel approach addressed here is in the dispersion of the active additive (photocatalyst or electrically conducting material) on the aggregate.
Preliminary experiments have shown important improvements in performance when compared with conventional functionalizing approaches. Much of this can be attributed to the stabilization of active additive dispersion prior to mixing in the wet concrete. Spatial distribution on the aggregate is preserved during mixing and casting and, in some cases, porosity control of the aggregate can be expected to limit the degree of cement occlusion. For electrically conducting materials, continuity of contact throughout the matrix is important. By combining conductive aggregate with conventionally mixed conductive materials in the binder fraction, a more continuous, and therefore more conductive matrix can be expected.
The project brings together groups from the UK and China with complementary expertise in photocatalysis and materials chemistry and focuses on photocatalytic and electrically conductive concretes. Each has applications with potentially high social and economic impacts. A concrete that electrically senses itself for structural defects or can de-ice itself without the use of de-icing salts would have obvious advantages and environmental benefits, and its improved electromagnetic shielding characteristics could be important in enhancing information security. Both the UK and China has centres of high population density and air pollution. China is also rapidly developing its built infrastructure, offering a significant and growing concrete surface area in intimate contact with the polluted atmosphere and an excellent opportunity for the support of photocatalysts. These are effective in the removal of environmental pollutants, particularly in urban areas where vehicular traffic emissions (containing volatile organic carbons (VOCs) and oxides of nitrogen (NOx)) are known to have adverse effects on the health of the population.

The successful demonstration of multifunctional concretes through the novel use of additive-aggregate composites is likely to have significant impacts. Concrete structures contribute significantly to the total surface area of the built environment and represent the interface between polluted air and a potentially effective solution in the form of a photocatalytic concrete substrate. Even where photocatalysts are inefficiently distributed via cement binder, improvements in air quality have been demonstrated, particularly in congested urban areas where high levels of traffic congestion (vehicle emissions). Photocatalysts supported on aggregates for optimal dispersion and exposure, are likely to be considerably more efficient bringing significant further improvements in air quality with consequent benefits to public health. The supported catalyst composite may also find application in other environmentally significant themes, e.g. water quality (a global issue), gaseous effluent treatment, etc. Electrically conducting concretes, anticipated from the inclusion of conductive elements, are also expected to contribute social and environmental benefits should they prove effective de-icing substrates and have the capacity to be self-monitoring, e.g. for structural damage. The improved conductive composite structure, i.e. optimally distributed conductors in porous aggregate and in binder phase, offers potential cost reduction relative to alternative integrative approaches due to a lower conductive materials content requirement.

From the examples given, there is evidently considerable potential for an effective 'multifunctional' concrete to have significant impact on society. The impacts of concrete-based photocatalysts on air quality alone represent not only a better quality of life for the population but could lead to alleviating the financial burden on the health service. Local authorities and airports may be expected to find a reduction in the need and costs of de-icing, should the resistive heating of electrically conductive concretes prove effective; this would also reduce the environmental impact of de-icing salts on water courses and greatly reduce the corrosion of steel in affected reinforced concrete. The project addresses the impact of functional aggregate incorporation, ensuring that formulations are optimised to maximise functionality but to minimise disruption to the practical mixing and placing characteristics of concrete.

The implementation of a successful research programme might be expected to be relatively rapid; the project will engage with the relevant industry representatives in UK and China, via well publicised workshops, to ensure that the benefits of the research are disseminated effectively to those who can drive implementation forward. Concrete technology is a well established industry and technologists are constantly modifying formulations to accommodate different exposure conditions and customer requirements. Consequently, end users will also be targets of the dissemination exercise.