The performance of a self-cleaning cool cementitious surface

Highlights

• Photocatalytic activity was tested using three TiO₂ anatase sources.

• Addition of 5% and 30% of TiO₂ in white cement paste was made.

• Dye degradation works better with 30% of TiO₂ than 5% for self-cleaning purposes.

• Self-cleaning by photocatalysis can maintain cool properties durability.

Abstract

Cool surfaces are designed to reflect more sunlight and absorb less heat. Environmental exposure can cause surface darkening. Microbiological growth and soiling can be partially controlled by periodic cleaning. A self-cleaning surface would be a suitable proposal for maintaining adequate reflectance.

The aim of this study is to obtain a cool, self-cleaning cement-based surface using TiO₂. Three samples of commercial TiO₂ (anatase) nanoparticles (P25, US NANO – IV and Millennium – TiONA) were mixed with white cement and water to produce a self-cleaning paste. The cement paste specimens were prepared by replacing 0%, 5% and 30% of the cement (by mass) by TiO₂.

The powder samples were characterized by X-ray fluorescence spectroscopy (XRF), and the photocatalytic efficiency of the specimens was tested using Congo Red dye degradation under UV and visible radiation. The loss of colour was measured by diffuse reflectance using a UV–vis spectrophotometer.

The cement specimens were characterized, before dye degradation, using UV–vis and Raman spectroscopy, and scanning electron microscopy to verify the presence of TiO₂ on the surface. The specimens’ characterization demonstrated high levels of TiO₂ on surfaces indicating conditions for photocatalytic reactions in case there was a presence of organism growth or deposits of atmospheric pollution.

Introduction

Increasing light reflectance and thermal emittance on the roof and façades of buildings can be a feasible strategy in the fight against global warming [1]. It can also help reduce urban heat islands, which can add up to 10 °C to maximum temperatures (Athens case) [2] thus, improving indoor and outdoor thermal comfort and reducing energy consumption. A cool roof can reduce indoor temperatures up to 3.3 °C [3], which can reduce the amount of energy used for cooling and even mean a better life quality for those who unable to afford air-conditioning. According to Giridharan et al. [4] a roof reflectivity of 0.6–0.7 is the optimum value to achieve energy savings in a cooled office and improve internal thermal conditions during summer. Optimum solar reflectance should be defined according to a building's use, insulation and climatic conditions [5]. A reported indoor temperature reduction of 2.3 °C, representing a 54% savings in cooling energy, used a double layer of milk and vinegar mixture which provided an 85.9% solar reflectance surface. A reduction in indoor temperatures was also reported by Pisello and Cotana [6] and reached a 4.7 °C decrease by using highly reflective, almost-white, clay tiles. Inorganic coatings contribute to energy efficiency and reach almost 20% in cooling reductions through the use of mineral based binders, aggregates and inorganic colour changing pigments [7].

Cool roofs and pavements constitute most of the horizontal surfaces of cities. The problem is to develop durable solutions. The cool properties are related to reflectance and emittance indexes. As roofs and pavements are exposed to natural environments they tend to darken with time. Darkening especially reduces the reflectance of surfaces, which compromises cooling properties [8]. Mainini et al. showed that cool roofs are most severely affected by weathering and soiling, particularly in polluted areas, reporting that roofing membranes having an initial high reflectance (0.80) lost 0.14 in Roma and 0.22 in Milano after two years of natural exposure.

TiO₂ anatase is an alternative to maintain cool properties in inorganic coatings. TiO₂ (anatase) has been of interest due to its capacity to generate functional surfaces. When excited by UV radiation, TiO₂ oxides pollutants [9], [10], [11], acts as a biocide [12] and generates self-cleaning inorganic surfaces [13]. It is clear that self-cleaning surfaces reduce the need for maintenance and, therefore, reduce costs; as a consequence, it is possible to lower the environmental impact produced by a building. Currently, there are commercial applications of TiO₂ as self-cleaning materials, including self-cleaning glass [14] and cement-based materials [10], [13], [15], [16].

The main goal of this study is to compare the photocatalytic eficiency of three TiO₂ nanoparticles embedded in a cool white cement matrix by measuring their capacity to degrade organic matter (Congo Red dye), which can be correlated to a reduced need for maintenance. The TiO₂ products used were P25 from Degussa, TiONA from Millennium and TIV from US NANO. P25 was used due to its common use in academic research on self-cleaning. The TiO₂ from Millennium and TiO₂ from US NANO were chosen for a comparative evaluation of the photocatalytic eficiency.

The experimental procedure was based on a previous work [13]. The laboratory execution was performed in two parts. The first part dealt with TiO₂ powder characterization, and the second part dealt with cement paste production (white cement + water + TiO₂), surface characterization and the evaluation of photocatalytic efficiency. UV–vis spectroscopy was used to measure the colour intensity changes due to dye degradation and to determine the best performance between the TiO₂ samples.