Lightweight pozzolanic materials used in mortars: Evaluation of their influence on density, mechanical strength and water absorption
Introduction
Lightweight aggregates are used in mortars to replace or combine with cement. By using these aggregates, such diverse properties as weight reduction, thermal insulation and fire resistance or workability may be improved. Thus, many lightweight materials (LWM) such as expanded perlite, expanded glass, hollow micro-spheres and expanded polystyrene are used without replacing cement, to improve these characteristics. Some authors [1], [2] have reported on the interfacial transition zone (ITZ) between lightweight aggregates and the paste in an attempt to understand their influence on the micro-structure and strength of concretes. Other researchers have reported on the positive influence of lightweight aggregates on mortar durability. The paste in contact with the lightweight aggregate seems to be denser than the paste located further away, which seems to be due to the water absorbed by the lightweight aggregate [3], which reduces the water:cement (w:c) ratio of the surrounding paste.
Many authors have also reported on natural and artificial pozzolanic materials such as metakaolin, silica fume and volcanic pumice [4], [5]. Expanded perlite, expanded glass and hollow micro-spheres (Fig. 1) are some lightweight materials that, because of their composition based on silico-aluminates, are potentially able to give pozzolanic reactions [6]. They can therefore, provide additional advantages in mortars by improving their mechanical strength. On the other hand, most LWM leads give low density pastes and high w:c ratios, which has the opposite effect on mechanical strength. Additionally, expanded polystyrene, is not pozzolanic but it is potentially more hydrophobic than expanded perlite, expanded glass or hollow micro-spheres, which could be useful in the prevention of the water absorption [7].
The goal of this paper is to investigate and compare some LWM and their effect on rendering mortars formulations. There are noticeable differences and similarities between the lightweight aggregates used in mortars. For example, both expanded perlite and polystyrene in mortars improve density and are able to increase the render covering capacity (surface covered). However, sometimes they may behave quite differently when exposed to similar environments. While the former remains inalterable to organic pollutants, heat or most atmospheric agents, the latter collapses due to heat and reacts with some organic compounds. However, the expanded polystyrene granules are flexible and hydrophobic, whereas perlite is fragile, rigid and water absorbent.
Expanded glass is a very realistic alternative to expanded perlite. Like perlite, the material is produced during expanding processes in rotating furnaces at 900 °C from powdered glass (wastes). The process yields rigid and round granules that are less water absorbent than perlite, entrapping minute air chambers inside. Despite their similarities, expanded glass is less fragile and less water absorbent than expanded perlite and so may be a good alternative to using perlite, partially avoiding some negative effects like high sorptivity and low mechanical strength.
Hollow micro-spheres are lightweight, low density aluminosilicates extracted from pulverised fuel ash from coal-fired power stations that are potentially very effective as pozzolanic agents. There are some similarities between expanded glass and hollow micro-spheres, for instance, both aggregates have a rounded geometry, their water absorption is not so high as perlite and their densities are similar. However, whereas expanded glass is richer in silica and sodium oxide, the percentage of alumina is higher in hollow micro-spheres.
Section snippets
Mix constituents and materials
Expanded perlite (EP), expanded glass (EG), hollow micro-spheres (HS) and expanded polystyrene (EPS) were the LWM selected to study their influence on renders. The SEM micrographs of the pozzolanic lightweight aggregates are shown in Fig. 1. The additions were made to simulate, as closely as possible, a typical render formulation. Some data about the LWM composition and their physical properties are shown in Table 1. Clean drinking water was used for all mixes.
Density and fluency
After mixing the mortars, some density tests were made to evaluate the influence of each LWM. The data were in accordance with the LWM studied, that is, fresh density was lower when expanded polystyrene and expanded perlite were used. Expanded glass and hollow micro-sphere samples showed similar values. Therefore, for the same dosages, the covering capacity should be enhanced by using expanded polystyrene followed by perlite. The other LWM should also increase the covering capacity, although to
Conclusions
- 1.
The studied lightweight materials: expanded perlite, expanded glass, hollow micro-spheres and expanded polystyrene are useful to improve the workability and covering capacity of mortars, which are important features in renders and single coat mortars.
- 2.
Expanded polystyrene and expanded perlite may excessively reduce some mechanical characteristics of the lightweight mortars, since density seems to strongly influence their mechanical strengths. Except the highest dosages of expanded glass and
Acknowledgments
The authors express their thanks to the CDTI Centre for the Development of Industrial Technology under the Spanish Ministry of Science and Technology.
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