Masonry and render mortars with tyre rubber as aggregate: Fresh state rheology and hardened state performances

Highlights

• Recycled Tyre rubber is mixed with sand as aggregate for masonry/render mortar.

• Mixing has been designed in order to not vary granulometry of the mixtures.

• With 20% rubber water penetration is minimized and strengths slightly depleted.

• Fresh state workability and thixotropy decrease as rubber content increases.

Abstract

This work shows how it is possible to tailor cement-rubber composites suitable as renders and masonry mortars with a reduced liquid water absorption. Recycled tyre rubber is used as aggregate in mixture with siliceous sand and its content is tuned in order to limit depletion of mechanical properties and, at the same time, ensure a sufficiently massive utilization of this secondary raw material. An extended investigation of the properties both at the fresh and hardened state is conducted. We show that presence of rubber sensitively reduces fluidity (hence workability) of the mortar. In particular, rubber modifies rheological properties such as thixotropy and dynamic torque, while does not affect viscosity. Overall, the fresh state properties are compatible with the casting and application of these mortar typologies, even for the highest rubber content (20–25% by volume). This rubber content is shown to be sufficient to ensure, just after few days of curing, an almost complete abatement of water drop absorption, specific functionality of these novel and more sustainable mortars.

Introduction

Tyre rubber is characterized by several features, which are beneficial during its on-road life such as resistance to mould, heat humidity, bacterial development, ultraviolet rays, some oils and many chemicals. These features become drawbacks in the phase of management of this material as a waste, boosting therefore, the development of new valorization solutions alternative to energy recovery [1].

One of the recycling routes involves the so called “granulate recovery” which includes tyre shredding and chipping, by which tyres are cut into small pieces of different sizes (shreds: 460–25 mm; chips: 76–13 mm; crumb rubber: 5–0.1 mm). After the removal of the steel and fabric components, the recycled tyre rubber (RTR) can be used for a variety of construction applications such as, i.e., soft flooring for playgrounds and sports stadiums, modifier in asphalt paving mixtures or additive/aggregate to cement concrete. Among these, the addition (as crumb rubber) to asphalt mixtures is highly diffused due to the good chemical interaction [1], [2].

The RTR as aggregate in cement structures has been proposed in various papers but it is still less attractive compared to applications in asphalt pavements. An important reason is the not favorable interaction with the matrix. Indeed, the cement paste is mainly characterized by hydrated metal /semimetal oxides, which make such a matrix hydrophilic (high surface energy) and well adhering to the conventional aggregates, generally based on quartz and/or limestone. Rubber, instead, made of organic polymers, is characterized by a low surface energy, and therefore, by a hydrophobic character. The interaction hydrophilic-hydrophobic is very unfavorable resulting in a poor adhesion between rubber particles and the cement matrix, which results in low compressive strengths of these composites [2], [3], [4], [5], [6], [7].

On the other hand, it has been recently shown that the low surface energy of the rubber particles represents also an advantage since it inhibits the absorption of water in rubber containing cement composites [4], [8].

This is a relevant feature since hydrophobic cement structures have: i) higher resistance to aggressive agents conveyed by water [9]; ii) longer durability upon freezing-thawing cycles; iii) self-cleaning ability; iv) resistance to paints/graffiti; may result ice-phobic [10], [11].

In particular, in our previous research, it was demonstrated the possibility of completely abating water drop penetration in cement composites by a full replacing (100% volume) of the natural aggregate (siliceous standard sand). An important aspect, not found in previous literature, is that this performance was proved both on the surface and in the bulk of the composite, i.e. by testing the property also on their fracture surface [4]. However, in that preliminary study a sensitive reduction of the compressive strength was observed in rubber containing composites compared to the standard one, in agreement with what previously found in literature and above explained [2], [3], [5], [6], [7].

Hence, we have addressed this work to investigate the feasibility of using recycled tyre rubber in a specific field of use, such as plasters, renders or masonry mortars, for which no strict restrictions for strength exist, characterized by a hydrophobic behavior.

At this purpose, in combination with a masonry cement, sand-rubber aggregate mixtures were used with a finely tuned rubber volume, up to a maximum of 25%, mixed together in such a way to not vary the granulometric curve of the full aggregate. This approach has been followed in order to preserve as much as possible the mechanical strength typically provided by the particle size distribution of sand aggregate mortars and, at the same time, to tailor a product where the use of the rubber can be massive, hence attractive for the recycling tyre plants.

According to literature [12], [13], [14], coatings and renders are usually applied in several layers, and consequently, each layer requires different features in terms of composition, thickness and dosages, depending on its position (base or surface). These kind of applications usually involves aggregates with small particle size as well as for masonry mortars, so in this research, aggregates with a maximum size of 2 mm were used, both RTR and sand.

In addition, properties of these mortars are relevant to the specific function they are designed. Unlike concrete, the strength is often not the main factor for consideration in plaster and masonry mortar, whereas in most practical scenarios, workability and water tightness are some of their most important properties. Articles dealing with the use of rubber in cement composites, probably due to the focus on structural applications of most of them, often reports detailed mechanical characterization while a lack of information exists about rheology and water tightness. When fresh state properties are taken into account, often these are limited to consistency and setting time [6], [15], [16], [17], [18], [19].

That is why, in order to fully characterize the material for applications as masonry and render mortars, we have performed a characterization of the fresh and hardened state behavior to determine how rubber inclusion affect: i) workability and rheological properties, such as yield stress and viscosity; ii) density and mechanical strengths and iii) wettability and water drops absorption.