Redispersible polymer powder modified cementitious tile adhesive as an alternative to ordinary cement-sand grout

With the advent of technologies on modern structural and building materials, adaptation of such technological features has been the pivotal concern of researchers. Ceramic tiles attained a distinctive focus due to its decorative feature for both indoor and outdoor conditions and also for their ease of implementation. The purpose of exploiting ceramic tiles is not only to impart structural strength but also the aesthetical characteristics that seem to matter a great deal. When it comes to the adhesion of these tiles onto the substrate wall, traditional method exerts the use of sand-cement grout. This faces some drawbacks including poor water retention property, hard and brittleness of the surface, much higher drying time, no flexibility, higher thickness of the paste and so on. These difficulties can be overcome by the addition of redispersible polymer powder (RPP) along with other cementitious constituents. The blended polymers interact with cement components to improve the physical and mechanical properties such as increased adhesion strength, reduced shrinkage and lower water absorption. This review article made an effort to provide the generalized idea about the cementitious tile adhesive (CTA) and its components. Focus was made onto the commercially available RPP and formulation of CTA with the inclusion of RPP. Critical analysis of the repercussions of RPP fortification was also carried out based on different researcher's findings.

1. Introduction

Being the oldest form of decorative art, ceramic tiles, from its inception to evolution through ages, is laden with history, from ancient Egyptian artifacts to modern household decoration. Their enthralling beauty and structural permanence made them treasured for centuries. The oldest evidence of ceramic tiles usage was dated from 4700BC in Egypt; subsequently, Romans and Greeks also started decoration with ceramic tiles. During the Islamic period, decorative ceramic tiles were frequent in mosques. Decorative tiles got admiration at Europe during the middle age and it was the industrial revolution of Britain which made the mass production possible as well as the affordability for the middle class people [1].

External cladding of tiles, also known as tiling system [2] has become the most picked construction choices not only for its aesthetic purposes but also for the patronage of wall from aggressive environmental conditions [3]. Ceramic tiles are the most adopted tiles around the world due to its versatility [2] and affordability [4]. The ceramic tiling system comprises of three interactive layers; the substrate layer, the tile layer and most importantly the adhesive layer [3, 5]. Adhesives refer to the substance, which has the ability of holding minimum two surfaces substantially in an indissoluble way. Adhesives that are used in structural purposes must possess high shear strength and a very strong resistance from aggressive environment [6].

Traditionally for the external cladding of ceramic tiles, a cheaper form of adhesive which is nothing but a mixture of cement and sand, is being used widely and much popular in Indian subcontinent. Traditional grout preparation involves mixing of ordinary portland cement and sand with water and the application of this grout is done by following the thick bed method, where the grout bed occupies 10–25mm of thickness from the base to the adherent [7]. This is a very time consuming process and requires much effort. Application of a polymer modified thin bed tile adhesive is a possible solution to all the problems raised by traditional cement-sand grout [8].

Polymer based adhesives are introduced to cement as a modifier for the betterment of adhesion, strength, waterproofing, durability, flexibility and deformity. Polymer modification of cement grout or paste for tiling and other purposes is not a contemporary conception; rather it had been performed since 1923. In 1924, the first patent regarding polymer modification in cement mortar was issued. Since then, polymer modified cement grout got the implementation in construction arenas for providing a decent performance compared to that of the ordinary cement-sand grout and eventually got the popularity [9, 10].

The basic principle of polymer modification involves the mixing of polymer or monomer in powdered or liquid form along with cement and other admixtures followed by curing. In case of monomers being used, in-situ polymerization of that monomer is required. The polymers or monomers with which polymer modification can be done are of four major types ( ), i.e., RPP, polymer latex, water soluble polymer and liquid polymer [9].

RPPs are simply the spray-dried powders which when mixed with water, produce stable dispersion just like the original dispersion and this is governed by the fact that it redisperse only once [11]. Originally, RPPs are synthesized from latex dispersions and this involves a two-step synthesis process. The first step includes preparation of polymer latexes via emulsion polymerization and the second step being the spray drying of this emulsion into fine powder. Prior to the spray drying, various admixtures are added to the formulation such as anti-foaming agents, accelerators, water retention agents etc. for improving the performance of the polymer. To prevent the cake formation of the polymer powder during storage, anti-blocking aids such as clay, silica and CaCO3 are added before or after the spray drying [9].

Polymer latexes or polymer dispersions are synthesized just like RPP by emulsion polymerization. Polymer latexes are characterized by the fact that they are of very small particles, typically possesses 0.05–5μm in diameter. Common examples of synthetic latexes include vinyl acetate, homo- and co-polymer latexes, acrylic copolymer latex, styrene butadiene copolymer latex etc. Due to chloride ion liberation and unsatisfactory resistance, polyvinyl acetate and poly (vinylidene chloride-vinyl chloride) latexes are not advised. In case of natural rubber and epoxy latexes, they are not compatible to be synthesized by emulsion polymerization, rather they are acquired naturally by tapping from the rubber trees, which is then concentrated and finally a solid mass is obtained. Cellulose derivatives, polyvinyl alcohol, polyacrylamide etc. are examples of water soluble polymer which are mixed with cement in powdered form with the expectation of preventing the dry out occurrence and to improve the serviceableness. This is due to the increasing viscosity of water by dissolving the polymer and the sealing property is exerted by the formation of a thin film. Overall, the modification with water soluble polymer doesn't contribute substantially to the strength of the system. Liquid polymers find the least of utilization due to its handling and storage difficulties. They are mainly epoxy and unsaturated polyester resins which are applied along with hardener or catalyst and accelerators [12].

CTA modified with RPP has got the special consideration because of its beneficial aspects. This whole concept is justified by the fact that, the hydration of cement and formation of polymer film occurs accordingly which begets a network structure of monolithic matrix phase through which the hydrated cement phase and polymer phase interpenetrate. Modification with RPP is akin to latex modification but with the advantage of the redispersion property [13]. Some of the features delivered by RPP are improved workability, improved adhesion, increased flexural strength, increased plasticity, improved abrasion resistance, improved water retention, reduced water absorption, increased viscosity and cohesion etc. Inclusion of RPP in the cement grout caused the formation of a film [14] as water evaporates and this acts as a binder. This polymer film interconnects all the cement particles and fillers [15]. Moreover, this is more suitable for household tiling than industrial or special type tiling and the low price for a high outcome is the pivotal reason for its popularity.

This article will give an introductory idea about the components that are being used in making ceramic tile adhesive, especially the inclusion of RPP.