Elsevier

Construction and Building Materials

Volume 217, 30 August 2019, Pages 363-371
Construction and Building Materials

Understanding the water absorption from MHEC modified glue mortar into porous tile: Influence of pre-drying

https://doi.org/10.1016/j.conbuildmat.2019.05.021Get rights and content

Highlights

  • Water from glue mortar transports to tile, during drying the water returns to the hydrating mortar.

  • A high viscosity of MHEC solution decreases the uptake of water in the tile and substrate.

  • Dry air induces a dry region at the mortar surface that inhibits wetting by tile.

  • Stagnant air delays formation of a dry region at the mortar surface, ensuring sufficient wetting by tile.

Abstract

This article presents an experimental investigation on the influence of Methylhydroxyethylcellulose (MHEC) modified mortar on water absorption into tile and base substrate using Nuclear Magnetic Resonance (NMR) imaging. Addition of MHEC shifts the absorption behavior from homogeneous to inhomogeneous as a result of increased viscosity of pore solution. The observed reverse flow from tile to mortar is a result of decrease in capillary pressure in mortar as a consequence of decrease in pore size due to hydration. Pre-drying of 1.3 wt% MHEC mortar with dry air shows little wetting by tile due to MHEC transport and formation of dry region at the mortar surface. On the contrary, pre-drying of 1.3 wt% MHEC mortar at 40% RH slows down MHEC transport and delays the formation of dry region at the mortar surface enabling higher wetting by tile.

Introduction

Cement based glue mortars are widely used as tile adhesives in buildings and constructions. Commercial glue mortars are complex systems, with Portland cement as a main ingredient, and a variety of inorganic calcium silicate and calcium aluminates [1]. Methylhydroxyethylcellulose (MHEC) is widely used as water retention agent in cementitious materials. The major applications are tile adhesives, wall renders, self-levelling underlayments, floor screeds and water-proofing membranes.

Critical for the use of mortar is the time to application, which is limited by the so-called open time, being the time during which a tile can be fixed to the substrate with sufficiently good adhesion. This open time can be modified by adding a small amount (1–2 wt%) of water soluble polymer, which might be caused by a change in drying behavior of cementitious glue mortar. Among others, the open time is determined by the drying process of mortar as a result of evaporation, internal moisture flow, hydration and film formation. Other key parameters that determine the open time are wetting of the tile during application and sagging of the tile. The degree of wetting relates to the water absorption and penetration of glue compounds into the tile, which determine the adhesion between tile and substrate to a large extent. The wetting behavior is influenced by the drying process of the mortar as a result of evaporation, internal moisture flow, viscosity of pore solution and polymer transport [2], [3].

Previously, drying of highly viscous MHEC/water solutions inside non-reactive porous media has been studied in detail [2]. It was found that MHEC increases the viscosity of the pore liquid four orders of magnitude and, as such, inhibits fluid flow. A transition from homogeneous drying to a front drying is observed for MHEC concentrations above 1.3 wt%. Consequently, a decrease of the evaporation rate is observed as the drying front retracts itself into the material. As MHEC addition leads to front drying, it reduces the drying rate [3].

Drying of glue mortar, a reactive porous material, in the presence of MHEC has also been studied previously [4]. In this study, we examined the effect of MHEC on the hydration characteristics of unsealed mortar. In absence of MHEC, mortar shows a homogeneous drying behavior and in presence of MHEC, mortar shows a front receding drying behavior. The drying time of pure mortar is shorter than hydration time, causing incomplete hydration. The drying time of MHEC modified mortar is comparable to the hydration time, ensuring good hydration. Again, a reduction in evaporation rate with increasing MHEC concentration is observed simultaneously accompanied by the formation of a film. The hypothesis is that MHEC forms a film at the surface [4]. This film will be hydrophobic and as such free of water, thereby possibly creating a barrier for water transport.

The immediate absorption of water into the tile has potentially a significant effect, because a threshold amount of available water is crucial for proper hydration of glue mortar. Water absorption in porous media is a complex process [5], [6], [7]. Most studies focuses on the capillary absorption of water in porous media. The mass of the liquid absorbed per unit area increases with the square root of the time elapsed until the liquid absorption reaches the boundary of the material, which may be described by both the Washburn and Richards equations [8], [9], [10]. The suction coefficient obtained from fitting the Washburn equation to such data depends on the physical properties of the liquid and properties of the porous materials. Only few studies relate the capillary absorption of water with the physical properties of liquid such as viscosity, surface tension and contact angle [11], [12], [13]. Gummerson et al. [12] showed that the rate of liquid absorption into brick ceramics scales approximately with (γ/η)1/2, where γis the surface tension and η the viscosity. No studies were reported on suction characteristics of MHEC solutions in building materials.

Measuring transport processes inside a porous material is challenging as most materials are non-transparent. Only a few techniques can measure fluid distributions inside 3D porous materials non-destructively and non-invasively, such as synchrotron, X-ray tomography [14] and Nuclear Magnetic Resonance (NMR) imaging [15]. In this study, we chose NMR imaging to monitor the water absorption processes of base concrete substrate and tile from a cellulose modified mortar, because NMR allows in-situ monitoring of water distribution with high spatial and temporal resolution non-destructively and non-invasively.

The aim of this study is to investigate the influence of MHEC on the water absorption from glue mortar into both the base concrete substrate and tile. The main research questions are: 1) What is the influence of MHEC added to the mortar mix on the water absorption process of a base concrete substrate and tile? 2) What is the effect of pre-drying of MHEC modified mortar on water absorption process before application of the tile? 3) What is the effect of humidity and air flow on the wetting and adhesion process of glue mortar? To answer these questions we have studied the water absorption from mortar with and without MHEC into a base concrete substrate and tile using NMR, with different drying conditions.

Section snippets

Materials and methods

This section focuses on the materials and methods used to understand the influence of MHEC on the moisture transport from the freshly prepared glue mortar into its neighbouring substrates, i.e., a base concrete slab and a tile (Fig. 1). In the first set of experiments, water absorption from mortar to the porous tile with increasing MHEC concentration was studied. In the second set of experiments, a pre-drying is performed on mortar paste before applying the tile in order to understand the

Capillary number: Key parameters to characterize absorption

This section is added to clarify the key parameters that play a role during absorption. This is achieved by investigating the dependence of the capillary number on the addition of MHEC. The capillary number allows clarifying the transport process, providing inside on whether or not homogeneous or front like absorption can be observed.

The absorption in a porous material can be characterized using the Capillary number (Ca). The Capillary number, (Ca=L/ξ) can be considered a ratio of length scales

Water absorption from mortar into concrete base substrate and tiles

This section focuses on understanding the role of MHEC on the moisture transport from the freshly prepared glue mortar into its neighbouring substrates, i.e., a base concrete slab and a tile. One dimensional NMR imaging was performed with mortars containing 0 wt% MHEC, 1.3 wt% MHEC and 2.1 wt% MHEC. In our experiments, the substrates were inserted in a cylindrical teflon sample holder on which a 5 mm thick mortar paste with different wt% MHEC was applied. After applying the mortar, immediately

Discussion

This section discusses the major results of this study.

Conclusions and practical consequences

This paper presents an experimental investigation of the water absorption behavior of a porous substrate and tile in presence of MHEC modified mortar using Nuclear Magnetic Resonance Imaging. The performed NMR study shows a clear effect of the addition of MHEC on the water absorption behavior of porous tile.

Our results show that the moderate addition of MHEC (MHEC < 1.3 wt%) results in a viscosity value low enough to enable the absorption of water by the porous tile, and as such provide better

Declaration of Competing Interest

None.

Acknowledgement

This research was carried out under the project number M81.6.08315 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl).

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