Effect of MHEC on evaporation and hydration characteristics of glue mortar 2017-09-13


  1. Miroslav
    Cement based mortars are widely used as tile adhesives in building and construction. Commercial mortars are complex systems, with Portland cement asmain ingredient, consisting of a variety of inorganic calcium silicate and calcium aluminates [1]. Together with water, the cement reacts via various pathways into different inorganic phases (calcium and aluminumhydrates, etc.) [2, 3]. Incase of a gluemortar, water soluble polymers, such as methyl cellulose andmodified cellulose ether (CE) (e.g.methylhydroxyethylcellulose (MHEC)) are added to influence the open time [4, 5]. However, thewayMHEC retainswater, and as such influences the open time (OT) of cementmortar, is not fully understood.

    Open time of the glue mortar is the time during which a tile can be fixed by mortar to the substrate with sufficiently good adhesion and without reduction of bond strength. Open time of the mortar determines the rate of application of tile, e.g. in case of short open time, only a small area can be tiled and exceeding the open timewould result in reduction in bond strength. Among others, the open time is determined by the drying process of mortar as a result of evaporation, internal moisture flow and hydration.Many questions are still open on how MHEC influences the drying behavior of a glue mortar, both by influencing the hydration as well as evaporation. Several aspects have been studied, of which an overview is given of the relevant papers for this study.

    Drying of high viscous MHEC solution inside non-reactive porous media has been studied in detail in a previous study by the authors [6]. In that study, a saturated fired clay brick (FCB) was studied as a model system excluding the influence of hydration. It was found that the dition of MHEC may increase the viscosity of pore liquid four orders in magnitude
    and, as such, influences moisture transport. Accordingly, the performed NMR drying experiments show a switch from homogeneous drying to an in homogeneous drying behavior, as a function of MHEC concentration.
    The research resulted in twomain conclusions: 1) The viscosity is the dominating factor influencing the moisture transport, resulting in a receding front type of drying at high concentration ofMHEC. 2) vaporation is also reduced by the addition of MHEC, however, the processes reducing the evaporation are not fully understood. This is surprising since Patural et al. [7] found that the addition of MHEC does not modify the self-diffusion coefficient of water, and our studies of MHEC solution in capillaries do not show any reduction in evaporation, which suggests some other mechanism at play.

    Another process influencing the available amount of water in a glue mortar is the hydration of cement, which evidently consumes free
    water. Several studies have shown that cellulose ether is an important
    ingredient reducing the speed of cement hydration [8, 9]. These studies
    show that methoxyl groups of hydroxyethylmethylcellulose (HEMC)
    and hydroxypropylmethylcellulose (HPMC) are responsible for the delay in portlandite Ca(OH)2 precipitation. The molecular weight and the presence of hydroxyl propyl seem to have negligible impact on cement hydration process. Silva et al. [10] studied the impact of hydroxyethylcellulose (HEC) on the pore size distribution of cement
    paste by MIP. The pore size distribution of cement paste without HEC
    in this study shows at least two peaks with a pore size of 4 nm and 75 nm. High concentration of HEC changes this distribution and gives two additional peaks at 100–500 nm, which correspond to capillary pores.

    The evaporation of water results in spatial variations in moisture content, pore structure, and hydration. To investigate these spatial variations, imaging techniques are necessary. Various techniques could be
    used, such as: synchrotron X-ray tomography, ultrasound andNMR. Nuclear magnetic resonance (NMR) has proven to be a powerful tool to
    probe simultaneouslywater content as well as pore evolution via relaxation analysis in a non-invasive and non-destructive mannerwith sufficiently high spatial and time resolution [11]. One dimensional profiling is adequate to follow the evaporation process at mortar/air interfaces and NMR relaxation provides information changing pore size distribution during curing. From NMR experiments, the obtained signal from hydrated cement paste belongs to different relaxation components

    The goal of this study is to investigate the role of MHEC on the evaporation and hydration characteristics of a glue mortar. The main research questions are: 1) What is the typical time scale of evaporation
    and hydration of mortar, and what is the dominant factor? 2) What is
    the effect of MHEC on the pore evolution of mortar in a sealed system?
    and 3)What is the interaction of unsealedmortar and its characteristics
    on the hydration and pore evolution both with and without MHEC? To
    that end, simultaneousmoisture distribution and hydrationwas studied
    on drying mortar with and without MHEC to understand the influence
    of the water retention ability of MHEC on the microstructure formation
    of unsealed mortar.