Impact of hydroxypropylguars on the early age hydration of Portland cement

doi:10.1016/j.cemconres.2012.10.010

Cement and Concrete Research

Volume 44, February 2013, Pages 69-76

https://doi.org/10.1016/j.cemconres.2012.10.010 Get rights and content

Abstract

Hydroxypropylguars (HPG) are used as admixtures in factory-made mortars. These molecules present water retention properties comparable to those obtained with commonly used cellulosic water-retaining agent.

The influence of HPG on cement hydration was investigated in order to improve understanding on the delayed effect induced by polysaccharides. Hydration kinetics were characterized by means of conductivity and isothermal calorimetry measurements. The influence of polymer concentration and predissolution was studied. A weak influence of HPG on the germination of hydrates was observed. In contrast, HPG induced a significant decrease in the hydrates growth rate. Strong effects of the polymer concentration and predissolution were noticed too. From these results, we supported the hypothesis that HPG adsorption on hydrated phases via polar interactions should be responsible for the delayed effect observed.

Introduction

Traditionally, mortars are based on a mixture of sand, a binder such as cement or lime, and water. Nowadays, modern factory-made mortars are complex materials, in which are added some kinds of admixtures used to exhibit various properties, from the fresh state to the hardened material.

Polysaccharides are one of these admixtures. They are frequently introduced into mortar formulations in order to improve water retention capacity of the freshly-mixed materials, which enhances cement hydration and adhesion to the substrate [1], [2], [3], [4]. These products are also expected to act as viscosity-enhancing admixtures to prevent segregation and thus improve the homogeneity and workability of the mortar [5], [6], [7], [8].

Among all polysaccharides, cellulose ethers (CE) are the most widely used when high water retention capacity is expected. Nevertheless, hydroxypropylguars (HPG) were recently presented as a promising new class of water-retaining agent [9]. HPG are already used in various industrial fields, such as textile printing, oil production or paper manufacturing, due to their thickening effect [10]. HPG were also the subject of some patents as admixture for mortars.

The effect of saccharides has been largely studied these fifty last years, in particular from a cement hydration delay point of view. In the first time, the delayed effect of monosaccharides was investigated [11], [12], [13], [14], [15], [16], [17], [18], [19]. It appears that sugars could be adsorbed to cement phases by a chelation process. It can thereby form a temporary barrier to further hydration. In broad agreement with Young [11], Thomas and Birchall [12] proposed that sugars would be able to poison the surfaces of the CH and CSH by complexation onto these phases, which inhibit their growth and therefore would be responsible for the delay observed. According to Taplin [13], sugars could bind to the cement phases if they have a HO single bond C double bond O group, or it could be formed in alkaline media, such as cement paste, by hydrolyzing [14].

Then, due to the use of polysaccharides in modern factory-made mortars, interactions between polysaccharides and cement have been reported recently. It is established that polysaccharides induce a more or less important delay of Portland cement hydration [20], [21], [22], [23], [24], [25], which depends on the nature of the polysaccharide [26]. Pourchez et al. [27], [28] have established a mechanism of interaction between CE and cement.

However, HPG have received scant attention by the academic community although they are already used industrially. As far as we know, only Izaguirre et al. [29] was interested on the impact of HPG, but in lime-based mortars. Thus, the aim of this work is to study the influence of HPG on the Portland cement hydration kinetics at early age.

Firstly, the water-retaining effect induced by the HPG in Portland cement-based mortars will be checked. Secondly, cement hydration kinetics in the presence of HPG will be presented. Hydration kinetics have been monitored by conductivity and isothermal calorimetry measurements. Finally, a special focus on the influence of the polymer predissolution has been performed.

Section snippets

Mineral product

The investigated ordinary Portland cement (OPC) was a CEM I 52.5 R CE CP2 NF type cement according to the EN 197-1 standard. Its chemical and phase compositions are given in Table 1. Oxide composition was determined by X-ray fluorescence spectroscopy (SRS3400, Bruker-AXS). XRD analysis (D5000, Siemens) also allowed quantifying the phase composition of the given cement by means of Rietveld refinement method (Siroquant V2.5 software).

Organic admixtures

Guar gum is a polysaccharide extracted from the seed endosperm

Water retention measurements

Water retention capacity of mortars was characterized according to ASTM C1506-09 standard [32]. This test is based on a measurement of the removed water after depression and was previously described by Patural et al. [3]. Water retention, noted WR, was calculated using the following equation: $W R (%) = \frac{W_{0} - W_{1}}{W_{0}} \times 100$ W₀ represents the initial mass of mixing water; W₁ is the loss of water mass after aspiration.

Mortars were prepared according to the following mixture proportions: 30% of cement, 65% of sand

Impact of HPG on water retention in fresh mortar

Water retention is a very sensitive mortar property. Indeed, it must be important enough to not disturb the hydration of the cement. It also limits the absorption of the mixing water by the substrate and thus provides good mechanical and adhesive properties to the mortar.

DTU 26.1 [38] specifies three classes of water retention of a fresh mortar. The first one represents mortars which water retention is lower than 86%; they belong to the low water retention category. The second class

Conclusions

Based upon this study, it appears that HPG delayed cement hydration with an influence on the growth of the hydrates, rather than on the germination of hydrates. Complexation of calcium ions does not explain this effect. A strong impact of the concentration on the hydration kinetics was observed. The results and the molecular similarity with CE suggest that the delay effect is due to the adsorption of the HPG via the interaction between hydroxyl groups and the highly polar hydrated phases. This

Acknowledgments

The authors would like to acknowledge Lamberti S.p.A. for the provided products.

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Impact of hydroxypropylguars on the early age hydration of Portland cement

Abstract

Introduction

Section snippets

Mineral product

Organic admixtures

Water retention measurements

Impact of HPG on water retention in fresh mortar

Conclusions

Acknowledgments

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