Turbidity-based measurement of bleeding in fresh cement paste as affected by cellulose nanofibres

Abstract

This study examined bleeding in cement paste as affected by cellulose nanofibres prepared with varying the content of carboxyl groups that were grafted onto the fibre surface. Alongside the conventional ASTM test method, bleeding was further evaluated using a turbidity scanner. A linear correlation was obtained between the bleeding heights as found from the two techniques. The results demonstrate that adding cellulose nanofibre (CNF) significantly reduces the bleed-water volume. Besides, the layer of bleed water in mixtures containing the CNF was more turbid. Clearly, the water retention capacity of CNF hinders the cement grains from settling down and thereby the nanofibres obstruct the upward migration of free water. As well, the higher the carboxyl content, the higher the fibre dosage permissible. And, although it was independent of the amount of carboxyl groups, bleeding was progressively less with an increase in the nanofibre dosage. However, in the presence of a superplasticizer, the cellulose nanofibres were less effective in reducing the bleeding.

Introduction

Portland cement and its composites are the most widely consumed materials of construction. Their estimated worldwide production in 2019, reached 3.7 billion metric tons [1]. As the binding process depends chiefly on hydration with water, both fresh and hardened properties are affected by the availability and transport of water, especially in the immediate hours after mixing [2]. Guidelines for designing concrete mixtures specify the amount of water required to achieve simultaneously, the desired compressive strength, the specific durability criteria and the requested workability [2]. Often, this amount exceeds the water demand as dictated by the associated stoichiometry for the hydration reactions [3]. This excess remains free and, after placement and compaction, it tends to migrate to the surface. As opposed to this upward migration of water, the heavier fractions including the cement particles and aggregates begin to deposit under gravity. So that, within a few minutes after placement, one might observe a thin film of water ‘bleed’ onto the surface. On its way up, a part of this bleed water may also collect under coarse aggregates and reinforcing steel bars, if any. Such sites create water lenses, and result in internal flaws that serve as weak links mechanically [[2], [3], [4]]. Bleeding occurs rapidly in the immediate aftermath of placement and compaction, and continues during the hydration of cement and the formation of fresh paste until final set, albeit at a reduced pace [2]. The rate at which bleeding occurs in this duration depends upon the amount of available free water, the rate of hydration and the physical properties of the solid grains, including their density, particle size and shape [4]. The complexity of the properties involved in the bleeding mechanism necessitates the development of newer, more accurate measurement methods [5]. One such is through measuring turbidity in a turbiscanner.

While bleeding must be accepted as an intrinsic phenomenon in fresh cement-based systems, its measure has the potential to cause rapid deterioration in the integrity of the hardened concrete. It can enlarge the internal pore network [6], and when the bleed water evaporates, there is a reduction in the volume of the as-yet plastic system that leads to cracking. This is commonly recognized as plastic shrinkage. Upon evaporation, a laitance is visible on the surface that is susceptible to abrasion. The latter quickly results in enhanced ingress of deleterious chemicals and a possible erosion of aggregates [[6], [7], [8], [9]]. It follows therefore, that competent mixtures must ensure that water evaporation is impeded and any moisture transport inside the system towards the outer surface is hindered. In this context, the present study identifies cellulose nanofibres as a viable admixture.

Cellulose nanofibres (CNFs) are bio-sourced and abundantly available, making them a sustainable additive that is also economical in comparison to other nanomaterials [[10], [11], [12], [13]]. These plant-based nanoparticles are produced by subjecting the source biomass through mechanical refinement after it has undergone a chemical pretreatment that helps loosen the lignin from the cellulose [[14], [15], [16], [17]]. The resulting nanofibre is typically 3–20 nm in diameter, and a few microns in length [[18], [19], [20]]. Prior studies report CNF with an aspect ratio between 30 and 300 and a specific surface area of 140–320 m²/g [13,20,21]. The latter is about 500 times greater than that for Portland cement. When the biomass is pretreated through TEMPO-mediated oxidation, a large number of active hydroxyl and carboxyl (-COOH) groups are introduced onto the nanofibre surface [14,18]. Together, these physical and chemical characteristics impart a unique hydrophilic capacity. Serra et al. [21] found water retention values to vary from 3.9 to 11.4 g/g of the nanofibre for CNF, where the content of carboxyl groups lay between 0.04 and 1.39 mmol/g, as measured by the conductivity method.

This suite of attributes has stimulated recent interest on the behaviour of CNF in cement-based systems [11,22] and the present authors are the first to illustrate its role in improving the durability by mitigating sulphate and chloride borne distress [23,24]. Owing to its nanoscale, CNF withstands degradation in the highly alkaline environment inside hydrating cement paste [[25], [26], [27]]. Studies on rheology [11,13,[28], [29], [30]] reveal an increase in yield stress and plastic viscosity with an increase in the CNF dosage in cement pastes, and a shear-thinning behaviour. This behaviour is credited to a decrease in the actual amount of water that is free to lubricate the cement particles, as the CNF retains water molecules through hydrogen bonds on the surface active groups that populate the nanofibre. While the workability may be restored through a water reducing admixture, the mutual interaction between the admixture and the CNF is not clear [10,28,31,32].

In addition to their role as fibre reinforcement [33], the water retention in CNF mitigates autogenous shrinkage of cement paste through internal curing [11,31,33,34]. Still, the effect of CNF on the setting time and strength gain is not clear either. Whereas Mejdoub et al. [19] report a shorter setting time, Jiao et al. [17] observed prolonged initial and final setting times in CNF-reinforced pastes. The present authors have reported that CNF scavenges calcium cations, Ca²⁺, which is likely to slow the setting process [23]. An increase in the setting time could lead to an increase in the bleed water [3]. However, to the authors’ knowledge it has not been investigated before. Therefore, this study examines the effect of CNF on the bleeding process of cement paste mixtures, with and without superplasticizers, and evaluates the effect of fibre dosage and its carboxyl content on this phenomenon.