Lignopolymers as viscosity-reducing additives in magnesium oxide suspensions

https://doi.org/10.1016/j.jcis.2015.07.037Get rights and content

Highlights

  • Lignopolymers have been synthesized as new superplasticizers for cement.

  • Lignopolymers were tested in model cementitious suspensions of magnesium oxide.

  • Comparisons with leading polycarboxylate ether (PCE) dispersant were performed.

  • At low concentrations, lignin-polyacrylamide was more effective than PCE.

  • Lignin-polyacrylamide was more effective at higher grafting densities than low.

Abstract

Lignopolymers are a new class of polymer additives with the capability to be used as dispersants in cementitious pastes. Made with kraft lignin cores and grafted polymer side-chains, the custom-synthesized lignopolymers were examined in terms of the molecular architecture for viscosity reducing potential in inert model suspensions. Lignin–poly(acrylic acid) (LPAA) and lignin–polyacrylamide (LPAm) have been found to vary the rheology of magnesium oxide (MgO) suspensions based on differences in chain architecture and particle–polymer interactions. A commercial comb-polymer polycarboxylate ester was compared to LPAA and LPAm at 2.7 mg/mL, a typical dosage for cement admixtures, as well as 0.25 mg/mL. It was found that LPAm was a more effective viscosity reducer than both LPAA and the commercial additive at low concentrations, which was attributed to greater adsorption on the MgO particle surface and increased steric dispersion from PAm side-chain extension. The influence of chain adsorption and grafted side-chain molecular weight on rheology was also tested.

Introduction

Portland cement, the main constituent in concrete for structural applications, relies on polymer additives to disperse particles and reduce viscosity to improve workability [1], [2]. For this goal polymer surfactants have evolved over the years from lignosulfonates to fully synthetic comb-polymers that are currently used due to their non-reactivity with concrete constituents and to meet the continual demand for better cement pouring and pumping [3], [4]. The sulfonate group chemistry of the lignosulfonates inhibits cement hydration, which can result in improperly cured concrete with subsequently poor mechanical properties [3]. Despite the non-reactivity and renewal resource of the lignin component of lignosulfonates, synthetic comb-polymers were adopted as a replacement to lignosulfonates due to their non-reactivity. Superplasticizer comb-polymers typically have a poly(acrylic acid) (PAA) main-chain with either poly(ethylene oxide) (PEO) or poly(methyl methacrylate) (PMMA) grafted side-chains and are known as polycarboxylate ethers (PCEs) [5], [6], [7], [8], [9], [10]. When added to alkaline cementitious mixtures, the PAA backbone becomes negatively charged and electrostatically adsorbs to the cement particles with side-chains extending into the water. These conventional superplasticizers are thought to prevent particle aggregation due to steric repulsion between side-chains on neighboring particles, [12], [14] but high concentrations of these superplasticizers (at least 2 mg/g cement) are needed to achieve this effect [8], [9]. The molecular architecture of comb-polymer additives has been explored and tailored for cement dispersion [7], but no new polymeric architectures have been extensively tested and recommended as alternatives to the current comb-polymers.

Furthermore, the analysis of a superplasticizer’s efficiency in improving cement workability has been irregular when conducted with empirical concrete-based tests, such as slump cones, which are unable to quantify the rheology of the paste [2], [15]. These methods have left gaps in the understanding of how polymer additives impact the many stages of cement processing during mixing, pouring, pumping, and placement. Rheometry of cements and model cementitious suspensions has consisted of many analyses, but connections between the macroscopic flow, polymer architecture, and polymer interactions have yet to be fully developed [16]. Herein we apply shear rate ramps to assess the pouring and placement of cementitious pastes along with creep tests (applied shear stress) as an analog for the pumping of cements. These rheometry methods provide detailed information on the polymer–particle flow dynamic that is unavailable from slump tests.

Magnesium oxide (MgO) suspensions have been used by both colloidal scientists and cement researchers [11], [13], [17], [18] to delineate the flow characteristics of cementitous pastes during short-term handling and placement. This model system is a simplified simulation of Portland cement with a uniform and unreactive particle composition. The physical interactions between particles and polymer can be investigated without the interference of the hydration reaction in cements [11], [13]. Although cement particles do undergo reactions upon mixing, MgO suspensions are employed to examine only the initial flow behavior without consideration of the longer-time effect of hydration. The particle size distribution of MgO can be tuned to match that of ordinary Portland cement [18], and MgO portrays similar surface chemistry to calcium silicate in cement [1]. However, the particle composition of the MgO suspensions leaves out the formation of hydration phases that interact with superplasticizer adsorption and effectivity [13], [15]. This aspect is not addressed herein but may be examined in future studies.

To explore the potential of a renewable material as a superplasticizer, new lignin-based polymers have been synthesized and tested for their flow properties in model cementitious suspensions. Previous results demonstrated that polyacrylamide-grafted kraft lignin significantly decreased the yield stress of Portland cement pastes to levels comparable with commercial polycarboxylate superplasticizers [19]. The processing characteristics are explored with rheometry for delineating the role of chain chemistry and architecture in reducing viscosity. With magnesium oxide (MgO) model suspension rheometry and chain characterization, the lignopolymers are shown to be a functional alternative to fully synthetic superplasticizers. Rheometry demonstrates that these lignopolymers can lower viscosity at concentrations below the current industry standards and that the tuned side-chain molecular weights also serve to improve viscosity reduction.

Section snippets

Lignopolymer synthesis and characterization

Lignopolymers were synthesized according to the procedures outlined in [19] for acidified (anionic) kraft lignin cores with grafted polymer chains. Kraft lignin with an estimated molecular weight of 25,000 g/mol without polymer side-chains was also prepared and used for comparison. Three lignopolymers with grafted polyacrylamide (PAm) chains were synthesized with different numbers of grafted PAm chains and different degrees of polymerization. From GPC analysis of cleaved chains using the method

Rheometry

An Anton Paar MCR 302 rheometer was used to perform shear rate ramps and creep tests of the suspensions. A four-vane fixture in a roughened Couette cup (24 mm vane diameter, 2 mm gap, 19.35 mL sample volume) was used to avoid wall slip [2], [20]. Wall slip was found to have influenced the Couette rheometry of 0.42 water:MgO suspensions when previously tested with ultrasonic speckle velocimetry [21], which prompted the use of the vane fixture. Preshear was applied before all tests by shearing the

MgO suspensions containing 0.25 mg/mL admixture

The suspensions with 0.25 mg/mL polymer had peak viscosities that were nearly an order of magnitude lower than that of the control suspension in viscosity curves (Fig. 1a) for the lignopolymers, acidified lignin, and commercial PCE. LPAm suspensions maintained the lowest viscosities over the ramped shear rate. Of the LPAm lignopolymers, LPAm-17-10k had the lowest viscosity by only a slight margin. Creep experiments (Fig. 1b) revealed that kraft lignin and LPAA-2-10k reached the greatest shear

Conclusions

Current comb-polymer additives improve cement workability but require higher concentrations to be effective [8], [9] and their synthetic chemistry is based on petroleum-derived monomers. Lignopolymers, with lignin cores and grafted polymer side-chains, offer good viscosity reduction at lower concentrations for improved cement workability. Rheometry was used for testing lignopolymers under simulated cement processing conditions using MgO suspensions, which replicate the ionic environment of

Acknowledgments

This study is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1333468. This work was also supported by the Pennsylvania Department of Community and Economic Development through the Discovered in Pennsylvania, Developed in Pennsylvania (D2PA) Program and the National Science Foundation (NSF) under Grant No. CBET-1510600 to NRW. The authors acknowledge Drs. Sebastian Manneville and Christophe Perge for their helpful discussions.

References (43)

  • J. Hot et al.

    Cem. Concr. Res.

    (2014)
  • S. Hanehara et al.

    Cem. Concr. Res.

    (1999)
  • A. Zingg et al.

    J. Colloid Interf. Sci.

    (2008)
  • Q. Ran et al.

    J. Colloid Interf. Sci.

    (2009)
  • L. Ferrari et al.

    J. Colloid Interf. Sci.

    (2010)
  • Y.F. Houst et al.

    Cem. Concr. Res.

    (2008)
  • L. Ferrari et al.

    Cem. Concr. Res.

    (2011)
  • A.M. Kjeldsen et al.

    Cem. Concr. Res.

    (2006)
  • D. Bedrov et al.

    Eur. Polym. J.

    (2010)
  • H.A. Barnes et al.

    J. Non-Newton. Fluid Mech.

    (2001)
  • E. Nägele

    Cem. Concr. Res.

    (1986)
  • H. Hoffmann et al.

    Colloids Surf. A Physicochem. Eng. Asp.

    (2010)
  • J. Ma et al.

    J. Eur. Ceram. Soc.

    (2003)
  • S. Chibowski et al.

    Colloids Surf. A Physicochem. Eng. Asp.

    (2002)
  • C. Autier et al.

    Colloids Surf. A Physicochem. Eng. Asp.

    (2014)
  • F. Winnefeld et al.

    Cem. Concr. Compos.

    (2007)
  • A. Kauppi et al.

    Cem. Concr. Res.

    (2005)
  • R.J. Flatt et al.

    Cem. Concr. Res.

    (2001)
  • R.J. Flatt et al.

    MRS Bull.

    (2004)
  • M.Y.A. Mollah et al.

    Cem. Concr. Res.

    (1995)
  • G.H. Kirby et al.

    J. Am. Ceram. Soc.

    (2004)
  • Cited by (15)

    • A novel shrinkage-reducing polycarboxylate superplasticizer for cement-based materials: Synthesis, performance and mechanisms

      2022, Construction and Building Materials
      Citation Excerpt :

      The fluidity loss of cement pastes containing PCA and SR-PCA is reduced by 39.1 % and 7.0 % within 2 h, respectively, indicating that the fluidity retention capability of SR-PCA is better than that of PCA. Generally, the hydration kinetics of the cement paste varies remarkably due to the presence of superplasticizers [9,51]. The heat flow and cumulative heat curves of mixtures containing 0.08% PCA and 0.4% SR-PCA are shown in Fig. 9.

    • Improvement in fluidity loss of magnesia phosphate cement by incorporating polycarboxylate superplasticizer

      2018, Construction and Building Materials
      Citation Excerpt :

      These results indicate that PCE is ineffective for dispersing MPC paste at the very beginning but has positive effect on reducing fluidity loss. In fact, PCE has been reported to be able to effectively plasticize MgO paste [33–35], and the reason is attributed to adsorption of PCE onto the surface of MgO particles. However, on the base of the studies about competitive adsorption between PCE and salt [36–40], one possible reason for the ineffectiveness of PCE can be deduced that the presence of borax or MDP in MPC system would significantly hinder the adsorption of PCE.

    • Performances and working mechanism of a novel polycarboxylate superplasticizer synthesized through changing molecular topological structure

      2017, Journal of Colloid and Interface Science
      Citation Excerpt :

      By means of regulating the molecular structure of PCE, some problems of PCE application can be specifically solved. Therefore, some researchers synthesized many novel superplasticizers with various kinds of molecular structures by structure design, which can achieve the breakthrough in performance [6–13]. However, there are still many deep problems needed to be solved.

    View all citing articles on Scopus
    View full text