Showing papers in "Cement and Concrete Research in 2018"

Eco-efficient cements: Potential economically viable solutions for a low-CO2 cement-based materials industry

Abstract: The main conclusions of an analysis of low-CO2, eco-efficient cement-based materials, carried out by a multi-stakeholder working group initiated by the United Nations Environment Program Sustainable Building and Climate Initiative (UNEP-SBCI) are presented, based on the white papers published in this special issue. We believe that Portland-based cement approaches will dominate in the near future due to economies of scale, levels of process optimisation, availability of raw materials and market confidence. Two product-based approaches can deliver substantial additional reductions in their global CO2 emissions, reducing the need for costly investment in carbon capture and storage (CCS) over the next 20–30 years: 1. Increased use of low-CO2 supplements (SCMs) as partial replacements for Portland cement clinker. 2. More efficient use of Portland cement clinker in mortars and concretes. However, other emerging technologies could also play an important role in emissions mitigation in the longer term, and thus merit further investigation.

3D printing using concrete extrusion: A roadmap for research

Abstract: Large-scale additive manufacturing processes for construction utilise computer-controlled placement of extruded cement-based mortar to create physical objects layer-by-layer. Demonstrated applications include component manufacture and placement of in-situ walls for buildings. These applications vary the constraints on design parameters and present different technical issues for the production process. In this paper, published and new work are utilised to explore the relationship between fresh and hardened paste, mortar, and concrete material properties and how they influence the geometry of the created object. Findings are classified by construction application to create a matrix of issues that identifies the spectrum of future research exploration in this emerging field.

One-part alkali-activated materials : a review

Abstract: Alkali-activated materials (AAM) are recognized as potential alternatives to ordinary Portland cement (OPC) in order to limit CO2 emissions as well as beneficiate several wastes into useful products. However, the alkali activation process involves concentrated aqueous alkali solutions, which are corrosive, viscous, and, as such, difficult to handle and not user friendly. Consequently, the development of so-called one-part or “just add water” AAM may have greater potential than the conventional two-part AAM, especially in cast-in-situ applications. One-part AAM involves a dry mix that consists of a solid aluminosilicate precursor, a solid alkali source, and possible admixtures to which water is added, similar to the preparation of OPC. The dry mix can be prepared at elevated temperatures to facilitate the reactivity of certain raw materials. This review discusses current studies of one-part AAMs in terms of raw materials, activators, additives, mechanical and physical properties, curing mechanisms, hydration products, and environmental impacts.

Rheological requirements for printable concretes

Abstract: We study in this paper the rheological requirements for printable concrete in terms of yield stress, viscosity, elastic modulus, critical strain, and structuration rate. We first discuss the extrusion/deposition process at the level of the nozzle from a material perspective. We then focus on the rheological requirements needed to prevent the flow of one layer or the strength-based failure of the rising printed element. We moreover discuss the rheological requirements needed to control the final geometrical dimensions of one layer and of the entire object, including buckling stability and surface cracking. We finally describe the requirement for a proper intermixing of the layers interface and also note that drying of the upper surface of the layer at rest could also play a major role on the interlayer bond. Finally, we evaluate the effect of the use of printing supports (i.e. non-direct printing) on the above rheological requirements.

Vision of 3D printing with concrete — Technical, economic and environmental potentials

Abstract: A vision is presented on 3D printing with concrete, considering technical, economic and environmental aspects. Although several showcases of 3D printed concrete structures are available worldwide, many challenges remain at the technical and processing level. Currently available high-performance cement-based materials cannot be directly 3D printed, because of inadequate rheological and stiffening properties. Active rheology control (ARC) and active stiffening control (ASC) will provide new ways of extending the material palette for 3D printing applications. From an economic point of view, digitally manufactured concrete (DFC) will induce changes in the stakeholders as well as in the cost structure. Although it is currently too ambitious to quantitatively present the cost structure, DFC presents many potential opportunities to increase cost-effectiveness of construction processes. The environmental impact of 3D printing with concrete has to be seen in relation to the shape complexity of the structure. Implementing structural optimization as well as functional hybridization as design strategies allows the use of material only where is structurally or functionally needed. This design optimization increases shape complexity, but also reduces material use in DFC. As a result, it is expected that for structures with the same functionality, DFC will environmentally perform better over the entire service life in comparison with conventionally produced concrete structures.

Early age mechanical behaviour of 3D printed concrete: Numerical modelling and experimental testing

Abstract: A numerical model was developed to analyse the mechanical behaviour of fresh, 3D printed concrete, in the range of 0 to 90 min after material deposition. The model was based on a time-dependent Mohr-Coulomb failure criterion and linear stress-strain behaviour up to failure. An experimental program, consisting of unconfined uniaxial compression tests and direct shear tests, was set-up and performed to obtain the required material properties. The material tests showed that the Young's modulus and cohesion linearly increase with fresh concrete age, as do the compressive and shear strength. The Poisson's ratio and angle of internal friction, on the other hand, remain constant. Subsequently, the model was validated by comparison to printing experiments. Modelling of the printed samples reproduced the experimental results qualitatively, but the quantitative agreement with the print experiments could be improved. However, the deviations can well be explained and the type of failure-deformation mode was predicted accurately.

Hydration and rheology control of concrete for digital fabrication: Potential admixtures and cement chemistry

Abstract: Concrete digital fabrication is an innovative construction approach where infrastructural elements can be built additively without using formwork. This represents a significant advantage, but also introduces materials engineering challenges, as the requirements normally fulfilled by the formwork are now imposed on the concrete. In this paper, it is discussed how admixtures can be employed to achieve the rheological and hydration properties necessary for printable concrete. An overview of various admixtures currently implemented in standard practice is presented. Then, the main required concrete states for extrusion and deposition processes are analyzed with respect to required performances and potential admixtures. Finally, possible side effects and incompatibilities are discussed, as well as how they could be unconventionally used for printable concrete purposes. The main objective is to demonstrate how admixtures will be critical in the development of concrete systems to realize digital fabrication, and to ultimately motivate investigation in the key areas discussed.

Investigation of the calcined kaolinite content on the hydration of Limestone Calcined Clay Cement (LC3)

Abstract: This study presents the influence of the calcined kaolinite content of calcined clays on the hydration of Limestone Calcined Clay Cements containing 50% of clinker (LC3-50). Above a calcined kaolinite content of 65% in calcined clay, further reaction of clinker is inhibited from 3 days onwards. Detail investigation indicates that this slowing down of clinker hydration is related to a significant refinement of pore connectivity. A limiting critical pore entry radius of 3–5 nm is reached, from which the porosity does not get further refined. The higher the calcined kaolinite content, the faster this refinement limit is reached. The formation of carboaluminate hydrates is also limited after reaching this refinement limit. As a consequence, the on-going reaction of metakaolin impacts C-A-S-H, mainly affecting gel porosity, which is not well characterized by MIP.

The role of early age structural build-up in digital fabrication with concrete

Abstract: The advent of digital fabrication for concrete calls for advancing our understanding of the entanglement of processing technology, rheology, admixture use and hydration control, in addition to developing novel measurement and control techniques. We provide an overview of recently proposed building processes, defining the type and range of yield stress evolution that they require for successful building. In doing so, we explain which mechanisms are at stake and how their consequences can be measured. Controlling the structural build-up of concrete with the precision required by digital processes will be at the heart of future progress. For this, chemically controlling cement hydration of concrete is essential and we provide an overview of admixtures, focusing on “set on demand” solutions, concluding that activators should be added as closely to the delivery point as possible. Advantages and limitations are discussed and showcased using recent successes in process scaling and material and process control.

Particle-bed 3D printing in concrete construction – Possibilities and challenges

Abstract: This article provides an overview of different particle-bed 3D printing techniques for the production of concrete elements. A classification is proposed which considers the direct production of concrete components, the production of formwork as well as composite components by means of a permanent formwork. Three techniques are considered as relevant for concrete construction, i.e. selective binder activation, selective paste intrusion and binder jetting. Design as well as material aspects are addressed. The underlying physics of fluid infiltration into the particle-bed and its effect on the properties of the hardened material are discussed on the basis of recent research results. Finally, the first applications of particle-bed 3D printing are presented which demonstrate the potential of this technique in concrete construction.

Rethinking reinforcement for digital fabrication with concrete

Abstract: The fabrication of novel reinforced concrete structures using digital technologies necessarily requires the definition of suitable strategies for reinforcement implementation. The successful integration of existing reinforcement systems, such as steel rebar, rods, wires, fibres or filaments, will indeed allow for printed concrete structures to be designed using standard structural codes. However, reinforcement integration has to be compatible with either the specific printing technique adopted for the structural element production or with its shape. This paper provides a systematic overview of a number of digital fabrication techniques using reinforced concrete that have been developed so far, proposing a possible organization by structural principle, or place in the manufacturing process.

Changes in microstructure characteristics of cement paste on carbonation

Abstract: This study investigates the influence of carbonation on the microstructure of cement paste cast with Ordinary Portland Cement (OPC), fly ash based Portland Pozzolana Cement (PPC) and Limestone Calcined Clay Cement (LC3) using X-ray diffraction (XRD), thermal analysis (TGA), scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP). A comparison is made between samples exposed to 3% carbon dioxide concentration and those exposed to natural CO2 concentrations. It is observed that the products formed on carbonation are similar in both the exposure conditions. Distinct rims corresponding to decalcified C-S-H are observed around clinker grains in the carbonated samples. Coarsening of pore structure is observed on carbonation in all the cements and an increase in the total porosity is observed in blended cements. Thermodynamic modeling indicates irrespective of the type of cement the total solid volume is reduced on complete carbonation. The volume change occurring on carbonation obtained from experimental results and thermodynamic modeling are compared.

Mechanical properties and durability performance of concretes with Limestone Calcined Clay Cement (LC3)

Abstract: The adoption of any binder system for structural concrete depends on the performance characteristics desired for addressing the long-term deformation and durability concerns. The major properties influencing the performance includes the shrinkage characteristics governing the long-term deformation, and durability characteristics related to various transport mechanisms, governing the performance in different service conditions. This paper describes the potential of Limestone Calcined Clay Cement (LC3) for use in structural concrete in comparison with Ordinary Portland Cement (OPC) and fly ash based blended cement (FA30). Three types of concrete mixtures were designed for the study, two based on achieving an equivalent strength grade (M30 and M50 concrete grade) with each binder, and the third with equal binder content and w/b ratio. Mechanical properties such as compressive strength and elastic modulus, and autogenous and drying shrinkage, along with various durability parameters of the different concretes were assessed. Oxygen permeability, rapid chloride penetration, chloride migration, resistivity development and water sorptivity were the various parameters considered for evaluation of durability performance. The results indicate the superiority of LC3 binder over other binders in producing durable concrete, especially in a chloride laden environment. The major reason for the better performance was attributed to the more compact and dense microstructure of the system with the LC3 binder against OPC and FA30. The drying shrinkage performance was seen to be similar for concrete with all three binders.

Effect of Graphite Nanoplatelets and Carbon Nanofibers on Rheology, Hydration, Shrinkage, Mechanical Properties, and Microstructure of UHPC

Abstract: This study evaluates the effects of two types of graphite nanoplatelet (GNP-C and GNP-M) and one type of carbon nanofiber (CNF) on rheological properties, hydration kinetics, autogenous shrinkage, and pore structure of ultra-high-performance concrete (UHPC). The dispersion method was optimized to secure uniform dispersion of the nanomaterials in the UHPC. The plastic viscosity decreased with the nanomaterials content as the content was increased from 0 to 0.05%. As the nanomaterials content increased from 0 to 0.3%, the duration of induction period was extended by the addition of CNF, but shortened by use of GNP-C or GNP-M; cumulative hydration heat release was increased by introduction of nanomaterials; the autogenous shrinkage of UHPC with CNF, GNP-C, and GNP-M was increased by 30%, 20%, and 20%, respectively. The use of 0.3% CNFs reduced the total porosity of the UHPC by approximately 35%, indicating that the presence of CNFs refined the microstructure of UHPC.

Effect of fly ash microsphere on the rheology and microstructure of alkali-activated fly ash/slag pastes

Abstract: The highly viscous property of alkali silicate-activated cements is one of the critical challenges that hinder their wide application. The present study focuses on ameliorating the rheological performance of sodium silicate-activated fly ash/slag pastes by using fly ash microsphere (FAM), which are highly spherical particles collected from fly ash with electrostatic adsorption classification technology. The FAM particles work as ‘ball-bearings’ in the pastes to reduce the internal friction between fly ash and slag grains, and meanwhile mitigate the agglomeration of flocs and fragmentation to release the locked water. The interrelationship between the FAM particle geometry and plastic viscosity of the paste is well described by the Krieger-Dougherty equation, which supports the proposed mechanisms of ‘ball-bearings’ effects. FAM can work as an inorganic dispersing agent to improve the workability of alkali-activated cement products for a variety of application aspects.

Concrete material science: Past, present, and future innovations

Abstract: Concrete is flying off, but it is simultaneously facing tremendous challenges in terms of environmental impact, financial needs, societal acceptance and image. Based on an historical approach of the science of concrete and reinforced concrete in particular, this paper calls for the exploration of radical changes in three key aspects of concrete use: reinforcement, binder content, and implementation methods. More precisely, it is suggested that, in parallel to the introduction of robotic fabrication methods, digital technologies may be key for the introduction several innovations like (i) rebar-free reinforcement using non-convex granular media; (ii) compression-optimized concrete structures, using topology optimization, architectural geometry, and 3D-printing or origami-patterned formworks; (iii) truly digital concrete through the coupling of massive data collection and deep learning.

Isothermal calorimetry and in-situ XRD study of the NaOH activated fly ash, metakaolin and slag.

Abstract: Much is unknown about the reaction processes responsible for the formation and polycondensation of geopolymers and other alkali activated materials. In this work, isothermal calorimeter and in-situ XRD were adopted to study the heat and mineral evolution of NaOH activated fly ash, metakaolin and ground granulated blast furnace slag. Both activator concentration and temperature have profound influences on duration of exothermal geopolymerization peaks. NaOH activated fly ash is more temperature dependable, with much higher activation energy than metakaolin and slag. The dissolution of source precursor is rapid and the formation of new phases can be detected by the end of the initial dissolution period. The in-situ XRD measurement together with the PONKCS analysis method promotes quantitative estimation of amorphous evolution during alkali activation.

Tensile performance of sustainable Strain-Hardening Cementitious Composites with hybrid PVA and recycled PET fibers

Abstract: Strain-Hardening Cementitious Composite (SHCC) is a type of advanced construction material that can enhance the resiliency and durability of structures. However, the high cost of the constituents limits the wide application of SHCCs. To reduce the material cost and improve the sustainability, this study explores the potential of replacing commonly-used polyvinyl alcohol (PVA) fibers by recycled polyethylene terephthalate (PET) fibers. The potential of fiber hybridization was first evaluated using micromechanical modeling, and the ultimate tensile strain of hybrid-fiber SHCCs was estimated using a semi-empirical method. Then the tensile performance of SHCCs after standard curing and accelerated aging was experimentally evaluated, and the crack pattern development with increasing tensile strain was recorded. Satisfactory mechanical performance can be achieved even when 50% of PVA fibers are replaced by recycled PET fibers with surface treatment. In addition, using recycled PET fibers in SHCCs can significantly reduce the material cost and environmental impact.

Computational design optimization of concrete mixtures: A review

Abstract: A comprehensive review of optimization research concerning the design and proportioning of concrete mixtures is presented herein. Mixture design optimization is motivated by an ever-increasing need for designers and decision-makers to proportion concrete mixtures that satisfy multiple – oftentimes competing – performance requirements, including cost, workability, mechanical properties, durability, and environmental sustainability. In this review, we first discuss common mathematical problem formulations, decisions, objectives, and constraints pertaining to concrete mixture design optimization. Subsequently, we examine the types of models employed to approximate properties of concrete, which include a variety of linear combination, statistical, machine learning, and physics-based models that are required to optimize the proportions of a mixture. We then review and discuss computational methods used to optimize concrete mixtures in the context of surveyed literature. Finally, we highlight and discuss current trends and opportunities for advancing the field of concrete mixture design optimization in context of the current state of the art.

Effect of slag content and activator dosage on the resistance of fly ash geopolymer binders to sulfuric acid attack

Abstract: Geopolymer (GP) binders are an appealing alternative to Portland cement (PC) binders as they have the potential to reduce the CO2 emissions associated with the cement and concrete industry. However, their durability in aggressive environments needs thorough examination if they are to become a viable alternative to traditional PC materials. This study investigated the effect of increasing slag content and activator dosage on the sulfuric acid resistance of fly ash GP binders. Their performance was also compared with that of neat PC mixes using various physical and microstructural techniques. The results show that increasing the slag content of fly ash GPs decreases porosity, but makes the reaction products more susceptible to sulfuric acid attack. It was also found that increasing the alkaline activator dosage of fly ash GPs has little impact on sulfuric acid resistance. Finally, GP binders displayed superior sulfuric acid resistance than their PC counterparts.

Corrosion rate of carbon steel in carbonated concrete – A critical review

Abstract: Reinforced concrete with lower environmental footprint (lower CO2 emission) can be obtained by reducing the clinker content in the cements. As the carbonation of concrete is faster, corrosion of steel in carbonated concrete during the propagation phase is becoming important both for science and practice. The present literature review summarizes the state of the art, reporting corrosion rate data for a broad range of cement types, w/b ratios and environmental conditions. Correlations between corrosion rate and the main influencing parameters are elaborated and discussed. It confirms that the corrosion rate of steel in carbonated concrete is not under ohmic control. More important are the degree of pore saturation and the effective steel area in contact with water filled pores. It also emerges that the new blended cements have to be systematically studied with respect to the corrosion behavior of steel in carbonated concrete in order to make reliable service life prediction.

Corrosion resistance of steel fibre reinforced concrete – a literature review

Abstract: Steel fibre reinforced concrete (SFRC) is increasingly being used in the construction of civil infrastructure. However, there are inconsistencies among international standards and guidelines regarding the consideration of carbon-steel fibres for the structural verification of SFRC exposed to corrosive environments. This paper presents a review of the published research regarding carbonation- and chloride-induced corrosion of SFRC, and proposes a deterioration theory for cracked SFRC exposed to chlorides and carbonation, based on the damage at the fibre-matrix interface. The review confirms an overall agreement among academics and regulators regarding the durability of uncracked SFRC exposed to chlorides and carbonation. Contrariwise, the durability of cracked SFRC is under discussion at the technical and scientific level, as there is a large dispersion on the experimental results and some of the mechanisms governing the corrosion of carbon-steel fibres in cracks and its effects on the fracture behaviour of SFRC are not fully understood.

Nucleation seeding with calcium silicate hydrate – A review

Abstract: The development of green cements, with the aim of reducing CO2 emissions, often results in reduced hydration activity, especially during the first hours and days. Nucleation seeding with C-S-H has enormous potential to accelerate hydration, which can compensate for the above-mentioned effect without compromising the long-term strength of seeded cements. In this work, the effects of calcium silicate hydrate are reviewed in detail, with a focus on synthesis, as well as their influence on the hydration mechanism and the development of mechanical properties, such as early and long-term compressive strength and porosity.

Autogenous shrinkage of alkali activated slag mortars: Basic mechanisms and mitigation methods

Abstract: This paper presents various mechanisms that can explain the cause of excessive autogenous shrinkage exhibited by alkali activated slag mortars (AASM) compared to Portland cement mortars (OPC). The influence of activator concentration parameters such as Na2O content and of the silica modulus (Ms) on the magnitude of autogenous shrinkage are evaluated, as are the proposed mechanisms. The results show that the shrinkage kinetics of AASM is strongly dependent upon the rate of reaction, internal relative humidity (RH) and surface tension of the pore solution. Pore size variation between AASM and OPC mortars and the corresponding tensile stresses based on the capillary tension approach were also calculated using experimentally measured internal RH and surface tension values. A higher amount of both Na2O and Ms. resulted in larger capillary stress resulting in greater autogenous shrinkage. Use of internal curing and of shrinkage reducing admixture were very effective in reducing the autogenous shrinkage of AASMs.

Application of organic- and nanoparticle-modified foams in foamed concrete: Reinforcement and stabilization mechanisms

Abstract: In this study, an ultra-stable foam used for foamed concrete was fabricated by modifying the gas–liquid interface by coupling the effects of an organic surfactant and nanoparticles. A serial of techniques was employed to study the products and microstructure development of foam and foamed concrete, respectively. The results show that nano-silica and hydroxypropyl methylcellulose can slow the coalescence and disproportionation of the bubbles by adsorbing at the air bubble surface and increasing the viscosity of cell wall paste, thus preventing gas transfer and hindering physical drainage between the gaseous and liquid phases. Also, a more homogeneous and finer pore structure is generated with the inclusion of the organic surfactant and nanoparticles. The pozzolanic reaction between the nano-silica and Ca(OH)2 results in an increased C-S-H content and the densification of the cell wall structure. In addition, the stability of foamed concrete with modification and relevant stabilization mechanisms were also investigated.

Comparison between natural and accelerated carbonation (3% CO2): Impact on mineralogy, microstructure, water retention and cracking

Abstract: The consequences of accelerated carbonation at 3% CO2 were compared with those of natural carbonation (0.04%). Cement pastes (CEM I and CEM V/A) as well as the three major constitutive phases (C-S-H of different C/S ratios, portlandite and ettringite) were used and changes in the mineralogy, microstructure, water retention and cracking were investigated. The main conclusion was that accelerated carbonation at 3% CO2 was representative of natural carbonation although it promoted the precipitation of metastable calcium carbonate (aragonite and vaterite) in place of calcite. The results also showed that the presence of aragonite and vaterite were characteristic of the carbonation of ettringite and C-S-H respectively.

Effect of alkali dosage and silicate modulus on carbonation of alkali-activated slag mortars

Abstract: The long-term durability and their mechanisms of alkali-activated cement based materials have remained largely elusive. In this paper, carbonation of alkali-activated slag (AAS) mortars activated by NaOH and waterglass with different alkali dosages and silicate moduli has been investigated after exposure to 3 ± 0.2% (v/v) CO2 at 20 ± 2 °C/65 ± 5% RH for 56 days. The results show that carbonation resistance of the AAS mortars increases with increase of not only alkali dosage but also silicate modulus. In addition to the higher pore solution alkalinity and slag reaction extent, the relatively higher carbonation resistance of the AAS mortars is attributed to the lower porosity and average pore size. The loss of compressive strength for the waterglass activated slag mortars after carbonation is due to decalcification of C-A-S-H phase, whereas the carbonation of katoite contributes to the increase of compressive strength of the NaOH activated slag mortars.

Application of neutron imaging to investigate fundamental aspects of durability of cement-based materials: A review

Abstract: Service life and durability of reinforced concrete structures have become crucial issues in all industrialized countries because of their economic and ecological relevance. Limited durability is frequently due to deterioration of steel and cement-based materials, such as mortar and concrete, by interactions with water and aggressive aqueous solutions. Neutron imaging has proved to be a powerful non-destructive technique to study quantitatively water content and water movement in porous materials. A neutron beam is much more attenuated by hydrogen in water than by most other elements present in cement-based materials. In this review, focus is placed on applications of both two-dimensional neutron radiography and three-dimensional neutron tomography to investigate specific aspects of durability and deterioration of cement-based materials. Examples of results obtained by qualitative and quantitative investigations of moisture movement in cracked and uncracked cement-based materials are presented. Self-healing, efficiency of water repellent treatment, internal curing, frost damage, fire spalling, ettringite formation and observations of various reinforced concrete components are addressed. The results obtained by neutron imaging provide a solid basis for better understanding of deterioration mechanisms of cement-based materials. Recent improvements of neutron imaging facilities have allowed unexpected possibilities to study complex processes in cement-based materials. The potential for further research based on this promising technology is outlined and discussed.

Influence of fly ash and metakaolin on the microstructure and compressive strength of magnesium potassium phosphate cement paste

Abstract: The influences of fly ash and metakaolin added as substitutions (by up to 50 wt%) of magnesium potassium phosphate cement (MKPC) on the microstructures and compressive strengths of the MKPC pastes were investigated. The results indicate that the aluminosilicate fractions of both fly ash and metakaolin are involved in the acid-base reaction of MKPC system, leading to a preferential formation of irregular crystalline struvite-K incorporated with Al and Si elements and/or amorphous aluminosilicate phosphate products. Metakaolin is more reactive than fly ash in the MKPC system. For the same addition dosage, the MKPC pastes containing metakaolin exhibit higher compressive strengths than the pastes containing fly ash. This is attributed to the formation of more highly reinforced microstructures and denser interfaces between the metakaolin particle and hydration products (e.g. struvite-K) in the MKPC paste containing metakaolin. Addition of 30 wt% metakaolin increases the compressive strengths of MKPC pastes at all test ages.

Enhancing thixotropy of fresh cement pastes with nanoclay in presence of polycarboxylate ether superplasticizer (PCE)

Abstract: Nanoclay, a thermal treated, purified attapulgite clay, has been used to increase thixotropy. However, the effect of nanoclay in presence of water reducing agent, such as PCE, on rheology, and the compatibility between nanoclay and PCE have not been well studied. The dynamic yield stress, thixotropic index, characteristic time, and microstructure of fresh cement pastes with combination of nanoclay and PCE addition are measured. It is found that nanoclay has a good compatibility with PCE. Nanoclay increases the dynamic yield stress and enhances the thixotropy of fresh cement pastes with and without PCE addition. The nanoclay addition agglomerates the microstructure at high PCE addition and increases the thixotropic index from bottom value to a high value. This study gives insight of achieving low dynamic yield stress yet high static yield stress and thixotropy mixtures.