Influence of Process Variables on Shrinkage in Low-Calcium Fly-Ash Geopolymers

The influence of process variables such as curing temperature and relative humidity (RH) on the shrinkage of alkali-activated fly ash (AAF) is examined in this work. The ambient conditions are varied after an initial accelerated moist curing at high-temperature. An analysis interlinking the effects of curing on AAF strength, shrinkage, reaction product content, and porosity is performed. Strength achieved and the pore structure formed for the different curing conditions depend upon the sodium alumino-silicate (N-A-S-H) gel content formed in the AAF. While water is not directly combined in the formation of N-A-S-H gel, its content is sensitive to the availability of moisture. The moisture loss due to drying during the geopolymerization reduces the N-A-S-H content formed in the AAF. Compared to the continuous moist curing at elevated temperature, there is a decrease in the N-A-S-H content on lowering the temperature or drying produced by the decrease in RH. Reducing temperature and RH following initial accelerated curing has the beneficial effect of reducing the shrinkage compared to drying at a higher temperature. Reduction in the N-A-S-H content due to decrease in temperature after the accelerated curing is more significant than the drying on lowering the RH to 50%. The autogenous shrinkage measured under sealed conditions contributes significantly to the total shrinkage in AAF. The shrinkage in the AAF is significantly lower than a comparable cement paste. While shrinkage is produced by drying, the moisture loss and shrinkage relationship is not unique. The shrinkage produced by moisture loss due to drying is primarily influenced by the pore structure formed in the AAF, which also depends on the N-A-S-H content. The influences of temperature and humidity on the strength, pore structure and shrinkage are determined by the N-A-S-H formed in the AAF

Influences of matrix strength and weak planes on fracture response of recycled aggregate concrete

The fracture behaviours of concrete made with natural aggregate and recycled coarse aggregate (RCA) derived from crushing old concrete are compared. The performances are evaluated using concrete proportioned for different compressive strengths. The RCA from crushed old concrete produces a composite aggregate with mortar and aggregate phases. An examination of the RCA shows pre-existing cracks in the aggregate phase. Crack propagation and crack opening profiles in the fracture response of concrete beams are evaluated using the Digital Image Correlation (DIC) technique. The displacement profiles across the beam obtained using DIC are evaluated to understand the crack growth in the concrete. The cohesive stress response and an energy measure determined from the fracture test are related to physical observations of the fracture surface. The crack path in the concrete and the contribution of the different interfaces depend on the strength of the matrix surrounding the aggregate. In concrete with lower cementitious and higher water contents, the new mortar interface with RCA and the pre-existing cracks in the RCA contribute to the fracture surface in the RCA. While a larger fracture surface area is created in concrete made with RCA, the energy measure and the cohesive stress determined from the fracture test are lower. In concrete proportioned for higher compressive strength, there is a densification of the RCA-mortar interface and the fracture plane is produced through the aggregates. The pre-existing cracks in the RCA create weak planes, which contribute to the fracture surface created. Improving the mortar-RCA interface does not result in an improvement in tensile strength or fracture characteristics since there is also a significant contribution of the weak planes in the aggregate phase of RCA to the failure surface. The measured fracture surface area does not correlate with the energy measure from fracture test response

Porosity and pore structure control in cellular geopolymer using rheology and surface tension modifiers

Cellular geopolymer is produced by aerating Alkali-activated Fly ash (AAF) binder paste using Aluminum powder. The cellular structure created in the geopolymer depends on the AAF binder paste rheology and the surface tension of the activating solution. The constant strain rate rheological behavior of AAF binder paste varies between Maxwell flow and yield type depending on the activating solution content. Aeration with a stable bubble structure is achieved in an AAF binder paste with a yield-type constant strain-rate response. AAF binder pastes which exhibit a Maxwell-flow type constant strain rate response cannot retain bubbles in suspension. Addition of clay transforms the constant strain rate rheological behavior of the AAF binder paste from Maxwell-flow to yield type response producing a stable aerated paste. Increasing the clay content in the AAF binder paste increases its yield stress. The total porosity in the cellular geopolymer is controlled by the size and quantity of aluminum powder. Equivalent total porosity is achieved with a lower dosage of finer aluminum powder. Yield stress and surface tension have opposing influences on the pore size without influencing the total porosity. Increasing the yield stress of the AAF binder paste by adding clay produces an increase in the mean pore diameter. Adding surfactant to the AAF binder paste decreases the mean pore diameter in the cellular geopolymer. The porosity and the mean pore diameter in the cellular geopolymer can be controlled with the use of clay, and surfactant and by regulating the content and fineness of the aluminum powder. © 2022 Elsevier Lt

Extrusion-Based Three-Dimensional Printing Performance of Alkali-Activated Binders

Printable alkali-Activated fly ash-slag mixtures, which are homogeneous under pressure and achieve buildability in the extrusion-based three-dimensonal (3D) layer printing process, are developed. A baseline mixture of fly ash and slag with a sodium hydroxide activator is modified to achieve extrusion-based printing requirements, including printability, shape retention, and buildability. The role of additional dry constituents such as microsilica and clay in reducing phase separation under pressure for producing printable mixtures is evaluated. Phase separation in the mixture under pressure is sensitive to the particle size distribution. Printable mixtures, which do not segregate under pressure, have a narrower distribution of particle sizes, indicated by the Rosin-Rammler fit. The link between the rheological behavior of the mixture and its performance in printing is evaluated. The constant strain rate rheological response of the mixtures is distinguished between the yield-Type and Maxwell-flow behaviors. Mixtures that exhibit a Maxwell-flow type response produce a steadily continuing deformation and are not buildable. The distinction between Maxwell-flow and yield-Type behaviors is essential for identifying buildable mixtures. Alkali-Activated mixtures exhibit a viscoelastic response with both elastic and viscous components. The proportion of the storage to the loss modulus from rheological measurements provides an index of buildability. Achieving buildability with multiple layers depends on an internal structure capable of resisting elastic deformation, which is indicated by the development of the storage modulus with time. The role of additives on specific aspects of the rheological behavior of the mixtures is evaluated. The rheological behavior required for printing is achieved using carboxymethylcellulose (CMC), which produces a yield-Type behavior, and enhances the storage modulus and thixotropy of the alkali-Activated mixture. © 2021 American Concrete Institute. All rights reserved

Embedded smart PZT-based sensor for internal damage detection in concrete under applied compression

An embedded PZT-based sensor for monitoring internal damage in concrete is presented. An experimental program for producing controlled increments in levels of stress and damage in the concrete is developed. Formation and coalescence of microcracks are evaluated using digital image correlation. The electrical impedance (EI) measurements are recorded from the embedded sensor for different levels of stress and damage in the concrete. Changes in the EI signature associated with the resonant conditions of the PZT patch produced by stress and damage in the concrete are identified. The EI measurements provide a sensitive indication of the applied stress. Applied stress and damage in the concrete produce two counteracting effects on the EI resonant frequency. The internal damage in the concrete is detected from the EI measurement significantly earlier than the occurrence of surface cracking. Embedded sensors are more sensitive to the internal cracking in concrete compared to surface mounted PZT patches

Rheology control of alkali-activated fly ash with nano clay for cellular geopolymer application

An efficient method of production for cellular geopolymer from an alkali-activated fly ash (AAF) binder paste by controlling its rheological behavior is presented. The AAF binder paste exhibits a complex rheological behavior, which is influenced by the solids loading and the composition of the activating solution. The constant strain rate response of the AAF binder paste varies between a yield-type and Maxwell flow behavior. Entrained porosity created in the hardened geopolymer matrix with the use of Aluminum powder exhibits a dependence on the constant strain rate response of the AAF binder paste. A cellular structure is achieved in pastes which exhibit a yield-type behavior. The addition of nano-clay fundamentally alters the constant strain rate response of AAF binder pastes producing a yield-type response in suspensions that exhibit a Maxwell flow behavior. There is also an increase in the peak stress in the constant strain rate response with nano-clay. A stable cellular structure is produced in the AAF binder pastes which exhibit a Maxwell-flow type response with the addition of clay. Montmorillonite is more effective than Bentonite in enhancing the yield stress of the AAF binder pastes. The importance of identifying the yield stress of the AAF binder pastes for creating a stable cellular structure within the hardened geopolymer matrix is established. The cellular structure in the geopolymer matrix can be tailored with the rheology control of the AAF binder pastes using nano-clay

Alkali-activated fly ash-blast furnace slag blend rheology: Evaluation of yield and Maxwell responses

The rheological behavior of pastes made with fly ash-slag blends in alkali silicate activating solutions is evaluated. The link between the activating solution composition and the fundamental rheological behavior of alkali-activated pastes are studied for different fly ash-slag blends. The transient response of alkali-activated pastes prior to initiation of flow under an applied strain rate varies between yield-type or Maxwell-flow behaviors depending on the silica content in the activating solution. Adding dissolved silica in the alkaline activating solution initially produces a decrease in the yield stress of the paste, but with further increase the transient response transitions to a Maxwell-flow type behavior. Maxwell flow behavior is a fluid-dominated response with an apparent yield stress in the transient response under constant applied strain rate. The apparent yield stress in the Maxwell flow response increases sensitively with increasing silica content in the activating solution. On increasing the silica content in the activating solution, there is an increase in the plastic viscosity of the pastes. The thixotropy of the paste is influenced by the silica content in the activating solution and the slag content in the blend. Blends with yield-type behavior exhibit a rapid yield stress recovery after shearing. The yield stress and the structural rebuilding energy (SRE) increase very rapidly with age in activated mixtures containing dissolved silica. The pastes which exhibit Maxwell flow response have a significantly slower structural buildup indicated by a lower rate of increase in the SRE with age compared to material with yield type of behavior. The slag content in the blend contributes to a rapid increase in the SRE. The requirements of rheology control of the paste for different processing requirements including pumping and 3D concrete printing are evaluated. © 2022 The Author

Improvement in early-age cracking performance of concrete with hybrid steel-macropolypropylene fiber blends

Cracking in concrete occurs at an early age due to volumetric changes in the material produced by environmental effects or internal mechanisms. The cracks developed at an early age adversely affect the durability and long-term performance of concrete. Cracks permit penetration of deleterious liquids and gases into concrete which seriously compromise the structural integrity, durability, and service life of the structure. In this paper, the early-age fracture response of concrete with steel (SF050) and hybrid blend of steel and macropolypropylene fibers (Hy050) is evaluated and related with the observed plastic shrinkage cracking. An improved performance is achieved from the hybrid blend with a smaller dosage of steel fibers when used in combination with macropolypropylene fibers. The cohesive stress-crack separation (σ-w) relationship obtained from the fracture tests indicate a higher residual stress at a smaller crack separation in Hy050 when compared with SF050. The synergistic effect of hybrid blend results in significant reduction in plastic shrinkage cracking and cracking density. © 202

Self-Leveling Geopolymer Concrete Using Alkali-Activated Fly Ash

Self-leveling concrete is developed with low-calcium alkali activated fly ash (AAF) binder paste. The rheological behavior of AAF pastes with different compositions is evaluated. AAF pastes are proportioned with alkali-silicate activating solutions to ensure specific reactive oxide ratios for comparable geopolymer strength. The yield stress and the viscosity of the AAF binder paste vary with the silica content and the silica modulus (SiO2/Na2O mass ratio) in the alkali-silicate activating solution. The slump and flow behaviors of concrete mixtures made with AAF paste are evaluated. The requirements of the AAF binder characteristics, paste content, and aggregate packing for achieving self-leveling flow characteristics under gravity-induced flow are assessed. The transition from a frictional to a flow-Type behavior in concrete mixtures depends on the AAF binder paste content. Self-leveling is achieved without the use of admixtures with an AAF binder paste of low yield stress and at a paste content of 45%. Improving the aggregate packing using the Fuller-Thompson curve and reducing the yield stress of the AAF binder paste increase the flow achieved in concrete mixtures. The specifications for cement-based self-consolidating concrete (SCC) are closely applicable for self-leveling AAF-based concrete

Experimental Investigation on Strengthening Of Soft Clay Brick Masonry Columns under Compression with Fiber-Reinforced Inorganic and Organic Matrixes

Masonry made with soft clay brick is commonly used in gravity load bearing of construction in India. The masonry piers and walls typically fail by vertical splitting. The purpose of this study is to improve the strength of masonry columns under compression using wrapping for additional confinement. The compressive load carrying performance and capacity of masonry columns wrapped with fiber reinforced composites in organic and inorganic matrixes are compared. For the purpose of overall improvements in cost and durability, glass and basalt fiber reinforcement is used. 30-40% improvement in the compressive performance of masonry prisms was achieved for both Organic and Inorganic matrixes. However, the specimens with inorganic matrixes were found to exhibit higher ductility compared to organic matrixes. Glass fibers were found to be more effective in wrapping masonry specimens compared to Basalt fiber specimens owing to its higher fiber count per unit length. Analytical models for predicting the compressive capacity of masonry columns with wrapping are verified against the experimental results. © 2022 Trans Tech Publications Ltd, Switzerland