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An experimental study characterized fresh and hardened ultra-high volume fly ash mass concrete (HP-HVFA, with 80% fly ash and a water-to-binder ratio of 0.26) and normal content fly ash mass concrete (NS-FA, with 35% fly ash and a water-to-binder ratio of 0.43). The creep and thermal cracking behaviors of the HP-HVFA and NS-FA concrete were evaluated using a temperature stress testing machine (TSTM). A series of TSTM tests were conducted under an adiabatic curing mode and a temperature matched curing (TMC) mode. The results indicate that HP-HVFA concrete had moderate strength at early age and desirable fresh and hardened engineering properties. Under the both curing modes studied, HP-HVFA exhibited lower hydration temperature rise. Under the TMC mode, both compressive and tensile creep of HP-HVFA were lower than that of NS-FA although their specific creep and creep-to-shrinkage strain ratio at time of cracking were almost the same. Under adiabatic mode, HP-HVFA had higher creep strain, specific creep, and creep-to-shrinkage strain ratio than NS-FA. [ABSTRACT FROM AUTHOR]

Abstract Recently, the concept of engineered cementitious composites (ECCs) has been extended to the creation of engineered geopolymer composites (EGCs). Although showing similar mechanical characteristics (e. g., strain hardening and multiple cracking) to conventional ECC, the strength of existing EGC is generally low, and this sometimes restrains its applications. In the present study, a low-calcium (Class F) fly ash-based, polyvinyl alcohol (PVA) fiber reinforced EGC was developed and further modified by a ground-granulated blast-furnace slag (slag). The slag was used to replace the fly ash at content of 0%, 10%, 20%, and 30% (by weight). The effects of the slag on the mechanical properties (e.g., compressive strength, modulus of elasticity, uniaxial tensile behavior, flexural bending strength, and pullout bond strength) of the EGCs were investigated. The results revealed that all EGCs studied exhibited a strain/deflection hardening behavior under tension/flexure, and all slag replacements for fly ash enhanced strength-related properties but reduced ductility-related properties of the EGCs. The EGC mix with 20% slag replacement for fly ash (FA-20%S) had 102.3 MPa compressive strength, 6.8 MPa tensile strength, and 6.2 MPa bond strength, while the EGC mix with no slag (FA-0%S) had 72.6 MPa compressive strength, 4.7 MPa tensile strength, and 3.5 MPa bond strength at 28 days. These strength enhancements were mainly attributed to the improved density of the EGC matrix and the bond between the matrix and fiber. There are close relationships between the bond strength and other strengths, especially the tensile and flexural strengths, of the EGCs. Highlights • A fly ash-based, engineered geopolymer composite (EGC) displayed a significant strain hardening behavior. • Replacing fly ash with 20% slag in the EGC increased the 28-day compressive strength from 73 MPa to 102 MPa. • Slag replacements for fly ash improved strength related properties but reduced ductility related properties of the EGCs. • The EGC mix with 20% slag showed more geopolymization products adhered on PVA fiber surfaces than the EGC mix with no slag. [ABSTRACT FROM AUTHOR]

Abstract In this study, flowability, compressive strength, drying and restrained shrinkage, cracking potential, and pore size distribution of fly ash based geopolymer (FG) pastes were investigated. The FG pastes were made with two SiO 2 /Na 2 O mole ratios (Module), two solute mass concentrations (Concentration), with and without shrinkage reducing admixture (SRA), and their properties were compared with those of Portland cement (PC) pastes. A liquid-to-binder ratio (L/B) of 0.33 was used for both PC and FG pastes. The drying condition of 25 °C and 50% RH was used for both free and restrained drying shrinkage tests. The results indicated that the Module and Concentration of activators affected all properties of FG pastes investigated. All FG pastes had a lower cracking tendency than the PC pastes, the latter of which cracked first although its shrinkage was not the highest. The drying shrinkage of FG pastes increased with decreasing Module and increasing Concentration. There was a critical pore size, d s , computed from both the water loss and pore size distributions, which was strongly related with free drying shrinkage of all the pastes studied. The smaller the d s value, the larger the free drying shrinkage a paste will have. SRA substantially coarsened pore structure, significantly reduced free drying shrinkage, noticeably reduced restrained ring shrinkage, distinctly delayed crack initiation time, and considerably decreased crack width of all pastes studied. Highlights • Shrinkage behavior of various fly ash based geopolymer (FG) pastes was compared with that of Portland cement (PC) pastes. • The drying shrinkage of FG pastes increased with decreasing SiO2/Na2O mole ratios and increasing solute concentration. • All FG pastes displayed lower cracking potential than PC pastes. • Shrinkage reducing admixture (SRA) distinctly reduced shrinkage of all pastes. • Free drying shrinkage increases with decreasing critical pore size (ds) of the paste. [ABSTRACT FROM AUTHOR]

The present paper evaluates the influence of the amount of quartz and water/binder ratio on microstructure development in cement pastes. Samples without quartz and with 10%, 20%, 30%, 40% and 50% cement replacement by quartz and with water/binder ratio of 0.1, 0.2, 0.3, 0.4, 0.5, 0.7 and 1.0 were autoclaved at 220 °C for 8 h under a 2.1 MPa pressure. The microstructure was characterized by elemental distribution X-ray maps, XRD, SEM and FTIR and the hardness and elastic modulus of the cement matrix were determined by dynamic hardness testing. The results showed that a significant fraction of quartz flour reacted under hydrothermal conditions, tobermorite and xonotlite were detected in samples with high quartz content and traces of α-C 2 SH were present in samples with low quartz content. Samples with a w/b of 0.2 and 30–40% of cement replacement by quartz exhibited the higher values of hardness and elastic modulus. [ABSTRACT FROM AUTHOR]

An experimental investigation has been carried out to repair pre-existing cracks in mortar using the microbiologically induced calcium carbonate precipitation (MICP) technology. In the study, 20 cylinder mortar samples (50 mm in diameter and 40 mm in height) were split to have cracks with various sizes. Sixteen of the cracked samples, an average width ranging from 0.15 to 1.64 mm, were repaired using the MICP method, while four cracked samples, with an average width ranging from 0.17 to 1.72 mm, were soaked under distilled water. The water permeability and splitting tensile strength (TS) of these repaired mortars were tested. The amounts of calcium carbonate (CaCO 3 ) precipitated on the cracked mortar surfaces were evaluated. The morphology of the CaCO 3 was observed under a scanning electron microscope (SEM). The results indicated that the MICP repair technique clearly reduced water permeability of the cracked samples. While water-treated samples were too weak to test, the MICP-repaired samples had TS ranging from 32 to 386 kPa after 21 cycles of MICP solution treatment. A relationship between the TS and amount of CaCO 3 precipitated was observed for samples with an average crack width between 0.52 and 1.1 mm, which indicated that TS increased with the amount of CaCO 3 precipitated on the crack surfaces. The SEM revealed that the precipitated CaCO 3 had possibly two forms: vaterite and calcite. [ABSTRACT FROM AUTHOR]

In the present study, unprocessed phosphogypsum (PG), a byproduct generated by the phosphorus fertilizer industry, is mixed with conventional oil-well cement (CM) and ground-granulated blast-furnace slag (GGBFS or SG), along with other activators and admixtures (such as silica fume (SF), retarder (USZ), and fluid loss control additive (BXF200-L)), to form a PGS slurry for oil well cementing. The mix proportion was optimized step-by-step according to the key properties of hardened PGS slurry, such as compressive strength, linear expansion, porosity, and permeability. The results indicate that the optimized PGS slurry (PGS-optimal slurry), made with the neat PGS:SF:USZ:BXF200-L (by weight) = 100:6:0.4:2, w/c = 0.44, where the neat PGS is consisting of a ratio of PG:SG:CM = 50:20:30 (by weight), had met major technical requirements for oil well cementing operation. Advantageous over the hardened CM slurry, the hardened PGS-optimal slurry generates significant expansion during hydration, thus providing the hardened slurry with excellent shrinkage compensation capability. The hardened PGS-optimal slurry has lower porosity, especially lower amount of harmful pores (>100 nm), than the hardened CM slurry. Consequently, the hardened PGS-optimal slurry also displays much lower permeability. [ABSTRACT FROM AUTHOR]

Predicting transient temperature developments of adiabatic calorimetry using experimental data of heat evolution of isothermal calorimetry can save engineering costs and improve efficiency. We proposed a multilayer physical model and mathematical solution to carry this task based on the laws of mass action and thermodynamics. We proved that the model is adequate and reported simulation results for cementitious materials. [ABSTRACT FROM AUTHOR]

Abstract: Raman spectroscopy has been employed to study the evolution of sulfo-aluminate and hydroxyl phases in pastes made with ordinary Portland cement (OPC) and fly ash (FA). Three fly ashes with different CaO contents were used as a cement replacement at the level of 0 and 50% by weight. The pastes were analyzed at 0, 0.2, 2, 4, 8, 12, 16, 20, 24, 48, 72h, and 7, 14, 21, 28, and 56days after mixing. The wavenumber ranges used for Raman spectroscopic analysis are 950–1050cm−1 for evolution of sulfates and 3600–3700cm−1 for evolution of hydroxides. Gradual disappearances of gypsum in parallel with the formation of ettringite (AFt) are clearly observed in most paste mixes. Evolution of hydroxides showed the gradual spatial growth of portlandite. In addition to the potential benefits, the limitations of using Raman spectroscopy in study of cement-based materials are also noted in this paper. [Copyright &y& Elsevier]

Abstract: The influences of colloidal nanoSiO2 (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO2 had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO2 on cement/FA hydration at later ages were proposed. [Copyright &y& Elsevier]

Abstract: The properties of cementitious materials in their dry state and their influences on viscosity of pastes were studied. The cementitious materials considered were Portland cement, fly ash, and ground granulated blast furnace slag (GGBFS). Both plain and blended mixes were tested, where plain mixes were made of a single material, while blended materials were made of the Portland cement blended with either fly ash or GGBFS. The properties tested for dry cementitious materials included the compression, recompression, swell indices, stiffness modulus, and coefficient of friction as well as the density and void ratio of gently consolidated bulk materials. Viscosity of the pastes, made with the plain and blended cementitious materials at a water-to-binder ratios (w/b) of 0.45 and 0.55 by weight, were measured. Correlations between the properties of the dry materials and viscosity of the pastes were analyzed. The results indicate that the size, shape and angularity of the cementitious materials greatly affect their dry-state properties as well as the flow behavior of the pastes. Cementitious materials that consolidate well in their dry state will produce a less viscous paste. A decrease in dry coefficient of friction and an increase in stiffness modulus will result in a decrease viscosity of the paste. [Copyright &y& Elsevier]

Abstract: The effects of ground clay brick (GCB) on alkali-silica reaction (ASR) expansion as well as on mechanical properties of ASR-affected concrete are investigated. Crushed red clay brick originated from demolished masonry was ground in a laboratory ball mill and replaced for portland cement at levels of 15% and 25% by weight in concrete mixes produced with alkali reactive sand. ASR expansion, compressive strength, flexural strength, and modulus of elasticity of the concrete mixes were evaluated. Effect of GCB on ASR gel chemistry was also studied on Pyrex glass-paste specimens using SEM/EDS (scanning electron microscope equipped with energy dispersive X-ray spectroscopy). The results indicate that GCB effectively reduces ASR expansion in concrete: associated cracking and loss on mechanical properties are also significantly reduced. SEM study suggests that GCB alters alkali silica gel chemistry thus resulting in a less expansive product. [Copyright &y& Elsevier]

Abstract: The shear behavior of fresh mortar is investigated using a force balance approach. In this approach, fresh mortar is considered as a two-phase material containing a matrix of cement paste and a group of rigid, spherical, non-cohesive aggregate particles. The shear resistance of a micro unit mortar is first assessed based on the friction between two contacted particles and the shear force carried by the cement paste of the micro unit mortar. The shear force of a macro-unit mortar is then calculated based on the number of the contacted aggregate particles in the mortar and the shear force carried by the rest of the cement paste. Forty-seven fresh mortar mixtures made with aggregate and paste having different characteristics and mix proportions are tested using a direct shear apparatus. The study indicates that the shear behavior of mortar follows the Mohr–Coulomb equation. The newly developed model can well predict the shear behavior of mortar materials and explain the direct shear test results. [ABSTRACT FROM AUTHOR]

• Dolostone (DS) concrete mixes showed a greater gain in surface resistivity (SR) after 28 days. • The DS aggregate-paste interaction was responsible for the rapid SR gain. • The DS aggregate-cement reaction produced calcite deposits and secondary ettringite crystals. Surface resistivity (SR) test measures the extent of ionic transport in cementitious systems. The present study sought to understand the influence of coarse aggregate type (limestone, dolostone), their chemical composition, and pore structure on the SR of high-performance concrete (HPC). In this study, water absorption and pore structure of five different coarse aggregates were measured. Concrete specimens were made with identical materials, mix proportions, and preparation methods, except for the difference in the sources of the coarse aggregates. Pore structure and SR of the concrete and the corresponding mortar were evaluated. The microstructures, especially the aggregate-paste interfaces, of the concrete were examined using scanning electron microscopy (SEM). The micromorphology of the specimens was also observed using thin section petrographic analysis. The results indicate that the SR of the concrete mixes decreased linearly (R2 >0.91) with the total porosity of all five coarse aggregate until 28 days. After 28 days, this trend no longer held likely due to the chemical interaction of dolostone aggregates with paste, while limestone aggregates did not react. SEM-backscatter (BSE) and SEM-secondary electron (SE) imaging coupled with energy dispersive X-ray spectroscopy (EDS) and thin section petrography suggested that dolostone aggregate-paste reactions resulted in precipitation of newer calcite deposits at aggregate-paste interfaces. This densified the interface and probably reduced the concentration of mobile ions in the pore solution within the concrete pore network resulting in higher SR after 28 days. Due to less stable nature of dolostone aggregates in highly alkaline conditions, HPC with dolostone aggregate also provided a conducive environment for the mineralization of secondary ettringite (calcium sulfoaluminate) within paste pore spaces. We contend that the greater SR gain by dolostone concrete mixes than that by limestone concrete mixes in later ages is explained by the combination of the aggregate-paste interface densification, reduction in ionic concentration of the concrete pore solution, and secondary ettringite mineralization. [ABSTRACT FROM AUTHOR]

Abstract: A particle–fluid model is developed for predicting the relationship between the shear stress and shear strain rate of highly flowable mortars. In this model, mortars are considered as a two-phase material, containing a fluid matrix (cement paste) and a group of well-graded, non-cohesive, and rigid particles (fine aggregate) that are uniformly distributed in the matrix. The mortar shear stress is assumed to be the sum of the shear stresses resulting from the paste flow, the aggregate particle movement, and the interaction between the cement paste and aggregate. The shear stress resulting from the paste flow is assessed using constitutive equations. The shear stress resulting from the aggregate particle movement is evaluated based on the probability and mechanical concepts of aggregate particle collision. The shear stress resulting from the interaction between the paste and aggregate is considered as the normal stress that the moving aggregate particles apply onto the cement paste. The shear rate of the mortar is obtained from the rheological definition of viscosity. Using this model, the effects of mortar mixture properties (such as aggregate size, volume, gradation, and friction as well as paste viscosity and yield stress) on mortar rheology are studied. [Copyright &y& Elsevier]

Abstract: The damaging impact of various deicing chemicals and exposure conditions on concrete materials was investigated. Five deicing chemicals (sodium chloride, calcium chloride with and without a corrosion inhibitor, potassium acetate, and an agricultural product) were studied. Freezing–thawing (F–T) and wetting–drying (W–D) exposure conditions were considered. Mass loss, scaling, compressive strength, chemical penetration, and micro-structure of the paste and concrete subjected to these deicing chemicals and exposure conditions were evaluated. Results indicated that the various deicing chemicals penetrated at different rates into a given paste and concrete, resulting in different degrees of damage. Among the deicing chemicals tested, two calcium chloride solutions caused the most damage. Addition of a corrosion inhibitor into the calcium chloride solution delayed the onset of damage, but it did not reduce the ultimate damage. Chloride-related deicing chemicals often brought about leaching of calcium hydroxide, as well as chemical alterations in concrete. Potassium acetate caused minor scaling, associated with alkali carbonation of the surface layer of concrete. Although producing a considerable number of micro-pores on the surface of the samples, the agricultural deicing product resulted in the least chemical penetration and scaling damage of paste and concrete. [Copyright &y& Elsevier]

Effects of curing temperature and NaOH addition on hydration and strength development of cement kiln dust (CKD)-fly ash (FA) binders were investigated. Pastes made with 50% CKD and 50% FA, having 0, 2, and 5% NaOH addition, and cured at temperatures of 24, 38, and 50 °C were evaluated. The hydration products of the binders were examined by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) tests. The results indicate that the major crystalline hydration product of the CKD-FA binders is ettringite, and the ettringite is stable in the CKD-FA system at age over 100 days. Curing at elevated temperature is more effective for CKD-FA binder strength improvement than NaOH addition, the later often depressing ettringite formation in a CKD-FA system. At a proper curing temperature (38 °C), addition of a small amount of NaOH (2%) may increase CKD-FA binder strength; while at a high curing temperature (50 °C), addition of NaOH (2%) may reduce the binder strength. [Copyright &y& Elsevier]

In this study, a highly flowable, rapid set crack repair grout powder (CRG) was used to facilitate mix design of 3D printing mortar. Various mortar mixtures with different amounts of CRG were evaluated for setting time, flowability, and printability. For the first time, pore/micro-structures of printed filaments and their bonds were examined in detail. The results have provided a better understanding of the anisotropic mechanical behavior of printed mortar. Plausible explanations have been made for the observations that printed samples displayed lower compressive but higher flexural strength than cast samples. It is demonstrated that CRG can be simply used as a raw material component to expedite the 3D printing concrete mix design process since finding the balance among various admixtures (e.g., water-reducing agent, thixotropic agent, accelerator, etc.) is not required. [ABSTRACT FROM AUTHOR]

A novel image analysis method integrated with SEM-EDS is proposed in this study for assessing the dispersion state of nanosilica (NS) particles in the hardened cement matrix. The method entailed capturing a statistically significant number of SEM micrographs of a cement-NS paste and their EDS elemental maps, the latter of which revealed that at 3-days, distinct C-S-H gel with a consistently low Ca/Si ratio (<0.9) had formed through pozzolanic reaction. Thereafter, pixels with Ca/Si ratio of 0.6–0.9 were extracted from a micrograph using a ternary plot of the atomic percentages of Ca, Si, and Al. Following this procedure, binary images were obtained, based on which, for the first time, the agglomeration and dispersion of NS particles in hardened cement paste were characterized using various quantitative descriptors, such as fractal dimension and Delaunay triangulation. A relatively small mean absolute error (1.16 vol%) validated the accuracy of the proposed methodology. [ABSTRACT FROM AUTHOR]

This paper provides an overview of the use of nanoclays in cement-based materials. The effect of nanoclays on shear rheological properties will be discussed, with a focus on structural build-up behavior. This macroscale flow behavior will then be tied to the effect of nanoclays on the fresh microstructure as characterized via in-situ particle size measurements and rheological techniques. This will provide insight on the mechanisms underlying the effect of nanoclays on fresh state properties through an improved understanding of their effect on flocculation and coagulation behavior. In addition, secondary effects nanoclays have on the hardening and hardened properties will also be discussed. Examples of casting applications in which nanoclays have been effectively utilized as rheological modifiers, i.e. reducing self-consolidating concrete formwork pressure, extrusion, semi-flowable concrete for slipform paving, and 3D concrete printing, will be presented. Finally, some thoughts on potential directions for future work will be shared. [ABSTRACT FROM AUTHOR]