Chemical Shrinkage Research Articles

Effects of fineness and content of pozzolan on mechanical property, microstructure, and early-age chemical shrinkage of cement-based material

Natural pozzolan is a promising supplementary cementitious material for hydraulic concrete due to the pozzolanic effect, especially in the area where fly ash is scarce. In this paper, the physical properties of pozzolan were evaluated. The effects of pozzolan fineness and content on the microstructure, compressive strength, and early-age chemical shrinkage of pozzolan blended cement pastes were investigated. The results showed that the addition of pozzolan decreased the compressive strength of hardened cement pastes. The pozzolanic reactivity was closely related to the fineness of pozzolan, and higher pozzolan content led to lower strength. X-ray diffraction (XRD) and thermogravimetric (TG) analysis proved that Ca(OH)2 was consumed by reacting pozzolan, forming new calcium silicate hydrated (C-S-H). Besides, the addition of pozzolan caused the increase in porosity, which was reflected by the increase in macro-pores content. The chemical shrinkage of cement pastes was mainly determined by the hydration rate of cement, so the early-age chemical shrinkage was notably reduced after adding pozzolan due to its lower reactivity than cement clinker. In addition, finer pozzolan particles resulted in less macro-defects in the cement matrix, which enhanced the deformation resistance of hardened cement paste, and thus the chemical shrinkage was also decreased.

Exploring sulfate optimization techniques in Limestone Calcined Clay Cements (LC3): limitations and insights

This study assessed the feasibility of different methods for sulfate optimization of limestone calcined clay cements (LC3). LC3-based pastes were studied with kaolinite and montmorillonite calcined clays and different gypsum contents. Quartz powder was used in reference systems. The sulfate optimization was done using isothermal calorimetry, compressive strength, thermogravimetric analysis, and chemical shrinkage analysis at different ages. Overall, there is no single best technique to predict sulfate optimization in LC3, carefully combining multiple techniques can assist in more coherent decision-making, such as isothermal calorimetry with compressive strength within 72 h. We recommend the optimum LC3 sulfate content based on criteria established by the decision-maker (academia or industry). Either based on mechanisms and kinetics reactions, phase assemblage, compressive strength, or shrinkage of LC3 mixtures containing different SO3 content. Also, the results of montmorillonite-LC3 systems were satisfactory without accelerated sulfate depletion.

Hydration behavior and strength development of supersulfated cement prepared by calcined phosphogypsum and slaked lime

The effect of the content of calcined phosphogypsum (CPG) and slaked lime (SL) on the fluidity, setting time, and compressive strength of supersulfated cement (SSC) were investigated. The hydration mechanism of the SSC containing CPG and SL was examined through electrical resistivity, hydration heat, and chemical shrinkage measurements and thermodynamic modelling was performed to predict the phase assemblages. The results indicate that SSC with CPG delayed the early age hydration and prolongs its induction period. The effect of CPG on delaying hydration was apparent in SSC with 5 % SL. CPG in SSC promotes the formation of ettringite (AFt), while excess SL inhibits it. With the increase of CPG, massive gypsum is produced, leading to the rapid setting and a decrease in fluidity. Excess AFt contributes to expansion and cracking, resulting in a decrease in compressive strength. With the incorporation of SL, the pH value of the pore solution increases, which is beneficial to the stabilization of AFt, thereby increasing the compressive strength. The compressive strength of SSC containing 10 % CPG and 10 % SL reached up to 35.2 MPa after 180 d. The study reveals the mechanism of the effect of CPG and SL on SSC hydration and provides theoretical guidance for preparing SSC for engineering applications.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Integration of monitoring indicators for self-sensing alkali activated cementitious materials: From electrical signals - resistivity to autogenous shrinkage

The chemical shrinkage and autogenous shrinkage of alkali-activated materials (AAMs) are usually more significant than that of ordinary Portland cement (OPC). Therefore, both searching for efficient methods to reduce shrinkage and exploring the shrinkage mechanism of AAMs are of great significance in promoting the application of AAMs. In this paper, metakaolin is used to reduce the chemical shrinkage and autogenous shrinkage of AAMs, while an innovative resistivity measurement is used to concretize the electrical signal of AAMs and the relationship between autogenous shrinkage and resistivity was explored. The early resistivity and heat of hydration curves of AAMs can be divided into five stages with high similarity, demonstrating the feasibility of indirectly quantifying the autogenous shrinkage and chemical shrinkage by resistivity. The results of TG-IR, MIP, and heat of hydration prove that the early reaction rate of AAMs decreases with the increase of metakaolin content, which further leads to the decrease of early chemical shrinkage and autogenous shrinkage. The reduction of autogenous shrinkage reached the maximum (49%) when the metakaolin content was 50%. A mathematical model between autogenous shrinkage and resistivity was established, which means that the resistivity of self-sensing materials can be used as an index to evaluate the early autogenous shrinkage of AAMs.

Study of synergistic effect of fly ash and superabsorbent polymers on properties of cement pastes

The main goal of this study is to observe changes in microstructural properties and the mutual interaction of cement pastes containing superabsorbent polymers (SAPs) and fly ash. SAP is a type of hydrogel additive, able to absorb large amounts of liquid in its structure. In cement-based materials, it is often used in the early stages of matrix hydration to absorb excess mixing water and to release it in the later stages, which is even more desirable when the material is exposed to higher temperatures. Another research goal is a partial substitution of binder (Portland cement) by the energy by-product of high-temperature ash, produced by the combustion of brown coal. A further goal is the observation of the interactions of these binders with the SAPs on the microstructural scale. For this purpose, computed tomography (CT), X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscopy (SEM) were performed. Basic physical and mechanical properties such as compressive strength, flexural strength, and bulk density were also determined. In the early stages of hydration, viscosity and chemical shrinkage were determined. From the results obtained, it is obvious that the presence of SAPs has a significant impact on the properties of cement pastes. From the two tested SAPs, SAP HM has a positive effect on volume stability and the flexural strength of the composite. The results of the mineralogical composition showed that the effect of SAP incorporation influences the crystallization of portlandite, alite, belite, and calcite. SEM analysis confirmed the results of XRD analysis and proved that hydration products can grow into SAP cavities. It is clear from the evaluation of the pore structure that SAPs increase the porosity of cement composites. The admixture of high-temperature fly ash has a positive effect on reducing the volumetric changes of mixtures. The mechanical parameters of the composite with fly ash were similar to that of the reference test specimens.

Fluidity, mechanical properties, shrinkage of alkali-activated slag/stainless steel slag mortars with composite activators

In order to efficiently utilize steel slag and reduce carbon emissions, the performance of alkali-activated slag/stainless steel slag mortar (ASM) prepared with a composite activator is investigated. In this work, CaO, Na2CO3 and water glass (WG) are used as composite activators, where the alkali equivalents of CaO and Na2CO3 are maintained at 4% and the alkali equivalents of WG (i.e. WG content) as variables were 0, 0.5%, 1.0%, 1.5% and 2.0% respectively. The properties of ASM, including mechanical properties, autogenous and drying shrinkage, are investigated. Results show that the mechanical properties of ASM decrease and then increase as the WG content increases, with the worst mechanical properties of ASM occurring when the WG content is 0.5%. The autogenous shrinkage of ASM decreases as the WG content increases from 0 to 0.5% followed by an increase as the WG content increases further to 2.0%. Autogenous shrinkage depends on chemical shrinkage, small pore (<50 nm) volume, MCL and capillary pressure. When the WG content is increased from 0 to 0.5%, the hydration reaction is inhibited by the reaction of the WG and CaO which reduces the pH of activator solution and gel amount, leading to a reduction in chemical shrinkage, small pore volume, MCL and capillary pressure. However, the high WG content accelerates the hydration reaction when the WG content is greater than 0.5%, which increases the autogenous shrinkage. The drying shrinkage of ASM decreases and subsequently increases as the WG content increases. The higher drying shrinkage is associated with weight loss due to the excellent connectivity of the pores.The pore structure also affects the drying shrinkage, which can be justified on the basis of capillary pressure.

Multiscale study of the process deformation for SiC/SiC composite materials in PIP process

In this paper, the process deformation of 2D SiC/SiC composite materials in the pyrolysis stage of precusor infiltration and pyrolysis process (PIP) is investigated experimentally and computationally through a multiscale study. The pyrolysis process of PCS was described by a kinetic model. The intrinsic properties and behaviors of pyrolysed PCS were characterized experimentally. The macroscopic properties and behaviors of 2D-SiC/SiC composite materials were obtained through the homogenization method. The proposed methodology can successfully predict the process deformation of an L-shaped 2D SiC/SiC composite structure in cyclic PIP process. It is revealed that the pyrolysis of precusor can generate significant chemical shrinkage and stiffness variation, leading to the process deformation of the entire composite structure. Unlike the precusor, the variation of properties and behaviors of composite materials are anisotropic. Furthermore, the cyclic process and the evolution of matrix composition complicate the process deformation. The behavior of composite material is dominated by pyrolysing PCS at first several cycles whereas by pyrolysed SiC at last several cycles. The fundamental understating on process deformation in this paper may benefit the design and the manufacture of SiC/SiC composite materials.

Experimental study of pure Class G cement hydration up to 150 °C and 50 MPa

A comprehensive experimental investigation of Portland cement hydration kinetics in a wide temperature range from 35 °C to 150 °C with pressure up to 50 MPa was performed by heat of hydration, chemical shrinkage and ultrasonic measurements. Increasing curing temperature was found to have a stronger acceleration effect during the main hydration period while increasing curing pressure had a stronger acceleration effect during the pre-induction period of cement hydration. By comparing chemical shrinkage test results with other methods of estimating cement degree of hydration, such as quantitative X-ray diffraction, non-evaporable water content measured by thermogravimetric analysis, as well as heat of hydration, it was observed that total chemical shrinkage of cement at complete hydration decreased with curing temperature from 35 °C to 75 °C and then stayed stable between 75 °C and 150 °C. Similarly, the P-wave velocity of cement (which is an indicator of physical and mechanical properties) as a function of hydration degree decreased with increasing curing temperature in the range between 35 °C to 75 °C, then remained relatively stable between 75 °C and 130 °C. The reduction of gel water content in C–S–H from 35 °C to 75 °C is believed to be the primary cause of these observed phenomena.

Mechanisms of Chemical and Autogenous Shrinkage in Alkali-Activated Hybrid Systems

This study aimed to investigate the role of chemical and autogenous shrinkage as well as microstructure of a hybrid system composed of alkali-activated ash blended with portland cement. Three alkali-activated binders were produced with 30% portland cement and different types of ash (70% of bottom ash, 70% of rice husk ash, or 35% of each type of ash). Chemical shrinkage was measured by the buoyancy method. The microstructural and surface analysis were investigated by scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherms, Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. Autogenous shrinkage, rather than chemical shrinkage, was mainly responsible for the shrinkage observed in these sealed systems. The results showed that the main mechanism involved in the shrinkage of this hybrid system is associated with the microstructural reorganization of the aluminosilicates phases resulting from polymerization.

Optimization of cement quantity through the engineering of particle size distribution – a sustainable approach

Cement is the most used building material on the planet and is responsible for 8% of the total CO2 released. In the coming decades, due to increased population and infrastructure needs, a surge in cement demand is expected. To meet the demand without affecting the carbon foot-print the efficiency of cement needs to be enhanced. This paper presents a novel approach to achieve enhanced efficiency through the engineering of particle size distribution. Since raw materials used in cement production vary in quality across the globe, the only feasible method of enhancement is through modification of mechanical parameters such as particle size and distribution. The paper explains the details of an experimental study on cement and concrete using a “Stimulator” produced by grinding commercially available OPC. Stimulator content was varied between 0 and 100% in the interval of 10%. Various tests such as water requirement, setting times, the heat of hydration, chemical shrinkage and compressive strength parameters were conducted, thus determining the optimum dosage of the stimulator. Despite the reduction in the average particle size of cement by 38–47%, water requirement has shown a nominal rise of 10–17% for S40 to S50 cement mixes, respectively. Concrete Samples with the optimum dosage of stimulator depicted 20–25% lesser requirement of cement to achieve strength equivalent to that of concrete containing OPC only. This reduction in the requirement of cement quantity leads to lower cost and lower amount of CO2 released. Implementation of this method of cement production does not require the introduction of new machinery. Unlike pure OPC, the cement & stimulator mix portrayed a double peak PSD arrangement representing the participation of 2 sets of particles in the cement matrix sizes. Durability tests conducted on concrete samples with the optimum dosage of stimulator and OPC have depicted better durability than concrete samples with OPC only.

Chemical ddmixtures used in 3D printing of building materials

Chemical admixtures present the key to successful 3D printing technologies. Moreover, many similarities between admixtures for dry-mix mortar and 3D printed mortar exist. This paper highlights current technologies and provides an outlook into which technologies might be required in the future. Generally, the kind of admixtures differ, depending on the type of 3D printing applied: extrusion, shot-crete or printing in particle bed. As of today, about 90% of actual work uses extrusion printing. In extrusion, immediate stiffening of the mortar is required once it leaves the nozzle. To achieve such sudden structuration, liquid alumina cement or crosslinking polymer systems are applied. Moreover, in order to avoid the sag of printed mortar layers, thixotropic agents based on nanoclays including attapulgite or meta muscovite are admixed. Additionally, adhesive polymers based on synthetic latex copolymers facilitate sufficient cohesion between individual printed layers and provide the necessary form stability of the structure. Early strength development can be achieved by nucleation enhancing admixtures ("seeding materials") such as those based on C-S-H-PCE nanocomposites. The working mechanisms of those admixtures and the benefits achieved from them will be disclosed in the presentation. A major hitherto unresolved problem is excessive shrinkage of 3D printed objects. It derives from chemical shrinkage (most mortars are very high in cement content) as well as physical/dry shrinkage (the absence of formwork to protect the surface from desiccation being the main reason here). So far, no technically satisfying and economical solution in actual application (i.e. not under controlled laboratory conditions) has been found, which requires future development work. In the future, more work will be devoted to print actual concrete which should be the ultimate goal, as it by far represents the most attractive application. However, to print concrete with steel embedded currently is considered to be unfeasible. To solve this problem, completely new approaches need to be taken to devise a realistic, field- suitable method.

Micro-mechanical analysis of the effect of ply thickness on curing micro-residual stresses in a carbon/epoxy composite laminate

During the manufacturing of thermoset-based carbon fiber reinforced polymers (CFRPs), micro-residual stresses are developed in the material, due a mismatch of the chemical-thermal-mechanical properties of the fibers and the polymer matrix. This ultimately leads to a reduction in the mechanical performance of the composite material. In this work, a representative volume element (RVE) of a composite sub-laminate with a 90° ply discretized at micro-scale level, stacked within homogenized plies, is proposed to study the micro-residual stresses and the ply mechanical response considering the in-situ effect caused by the constraining adjacent layers. A model framework is developed using finite elements in Abaqus®/Standard with user-subroutines. The retrieved micro-residual stresses in the 90° layer are generated by the dissimilar chemical shrinkage and thermal expansions, plus the additional residual meso-scale stresses caused by the constraining effect of adjacent plies. The residual stresses in the constrained condition can achieve almost 80 % of the ply failure load for the 0/90/0 sub-laminate, meaning lower sub-laminate strain to failure. Thick laminates retrieve more residual stresses in the transverse 90° layer and exhibit lower constrained strength and more unstable post-failure response, demonstrating that thin-ply laminates exhibit beneficial constraining effects during the curing process.

Hydration and chemical shrinkage of Portland cement with bauxite residue from Bayer process

Hydration and chemical shrinkage of Portland cement with bauxite residue from Bayer process

Understanding the role of a novel internal conditioning technique with functionalized montmorillonite in cement hydration kinetics

A novel internal conditioning (InCon) technique based on saturated sodium montmorillonite (sMT) functionalized with two non-ionic surfactants, polyoxyethylene (9) nonylphenylether and t-octyl phenoxy poly ethoxyethanol, is investigated in this study. With the integration of water for internal curing and pozzolanic reactivity in a single system, the role of InCon in modifying cement hydration kinetics is comprehensively elucidated. The results indicate that, in the presence of InCon, both silicate reaction and secondary aluminate reaction rates are enhanced, and the apparent activation energy (Ea) of cement hydration was decreased from 34.3 KJ/mol to 28.7 KJ/mol indicating a lower temperature sensitivity and threshold of the cement hydration reactions. In addition, decreased CH contents, improved degree of hydration, increased chemical shrinkage, and the formation of additional CSH and aluminum-containing phases were obtained from the cement with InCon. The autogenous shrinkage of cement and the negative impact of dry sMT on the early age strength of cement can be offset by InCon paving a new path to improve the overall properties of concrete.

Ultra-high performance concrete prepared with ceramic polishing waste: Hydration, microstructure and mechanical property

This study aims to clarify the effects of ceramic polishing waste (CPW) on the hydration properties, microstructure development and mechanical properties of ultra-high performance concrete (UHPC). In present work, CPW was used as an auxiliary cementitious material to replace cement(C), fly ash (FA) and silica fume (SF). Firstly, the pozzolanic activity of CPW was examined via Frattini test method. Then, the hydration kinetics of UHPC with CPW was characterized by isothermal calorimetry, low-field NMR and chemical shrinkage. The hydration product and microstructure of UHPC with CPW were also studied by XRD, TGA and SEM. Finally, the effect of CPW substitute on workability and mechanical properties of UHPC was investigated. Results showed that CPW performed excellent pozzolanic activity, and it could participate in the hydration reaction at the late age. When the replacement ratio of cement with CPW was less than 20%, the compressive strength of UHPC was slightly decreased in the early stage, however, the strength could be comparable to that of UHPC without CPW at 56 d. Besides, it is noteworthy that the compressive strength of all UHPC with CPW exceeded 120 MPa at 28 d. This paper developed a novel application scenario for CPW that would help alleviate the problem of tight supply of raw materials such as SF and FA.

Performance of clinoptilolite zeolite after milling as a pretreatment on hydration kinetics, shrinkage, and alkali-silica reaction of cementitious materials

Natural clinoptilolite zeolite has been a popular supplementary cementitious material (SCM) due to its acceptable pozzolanic performance and the overall lower environmental footprint. Previous research established that milling is an effective pretreatment technique to further increase the pozzolanic reactivity of zeolitic tuffs leading to an increased specific surface area and amorphous contents. Therefore, the present study characterized the zeolite particles after ball milling for 1 and 3 h using phase analysis by X-ray diffraction (XRD), particle size distribution by laser diffraction, microstructural analysis by scanning electron microscopy (SEM), moisture absorption rate, and relative chemical dissolution. The performance of milled clinoptilolite zeolite as a SCM with the replacement of up to 20% portland cement was evaluated through hydration kinetics (heat of hydration, setting time, chemical shrinkage, degree of hydration), workability, compressive strength, autogenous shrinkage, drying shrinkage, and alkali-silica reaction (ASR). Results revealed that 1 and 3 h of milling led to an increased specific surface area, moisture absorption capacity, and relative dissolution of particles, but had no visible effects on the crystalline structure of zeolite particles compared to the unmilled zeolite particles. For the hydrated system, both 1 and 3-h milled zeolite increased the overall heat of hydration leading to an increased silicate and aluminate reaction along with the acceleration effects in the setting time. The compressive strength of up to 20% milled (1 and 3 h) zeolite samples was increased by about 20 to 25% compared to the unmilled zeolite samples at an early age which suggested an increasing pozzolanic response of milled zeolite particles in the system due to an increased volume of hydrated phases and degree of hydration. Milling slightly decreased the workability by demanding a higher content of fresh water which was released at a later age leading to a higher drying and autogenous shrinkage. In addition, milling reduced the internal curing capacity leading to damage to the porous structure of zeolite particles. The use of up to 20% 3-h milled zeolite reduced the deleterious expansion by about 80% due to ASR compared to the control sample and the overall performance of milled clinoptilolite zeolite as the SCM was satisfactory in the hydrated system.

Effect of diatomite on the reaction kinetics, early-age chemical shrinkage and microstructure of alkali-activated slag cements

Understanding and revealing the relationship between the early-age chemical shrinkage, reaction characteristic and microstructure of alkali-activated slag (AAS) pastes are of importance for the application of this cementitious material. This study investigates the effect of diatomite on the early-age compressive strength and the first 72-h chemical shrinkage of AAS pastes. A novel approach is proposed based on isothermal conduction calorimetry and 1H low-field nuclear magnetic resonance analyses to identify the internal correlation between chemical shrinkage and reaction situation. Results show that the early-age chemical shrinkage is dominated by the reaction process and reaction degree of AAS pastes. The incorporation of diatomite makes a great contribution to the control of chemical shrinkage even though it slightly sacrifices the development of compressive strength. This suggests the controls in the reaction process and chemical shrinkage of AAS pastes can be achieved by incorporating diatomite. The proposed approach can be used to assess chemical shrinkage of AAS, which contributes more to deeper understanding in the early-age volume change of alkali-activated cements.

Sustainable approach to simultaneously improve the pozzolanic activity of sugarcane bagasse ash and the vinasse fertilization potential

The purpose of this study was to evaluate the viability of sugarcane bagasse ash as a sustainable supplementary cementitious material and its association with improving the fertilizing properties of vinasse, used as a leaching agent. Thus, a bagasse ash with low quartz contamination was divided into three parts: one kept in natura, one submitted to citric acid leaching and the third to vinasse leaching. After production, which was concluded with re-calcination and ultrafine grinding, the three ashes were characterized according to their chemical and mineralogical composition, particle size, porosity, density, and pozzolanic activity. Paste hydration with 20 wt% cement replacement by each ash was assessed in isothermal calorimetry and chemical shrinkage tests. The elemental and oxide composition of vinasse before and after leaching was also obtained. The results showed that vinasse leaching promoted an increase in specific surface area and pozzolanic activity of ash, comparable to that observed for citric-acid leaching procedure. Tests with pastes containing vinasse-leached ash confirmed the improvement in cement hydration. In addition, after leaching, vinasse exhibited a decline in heavy metal concentration, increased pH (neutralization) and macronutrient concentration. Thus, vinasse leaching improved the pozzolanic characteristics of ash, making vinasse a more environmentally friendly fertilizer.

Post-repair residual stresses and microstructural defects in wind turbine blades: Computational modelling

Wind turbine blades require regular maintenance and repair. The post-repair reliability and lifetime depend on the quality and method of repair. Residual stresses present in the repaired sections of the blades are one of the reasons for reduced lifetime and premature failure of repaired blades. Residual stresses occur due to the thermal cycle used to cure the composite repair patch or adhesive and chemical shrinkage during curing. The presence of voids further aggravates the magnitude of the residual stresses. To investigate these effects, analytical models are used to study the cure kinetics and void evolution while the development of residual stresses during the post-gelation period of curing and the effect of the voids are obtained using coupled temperature-displacement analysis in ABAQUS. User subroutines (UEXPAN, HETVAL and UMAT) are used to define the chemical shrinkage, heat generation due to curing and cure-dependent modulus. Both hard and soft patch repair models are considered. It is found that the temperature cycle used affects both the time taken for complete cure and the residual stresses developed in the adhesive. Increasing the first dwell temperature decreases the time to cure but increases the residual stresses. Also, the presence of voids is found to cause stress concentration regions, with stresses increasing up to 2 to 2.5 times the average stresses. Therefore, a balance is needed to minimise the time taken for the repair process while maintaining residual stresses as low as possible, especially in wind turbine repair applications.