Unhydrated Cement Particles Research Articles

New insights into autogenous self-healing in cement paste based on nuclear magnetic resonance (NMR) tests

The aim of this study is to investigate the effect of water migration from cracks into the bulk paste on autogenous self-healing. Nuclear magnetic resonance (NMR) technique was utilized to monitor water migration from cracks into the bulk paste during the process of autogenous self-healing. NMR results show that initially the water in the crack migrates into the bulk paste and the water content of the bulk paste increases significantly. However, after 5-h autogenous self-healing, the amount of non-chemically bound water in the bulk paste (adjacent to the crack surfaces) determined by NMR decreased instead. It indicates that some of the water coming from the crack was used for additional hydration of unhydrated cement particles in the bulk paste (during the process of autogenous self-healing). Before this study, in term of autogenous self-healing only the recoveries that related to the filling of cracks were concerned. The observation and quantification of densification of cement paste adjacent to the crack surfaces provides a new insight into autogenous self-healing.

Creep of 45 year old cement paste: the role of structural water

Microindentation creep and dynamic mechanical thermo-analysis measurements were carried out on compacted specimens of cement paste prepared at water/cement ratios of 1.00, 0.50 and 0.40 and hydrated for 45 years. Similar data was obtained for pastes prepared at w/c = 0.40 and hydrated only 2 months. Creep modulus–mass-loss curves were plotted for all the systems studied. These curves were used to emphasize salient differences between all the paste systems. The influence of degree of hydration, volume concentration of C–S–H, capillary porosity, C–S–H chain-length and the restraining effect of the unhydrated cement particles on creep are parameters that are used to provide a possible explanation for the observation that the creep of the old pastes is higher than creep of the young pastes. The structural role of water in the creep process at various stages of equilibrium drying from the 11 % RH condition is discussed.

Dimensional stability of 1·4 nm tobermorite, jennite and other layered calcium silicate hydrates

Length-change measurements of 1·4 nm tobermorite, jennite and calcium silicate hydrate preparations (with varying calcium/silicon ratio) subjected to drying from the 11% relative humidity condition are reported. The dimensional stability investigation is focused on the role of adsorbed and interlayer water as shrinkage mechanisms involving menisci effects in capillary pores are eliminated in this humidity range. Length change against mass loss curves and the corresponding d002 basal spacing change (obtained by X-ray diffraction) with mass loss are assessed and inferences drawn regarding nanostructural models for calcium silicate hydrates in hydrated Portland cement paste. Length changes of the pure phases were significantly greater than those for hydrated cement paste due to the restraining effect of unhydrated cement particles and calcium hydroxide crystals in the paste. The dimensional stability and the role of the removal of ‘structural water' from the systems studied are discussed in terms of its relevance to cement-based materials.

Micro- and nano-scale characterization to study the thermal degradation of cement-based materials

The degradation of hydration products of cement is known to cause changes in the micro- and nano-structure, which ultimately drive thermo-mechanical degradation of cement-based composite materials at elevated temperatures. However, a detailed characterization of these changes is still incomplete. This paper presents results of an extensive experimental study carried out to investigate micro- and nano-structural changes that occur due to exposure of cement paste to high temperatures. Following heat treatment of cement paste up to 1000°C, damage states were studied by compressive strength test, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) atomic force microscopy (AFM) and AFM image analysis. Using experimental results and research from existing literature, new degradation processes that drive the loss of mechanical properties of cement paste are proposed. The development of micro-cracks at the interface between unhydrated cement particles and paste matrix, a change in C–S–H nano-structure and shrinkage of C–S–H, are considered as important factors that cause the thermal degradation of cement paste.

2D cohesive zone modeling of crack development in cementitious digital samples with microstructure characterization

This paper presents two-dimensional (2D) cohesive zone modeling (CZM) techniques to simulate microscale crack propagation with cementitious digital samples under various loading conditions. The 2D multiphase bilinear cohesive zone models were employed to investigate the fracture behavior within heterogeneous cementitious material samples. The microstructure of concrete and cement paste samples were characterized with scanned surface (SS) and scanning electron microscope (SEM) imaging techniques, respectively. The digital concrete sample was generated from gray-scale SS images in millimeter. The single-edge notched beam (SEB) simulation on crack propagation within aggregate-cement microstructure was favorably compared with the tested sample. The digital cement samples with pores and unhydrated cement particles were generated from SEM images in micron scale. The compact tension (CT) test was simulated to predict crack propagation through pores. The study finally simulated the internal frost damage caused by ice crystallization with the SEM digital sample. The pore pressure calculated from thermodynamic analysis was input for model simulation. The CZM predicted the crack initiation and propagation within cement microstructure. The favorably predicted crack paths in concrete and cement paste samples indicate the developed CZM techniques have the ability to capture crack initiation and propagation in concrete or cement microstructure system with multiphase and associated interfaces.

Effect of silica fume and fly ash on hydration and microstructure evolution of cement based composites at low water–binder ratios

The hydration process of ultra-high performance cementitious composite (UHPCC) is distinctly different from common concrete owing to its low water–binder ratio (w/b). The effects of silica fume and fly ash on the hydration process and microstructure were investigated via different characterization methods in this work. The results indicated that the hydration process could be accelerated at the beginning by addition of silica fume, and fly ash, on the other hand, retarded hydration mainly in the dormant and acceleration periods. When they were incorporated together in cement, the result differed significantly from cement hydration at higher w/b (>0.35). As a result of Ca(OH)2 consumption by pozzolanic reaction, the porosity reduced (<4%) and the interface strengthened as the curing time increased. Nanoindentation results revealed that most hydration products of UHPCC were UHD C–S–H and unhydrated cement. The unhydrated cement particles were surrounded by UHD C–S–H, implying that the UHPCC had excellent mechanical performance and durability.

Characterization and quantification of self-healing behaviors of microcracks due to further hydration in cement paste

It has been reported by previous studies that cracks in cementitious materials can be healed by further hydration of unhydrated cement particles. However, by now, neither the physicochemical process nor the potential of self-healing due to further hydration is completely understood. In this paper, in order to gain insight into self-healing by further hydration, healing behaviors due to further hydration were characterized and quantified. The mineralogy of healing products was qualitatively determined and the percentage of each mineral was specified. The formation of healing products as a function of time was quantified as well. Moreover, self-healing of microcracks was simulated by a reactive transport model. The calculated filling fraction by the healing products in microcracks was consistent with the experimental results. The healing process slowed down markedly after 300h. In addition, in younger cement pastes, larger amounts of unhydrated cement lead to greater filling fraction of microcracks.

Atomic force and lateral force microscopy (AFM and LFM) examinations of cement and cement hydration products

The objective of this work was to better understand how atomic force microscopy (AFM) and lateral force microscopy (LFM) techniques can be used as tools to understand the nanostructure and microstructure of cement and cement hydration products. AFM and LFM techniques were used on mortar samples to distinguish between CSH, CH, and unhydrated cement particles. The LFM technique appears to be more sensitive to topographic changes than conventional AFM and it can more clearly distinguish between the different phases at high magnification (low scan range). AFM could also be used to calculate the roughness of the interfacial transition zone (ITZ) between aggregate and the cement paste at different ages. The rough surface at the interface of the paste and aggregate is generally interpreted as higher porosity. It was found that a reduction in roughness (i.e., porosity) occurred for samples that were cured for a longer time which are consistent with the explanation of porosity.

Influence of mix composition on the extent of autogenous crack healing by continued hydration or calcium carbonate formation

Crack formation is an inherent property for cementitious materials, however, these materials also possess the ability to heal cracks in a completely autogenous way. Upon ingress of water, unhydrated cement particles, available at the crack faces, may further hydrate and cause crack healing. When both water and carbon dioxide become available inside the crack, healing occurs due to carbonation of leached calcium hydroxide.The influence of using alternative binder materials, such as blast furnace slag and fly ash, on the extent of autogenous crack healing was investigated in this study. Blast furnace slag and fly ash react slower compared to cement and thus more unreacted binder material remains available within the hardened matrix to cause crack healing by further hydration. However, slag and fly ash consume calcium hydroxide during their reaction and thus the potential for calcium carbonate formation may be reduced.Isothermal calorimetry and water permeability measurements showed that replacement of Portland cement by these supplementary cementitious materials resulted in higher degrees of ongoing hydration upon contact of hardened cement paste with water. The effect of adding blast furnace slag or fly ash on calcium carbonate precipitation was evaluated through microscopic analysis of the change in surface crack width of specimens subjected to wet/dry cycles. Cement replacement by blast furnace slag or fly ash did not seem to improve crystal precipitation. Besides, it was shown from this experiment that smaller cracks closed more completely and faster compared to larger cracks. Also when regain in strength of samples subjected to wet/dry cycles was evaluated, no differences were seen between the different test series.In general, it can be concluded that cement replacement by blast furnace slag or fly ash improves autogenous healing by enhancing further hydration, while it does not affect the amount of calcium carbonate precipitation. The advantageous effect is most pronounced when blast furnace slag is used as cement substitute.

Growth of Calcium Hydroxide Islands in Tricalcium Silicate‐Based Cements at Early Age

Microstructural characteristics of hydrated triclinic tricalcium silicate [C3S(t)], monoclinic tricalcium silicate (alite), and type I portland cement at ages between time zero and 28 days were observed using electron microscopy. Image analysis was used to follow the development of calcium hydroxide (Ca(OH)2) and calcium silicate hydrate at micro‐ and meso‐length scales. In all cases, easily recognizable islands of Ca(OH)2 were noted to form, though the morphology and rate of formation differed for the three cement types. The formation of these islands was found to influence hydration by hindering the reaction of unhydrated cement particles that are embedded within their largely Ca(OH)2‐bearing matrix. Such may be important features to include and validate in developing microstructural and other forms of computational models. The extent to which such microstructural features influence the local rates of reaction seems to depend, at least in part, upon the crystallography of the C3S.

The influence of admixed micelles on the microstructural properties and global performance of cement-based materials

In this study, the influence of the admixed micelles (poly(ethylene oxide)-block-polystyrene micelles) on the microstructure and hydration rate of cement paste and more global performance of mortar were investigated. In cement paste, the cement hydration rate was only slightly influenced by the admixed micelles; on the other hand, the micelles lead to a reduced porosity and a denser matrix. The nano indentation combined with MIP and microstrutural analyses proves that the admixed micelles lead to a more uniform distribution of hydration products. In mortar, the compressive strength of the micelle-containing mortar was almost unaffected; the coefficient of water permeability was 3 orders of magnitude lower for the micelle-containing specimens, compared to the control (micelle-free) specimens. All experimental results indicate that the micelles act as nucleation sites for the formation of new hydration products and possibly induce defloculation of un-hydrated cement particles, resulting in a more uniform and homogeneous cement matrix.

순환잔골재와 플라이애시를 사용하는 무 시멘트 모르타르의 특성에 미치는 순환골재 미분말의 영향

The purpose of this study was to investigate the influence of recycled aggregates powder (RP) contents on recycled aggregates (RA) using fly-ash (FA) mortar in a condition of zero cement targeting earth filling materials, and the results can be summarized as follows. First, there was a tendency that as RP contents increased, W/B and air contents increased also. In the case of compressive strength, the strength was hardly developed at the early age, which was prior to 14 days; however, with the starting point of 14 days of age, strength of around 1.5~2.0 MPa was developed when it arrived at 28 days. At a curing temperature of , the more RP contents increased, the more the compressive strength increased in comparison with FA 100% at all levels except RP 100 %, showing the highest compressive strength at RP 25 %. At a curing temperature of , the temperature-dependence appeared to be large, as the RP contents were decreased compared to the curing temperature of . In addition, based on SEM analysis, this study was able to confirm that a pozzolanic reaction formed by an alkali stimulus of RA with the lapse of certain days even in 100 percent FA, causing the densification of tissues, and with RP 25%, hydrate was created the most densely by the hydration of unhydrated cement particles and pozzolanic reaction of FA.

Simulation of self-healing by further hydration in cementitious materials

Cracks, caused by shrinkage and external loading, facilitate the ingress of aggressive and harmful substances into concrete and indeed reduce the durability of the structures. It is well known that self-healing of cracks can significantly improve the durability of the concrete structure. In this research, self-healing of cracks was proposed to be realized by providing extra water for further hydration of unhydrated cement particles. In order to provide theoretical guidance for the practice, self-healing by providing extra water to promote further hydration was simulated. The simulation was based on water transport theory, ion diffusion theory and thermodynamics theory. In the simulation, self-healing efficiency under different conditions as a function of time was calculated. The relationship between self-healing efficiency and the amount of extra water from the broken capsules was determined. According to the results of the simulation, the amount of extra water can be optimized by considering self-healing efficiency and other performances.

Elastic modulus change investigation of cement paste before and after carbonation using nanoindentation technique

The elastic modulus of OPC cement pastes before and after carbonation was tested by nanoindentation technique. The test results show that carbonation reaction causes significant reduction of the frequencies of elastic modulus in the 7∼34GPa and 83∼160GPa, meanwhile a dramatic increase of the frequency of elastic modulus in the 34∼83GPa. Both portlandite, C-S-H gel and unhydrated cement particles are susceptible to carbonation. The elastic modulus frequency value of LD C-S-H and HD C-S-H after carbonation decreases by 85.4% and 55.5%, respectively. LD C-S-H is more heavily affected by the carbonation degradation compared to HD C-S-H. Before carbonation reaction, the size of unhydrated cement particles is about 15∼30um, and the hydrated products wrap around the unhydrated cement particles and the elastic modulus decreases progressively outward from the center of cement particles. After carbonation reaction, the number and size of unhydrated cement particles decreased obviously.

Modeling of self-healing efficiency for cracks due to unhydrated cement nuclei in hardened cement paste

Nuclei of unhydrated cement particles left in hardened cement paste after maturation can provide self-healing capability to micro-cracked matrix under favorable conditions. In this present study, a model study is conducted to characterize the influence of hydration reaction of the unhydrated cement nuclei on self-healing efficiency of cracks in hardened cement paste. Based on two different cracking modes of unhydrated cement nuclei, i.e., the splitting crack mode and the dome-like crack mode, the mathematical expressions on the self-healing efficiency are presented from the viewpoint of geometrical probability. Recurring to a generalized hydration reaction model of cement particles, the self-healing efficiency model quantitatively consider the influence of the volume fraction, particle size distribution and cracking modes of unhydrated cement nuclei randomly distributed in hardened cement paste.

Fractal Analysis on Microstructure Characteristics of Concrete Materials

The fractal characteristics of concrete materials microstructure are analyzed with fractal theory. The fractal dimension of concrete materials microstructure in the different conditions are tested and counted by code and box Dimension. The result shows that the fractal characteristics of concrete materials microstructures are highly notable, as interface of cement clinker mineral, dispersed state of unhydrated cement particles, crack of hardened cement paste, hydration product and pore structure of hydrated cement paste section. Proving it is effective to measure surface of cement clinker mineral, figures of hydration C-S-H grains and outlines, pore structure of concrete section. Using box dimension is effective to test microcracks and pore structure of section of motor samples and fiber-hydrate. In the course of fractal analysis, the dimensions of fractal are bigger than their topologic dimension, and they are smaller than their space dimension, they are keeping with the fractal theory.

Self-healing of polymer modified concrete

Self healing phenomenon of concrete has been observed in traditional, fibrous, self compacting concrete. This phenomenon occurred mainly due to the presence of unhydrated cement particles in the presence of water. Mechanism of polymer in concrete depends on creating a layer and net of polymer around cement particles which enhances the properties of polymer modified concrete. This mechanism may affect the self healing of this type of concrete. This work aims to study the presence of the self healing phenomenon in polymer modified concrete and the related parameters. An experimental investigation on self healing of polymer modified concrete was undertaken. In this research work, effect of polymer type, polymer dose, cement content, cement type, w/cm ratio and age of damage were studied. The healing process extended up to 60 days. Ultrasonic pulse velocity measurements were used to evaluate the healing process. Results indicated that, the self healing phenomenon existed in polymer modified concrete as in traditional concrete. The increase of polymer dose increases the healing degree at the same healing time. This increase depends on polymer type. Also, the decrease of w/cm ratio reduces the self healing degree while the use of Type V Portland cement improves the self healing process compared with Type I Portland cement. Cement content has an insignificant effect on healing process for both concrete with and without polymer. In addition, the increase of damage age decreases the efficiency of self healing process.

Microstructural Analysis of Paste and Interfacial Transition Zone in Cement Mortars Modified with Water-Soluble Polymers

The presence of water-soluble polymers affects the microstructure of polymermodified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks.

Analysis of strength development in cement-stabilized silty clay from microstructural considerations

This paper analyzes the strength development in cement-stabilized silty clay based on microstructural considerations. A qualitative and quantitative study on the microstructure is carried out using a scanning electron microscope, mercury intrusion pore size distribution measurements, and thermal gravity analysis. Three influential factors in this investigation are water content, curing time, and cement content. Cement stabilization improves the soil structure by increasing inter-cluster cementation bonding and reducing the pore space. As the cement content increases for a given water content, three zones of improvement are observed: active, inert and deterioration zones. The active zone is the most effective for stabilization where the cementitious products increase with cement content and fill the pore space. In the active zone, the effective mixing state is achieved when the water content is 1.2 times the optimum water content. In this state, the strength is the greatest because of the highest quantity of cementitious products. In the short stabilization period, the volume of large pores (larger than 0.1 μm) increases because of the input of coarser particles (unhydrated cement particles) while the volume of small pores (smaller than 0.1 μm) decreases because of the solidification of the cement gel (hydrated cement). With time, the large pores are filled with the cementitious products; thus, the small pore volume increases, and the total pore volume decreases. This causes the strength development over time.

Hydration Characteristics of Calcium Silicate Cements with Alternative Radiopacifiers Used as Root-end Filling Materials

Introduction Mineral trioxide aggregate (MTA) is composed of calcium silicate cement and bismuth oxide added for radiopacity. The bismuth oxide in MTA has been reported to have a deleterious effect on the physical and chemical properties of the hydrated material. This study aimed to investigate the hydration mechanism of calcium silicate cement loaded with different radiopacifiers for use as a root-end filling material. Methods Calcium silicate cement loaded with barium sulfate, gold, or silver/tin alloy was hydrated, and paste microstructure was assessed after 30 days. In addition, atomic ratio plots of Al/Ca versus Si/Ca and S/Ca and Al/Ca were drawn, and X-ray energy dispersive analysis of the hydration products was performed to assess for inclusion of heavy metals. The leachate produced from the cements after storage of the cements in water for 28 days and the leaching of the radiopacifiers in an alkaline solution was assessed by using inductively coupled plasma. Results The hydrated calcium silicate cement was composed of calcium silicate hydrate, calcium hydroxide, ettringite, and monosulfate. Unhydrated cement particles were few. No heavy metals were detected in the calcium silicate hydrate except for the bismuth in MTA. Calcium was leached out early in large quantities that reduced with time. The barium and bismuth were leached in increasing amounts. Copper was the most soluble in alkaline solution followed by bismuth and barium in smaller amounts. Conclusions The bismuth oxide can be replaced by other radiopacifiers that do not affect the hydration mechanism of the resultant material.