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In this study, the autolytic clinker microsphere with clinker as core and polyvinyl pyrrolidone (PVP) as coating film was prepared. Pretreatment of clinker with silane coupling agent was firstly processed during the preparation. To investigate the autolytic mechanism, the microstructures of the autolytic clinker microsphere at different curing ages were observed using environmental scanning electron microscopy (ESEM), equipped with an energy dispersive spectrometer (EDS). The autolytic stages were also identified based on the microstructural evolution. The influence of pretreatment degree on autolytic behavior was also studied by measurements of micro-morphology and isothermal calorimetry. Experimental results indicated that the compressive strength recovery of specimens was increased by 15–19% due to the addition of autolytic clinker microspheres. The recovery of compressive strength was also improved with the increase of pH value. The improvements in compressive strength recovery of specimens with microspheres were in the range of 15–19%, 15–31%, 25–36%, and 29–50% with the pH value of 7, 8, 10, and 12, respectively. It was also found that inner damage of cement-based matrix had greater recovery when pre-cracked specimens were cured in alkaline environments. [ABSTRACT FROM AUTHOR]

This paper investigates damage and healing of cementitious materials based on fly ash (FA) with a special focus on the effect of repeated loading on the self-healing performance. In total, 90 prism specimens and 33 cylinder specimens were prepared and tested. Variables including the FA replacement ratio and preloading level were considered. Different methods were adopted to evaluate the self-healing performance, i.e., mechanical tests, acoustic emission (AE) analysis, water absorption measurement, and scanning electron microscope (SEM) observation. Test results revealed that an appropriate amount of cement replaced by FA was beneficial to the self-healing performance of the specimens subjected to a high level of repeated preloading, where the strength increased by 11% and the water absorption rate decreased by 12%, in comparison with that of the undamaged specimens. This was attributed to the active hydration of FA, which was confirmed by an energy dispersive X-ray spectroscopy (EDS) analysis. This study extends understanding of the self-healing performance when subjected to repeated loading and provides some useful suggestions for the self-healing technique based on mineral additives. [ABSTRACT FROM AUTHOR]

Self-healing of cracks in cementitious composites is of great significance to improve the serviceability of concrete structures. In this study, poly urea–formaldehyde (PUF) microcapsules enclosing epoxy resins were synthesized. The damage- and healing process of cement paste incorporating microcapsules was in situ detected by acoustic emission (AE) technique, in which passive AE and active AE were combined to provide complementary information about the damage and crack formation. The two representative AE signals, i.e., matrix cracking and debonding of the interface, were used as calibration for further AE post-processing analysis. The effects of the concentration of microcapsules and the level of pre-damage were investigated. The results revealed the distinguished cracking mechanisms according to the differentiated feature of the signals in terms of the temporal and spectral AE descriptors. The plot of average frequency (AF) versus RA index (rise time/amplitude) confirms that the cracking modes contribute to the characteristic spectrum. [ABSTRACT FROM AUTHOR]

The effective mechanical properties of matrices healed by embedded microcapsules were investigated using Eshelby theory based on the microscale healing mechanisms, referring to the mechanical properties of the adhesive and the proportion of cracks that could be healed. This was determined by measuring the water absorption and vacuum saturation of precracked matrices. The microcapsules or cracks should not exceed 4 vol.% to ensure that approx. 90% of the modulus is retained even if the cracks cannot be reattached at all. Both the healed cracks and unhealed cracks contribute to the anisotropy, with a greater contribution from the unhealed cracks. The elastic modulus of the adhesive has a more significant influence on the anisotropy as the applied load increases. The measured stiffness recovery indicates that the cracks were only partially healed. This value corresponds to a healing efficiency index of 0.2–0.4 for the present conditions. [ABSTRACT FROM AUTHOR]

The effects of the addition of minerals of various compositions (i.e., silica-based materials, chemical expansive agents, swelling minerals and crystalline components) on the self-healing performances of cementitious materials were investigated. The self-healing capabilities were assessed by performing water permeability tests, quantifying the widths of the surface cracks and studying the water absorption of mortars that were pre-cracked by either splitting or compression. The results showed that the cracks that appeared early on healed more efficiently when they were cured in still rather than flowing water. High pHs and high temperatures accelerate crack healing. The healing efficiency can be further improved by utilizing a combination of minerals rather than a single mineral. A self-healing mechanism was discussed by combining these results with micro-observations. The precipitation of calcium carbonate, which is aided by higher pH values and higher calcium ion contents, was found to be the main contributor to the healing of surface cracks. [ABSTRACT FROM AUTHOR]

Capsule-based self-healing of cementitious materials is a promising solution to improve the serviceability and to prolong the longevity of concrete structures. The successful use of mechanical triggering microcapsules to achieve cracks healing is still challenging and is closely dependent on the breakage of capsules upon crack formation. In this study, self-healing microcapsules were synthesized via complex coacervation using a styrene maleic anhydride (SMA) shell and an epoxy resin adhesive. The SMA/gelatin bi-layer wall formed with gelatin as the out layer for better trigger sensitivity under crack formation. The effects of various preparation parameters, referring to stirring rate, temperature, core-shell ratio and pH, have been investigated by characterizing the microcapsule morphology, shell thickness, size distribution, and chemical nature. The morphology demonstrates the encapsulation ability of SMA/gelatin and the spherical shape facilitates the rupture under crack formation. The elastic modulus and hardness of the microcapsules were determined using nano-indentation to assess the interfacial bonding and likelihood of mechanical triggering. Self-healing functionality of the developed microcapsules were validated in paste according to the in situ Acoustic Emission measurement. [ABSTRACT FROM AUTHOR]

This paper presents a bio-inspired autolytic mineral self-healing method for cementitious materials. The autolytic mineral microsphere (AMM) consists of porous silica microsphere adsorbed with sodium silicate as healing agent and polyvinyl pyrrolidone (PVP) as resoluble coating material which will be autolyzed gradually under alkaline solution. The porous silica microsphere and coating film were prepared by sol-gel method and solvent evaporation respectively. The microstructure of AMM were characterized by environmental scanning electron microscopy (ESEM) and specific surface & pore size analysis instrument. The PVP thickness was measured by transmission electron microscope (TEM). Autolytic behavior of AMM was studied using Ultraviolet–visible (UV-VIS) spectrophotometer under different pH values, temperature and PVP thickness. Experimental results show that autolytic behavior is mainly controlled by PVP thickness, and the dissolution rate of sodium silicate is affected by pH values and temperature. Compressive strength recovery of cementitious materials is greatly enhanced by adding AMM. [ABSTRACT FROM AUTHOR]

Highlights • The self-healing capability of steam-cured mortar was investigated for the first time. • AE signals can distinguish damage stage between healing products and host matrix. • The amount and energy of healing products can be qualitatively assessed by AE signals. Abstract The objective of this work is to investigate the self-healing performances of steam-cured mortars with mineral additives (e.g., calcium-based materials, chemical expansive agents and crystalline components). The self-healing capabilities of crack was evaluated by transport tests and acoustic emission (AE) analysis for cracked mortars. The two representative AE parameters, i.e., amplitude and energy, were used to reveal the characteristics of healing products. The results showed that the improvement of transport properties of cracked steam-cured mortars were more effective compared to standard-cured mortars. The healing efficiency can be further improved by utilizing mineral additives. The differences of damage stage between healing products and host matrix can be distinguished by AE signals. Upon reloading of steam-cured mortar with mineral materials, the number of amplitude were much larger and the cumulative energy was much higher compared to other mortar. It indicated that the steam-cured mortar with mineral materials possessed larger amount and much higher energy healing products in the crack to some extent, which were well consistent with that of transport properties and microscopic observation. [ABSTRACT FROM AUTHOR]

This work proposes a novel insight into the autolytic mineral self-healing strategies for cementitious materials. The autolytic mineral microsphere (AMM) was prepared via the fluidization technique with cement clinker/calcium oxide as the healing substances and amino trimethylene phosphonic acid as the autolytic coating film. The basic characteristics of AMM and its hydration influence on the cementitious composite were analyzed by the laser particle analyzer, calorimetry, X-ray diffractometry and scanning electron microscopy. The autogenous self-healing capacity of the cementitious composite incorporating AMM was assessed by the crack closure, flexural property restoration and electrochemical impedance spectroscopy. Through the autolytic procedure, AMM regulated the hydration evolution and enhanced the self-healing performance of cementitious composite. The AMM containing cement clinker steadily healed the crack, leading to a well regeneration of fractured structure and restoration of flexural property. The partial substitution of cement clinker by calcium oxide in AMM further increased the crack filling, but induced a volume instability of the cement matrix as well, resulting in the strength decrease under a long-term secondary hydration. The structural alteration of cementitious composite was predominately impacted by the self-healing effect in the first 28 d re-curing, whilst the volume instability effect played a dominant role subsequently. [ABSTRACT FROM AUTHOR]

Secondary hydration is the main driving force of autogenous self-healing and directly impacts the closure of cracks in damaged cementitious materials. In fiber-reinforced cement composites, secondary hydration also dominates the regeneration of the fiber-matrix interface during the self-healing processes. As an important factor influencing the cement hydration, silica fume (SF) addition can be crucial to the self-healing performance of cementitious materials and composites. In this paper, the effect of SF on the autogenous self-healing of ultra-high-performance fiber-reinforced concrete was investigated. With a ≥20% SF addition, the secondary hydration the cement matrix adjacent to the cracks was accelerated. The dissolution of amorphous silica and the formation of Si-containing healing phases was observed. These effects predominated the closure of the mouth and the inner part of the cracks. SF addition also enhanced the regeneration of fiber-matrix interface by promoting the growth of self-healing products in these areas. The closure of cracks and the regeneration of fiber-matrix interface are both responsible for the restoration of flexural properties. • The secondary hydration acceleration of the cement matrix after cracking was found with a ≥20% SF addition, which promoted the self-healing of the cracks in UHPFRC. • The effect of SF on the structural regeneration of the cracks in UHPFRC was elucidated with a self-healing mechanism of the amorphous Si dissolution. • The self-healing of the deep matrix crack and fiber-matrix interface together induced the flexural property restoration of the SF-containing UHPFRC. [ABSTRACT FROM AUTHOR]

• A novel rapid crack-healing approach is proposed by electric-induced in-situ synthesis of ettringite. • Electric-induced in-situ synthesis of ettringite to heal concrete cracks is feasible under the asymmetric healing condition. • Electric field-induced ettringite can significantly shorten the healing time. • The rapid healing mechanism was disclosed through both the macroscopic and microscopic tests. Cracking is inevitable in concrete and can seriously threaten the durability of the structures. This paper proposed electric-induced in-situ synthesis of ettringite as a crack healing strategy for concrete structure. Composite solutions of aluminum sulfate and calcium acetate with four different molar ratios of Ca to Al (1, 3, 5, 7) were used as electrolyte solutions. The effectiveness of healing was evaluated by crack closure ratio, permeability, and coverage ratio of the crack section, while the composition and microstructure of products were also investigated. From the results, when the Ca-Al ratio is 3 or 5, the crack closure time shortened to 1 day, and the permeability coefficient reduced by nearly 4 orders of magnitude compared to the unrepaired specimen after healing for 7 days. Of all the groups, the products penetrated the entire crack, and the coverage ratio of crack section can reach more than 60 % when the Ca-Al ratio is 5. Additionally, ettringite was detected as the main phase of products by X-ray diffraction and thermogravimetric analysis (TGA). Needle-shaped ettringite can be observed during the whole healing period through scanning electron microscopy (SEM). In-situ synthesized ettringite crystals evolved from small and short prismatic shapes to long and acicular shapes. These crystals overlapped with each other and formed a dense network structure. [ABSTRACT FROM AUTHOR]