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Geopolymers, combining some characteristics of organic polymers, cements and ceramics for its special poly-condensed network structure, have received extensive attention of the researchers as a green cementitious material because of favorable and unique characteristics. This study mainly presents an overview on the progress of geopolymers research and development in recent decades. The works on raw materials, geopolymerization kinetics, the applications development and challenges of geopolymers are reviewed. This paper focuses on the applications progress of geopolymer as green civil engineering materials, rapidly-repair and dealing with poisonous and radioactive waste materials, coatings, novel ceramic materials, functional composite, three-dimensional printing materials and biological materials, etc. The main contribution of this review is to systematically summarize the complex and diverse of published results on geopolymers application in reality and present future challenges, which is beneficial to the further research activities of civil engineers, environmental scientists, chemists and materials scientists. The current investigation indicates that the geopolymers not only exhibit widespread application prospect to be used as sustainable construction materials, but also have great potential in the fields of three-dimensional printing materials, biological materials and other emerging geopolymer functional materials.

Recently, with the development of accelerating mineral carbonation technology, the preparation of building materials by using carbonation curing cement-based materials has attracted wide attention. The diffusion and transfer mechanism of carbon dioxide in porous cementitious materials was the key point that restricts the improvement of materials carbonation rate. In order to obtain comprehensive information that affects accelerating carbonation rate of porous cement-based materials, this paper talks about the influences of paste porosity controlled by water to cement ratio (w/c) on carbonation and the paste micro-structures evolution. Also, mineral compositions before and after carbonation curing were evaluated. Results show that the carbonation rate, carbonation depth and compressive strength are improved after carbonation, while total porosity of complete carbonation zone decreases with the growth of paste porosity. The pores with diameter larger than 200 nm played a leading role in carbon dioxide diffusion, which were reduced by 84.25% of the total volume after carbonation curing. The carbonization reaction mainly occurred in the first six hours in carbonation curing, while the subsequent carbonation curing were unprofitable to the carbonation effect due to the generation of calcium carbonate prevents the diffusion of CO2, making the subsequent carbonation become difficult. XRD results revealed that during carbonation curing the major reactants were C2S, C3S and some hydration products of Ca(OH)2 after 1 d hydration.

Hydration and micro-structure are important in the study of alkali-activated materials, especially used in building materials. To obtain information about hydration processes, we have investigated the hydration rate and micro-structural evolution. In this research, we evaluated the performances of nano-silica composite, which is produced by adding nano-silica into alkali activated slag. We also tested the impacts of nano-silica on hydration, micro-structure and porosity. Results of the experiments shows that the comprehensive strength is enhanced, the microscopic morphology and hydration characteristics are optimized. Also, hydration rate suggested that the impact of nano-silica on alkali activated slag early hydration is far greater than that of later hydration. Furthermore, the nano-silica action mechanisms on alkali-activated slag discussed in this paper mainly are that the nano-silica supplied siliceous precursor and served as nucleation seed and its accelerated early hydration effects.

Belite-ye’elimite-alite (BYA) cement is a type of an environment-friendly binder with less CO2 emission in manufacturing process and has the durative development of strength in hydration stage. In this paper, BYA clinkers were well prepared by adding barium oxide. A combination of X-ray diffraction (XRD), the Rietveld methodology, differential scanning calorimetry (DSC)-thermogravimetric (TG) analysis and scanning electron microscopy (SEM)-energy dispersive spectra analysis (EDS) was used to investigate the composition and quantitation of clinker phase. Strength tests were also conducted to study the mechanical properties in the hardened cement pastes. The results indicate that BYA clinker prepared at 1380 °C with 5 wt% of ferrite phase contained 25.6 wt% of ye’elimite coexisting with 9.3 wt% of alite. The characteristic diffraction peak of ye’elimite shifts to the left as barium replaces calcium in all the clinker specimens. In addition, the trace element of barium is more tend to incorporate into other clinker phases than alite. Compressive strength of BYA cement increases by 20.4 MPa from 3 d to 28 d. It is beneficial for the improvement of the mechanical properties of BCSA cement at medium hydration ages.

The efflorescence caused by carbonate weathering is one of the dominant causes of deterioration of alkali-activated inorganic materials. In order to inhibit the efflorescence of waste based alkali-activated inorganic binder, the effects of nano -silica on the compressive strength, carbonate ions concentration, hydration rate and pore size distribution of the obtained alkali-activated inorganic binder have been investigated, and the efflorescence inhibition mechanism has been also analyzed. The results revealed that compressive strength and microstructural properties could be further developed with inclusion of nano-silica in alkali-activated inorganic binder. In addition, the efflorescence decreased with increasing nano-silica content and decreasing of the nano-silica particle size. Besides, the hydration rate results indicated that the hydration of waste based alkali-activated inorganic binder was accelerated before hydration for 3d with the incorporation of nano-silica. The suggested inhibition mechanisms discussed were mainly; micro-aggregate filling effect, induced nucleation, and accelerated hydration effects of nano-silica.

Alkali-activated inorganic binder has been a research hot spot in the field of building materials owing to its wide range of raw materials, low energy consumption, less pollution and superior performances. The bicarbonate salt weathering is one of prime cause of deterioration of porous materials. To inhibit the efflorescence of waste based alkali-activated inorganic binder, 5A zeolite powder was blended and the efflorescence inhibition mechanism was analyzed. Effect of 5A zeolite content on the mechanical properties, bicarbonate ions concentration, pH value and pore size distribution of the obtained alkali-activated inorganic binder were investigated. The results showed that the compressive strength and bending strength of the waste based alkali-activated inorganic binder are improved with adding moderate content of 5A zeolite powder, and the pH value of specimens decreases with increasing amount of 5A zeolite powder content, the average pore size of samples decreases with adding moderate content of 5A zeolite powder due to its micro-aggregate filling effects. The efflorescence of waste based alkali-activated inorganic binder is effectively reduced with the adding of 5A zeolite powder, and the inhibition mechanisms mainly are that the special ion-exchange properties and micro-aggregate filling effects.

Nowadays, self-healing properties of concrete are emphasized in addition to the mechanical properties and durability, especially in the design of concrete for major project in harsh environment. Therein, aiming to improve intrinsic self-healing ability, aggregates gradation (AG) and sand ratio (SR) were adjusted, and the maximum aggregate size (Dmax) was optimized. The results showed that proper aggregate gradation and sand ratio not only contributed to high compressive strength, but also to enhance intrinsic self-healing ability. Especially for the concrete with small Dmax and the SR of 37%, the self-healing ability was improved according to the ultrasonic waveform results. Furthermore, the cracks of specimens with SR of 33% could be healed completely on the basis of the ultrasonic pulse velocity (UPV) and the measurement of crack’s closure. Better self-healing ability of concrete could be obtained by adjusting Dmax and SR properly. Thus, intrinsic self-healing ability should be taken as an index in the design and preparation of concrete.

Ultra-high performance fiber reinforced concrete (UHPFRC) is a new kind of cement-based composite material with high mechanical properties and excellent durability. However, the poor bonding properties between fiber and matrix in UHPFRC give rise to a weak fiber reinforcement effect. Therein, the surface of steel fiber was treated with γ-aminopropyl triethoxy silane to improve the interfacial bonding property of steel fiber. Optimal treatment of steel fibers with a silane coupling agent was determined through an orthogonal experiment. The mechanical properties, bending toughness, and energy absorption capacity of UHPFRC are investigated. The study found that bending toughness and energy absorption capacity of UHPFRC are increased by 47.6% and 78.0% with 2 vol% of treated steel fibers. The interface microstructure of steel fibers and matrix was characterized using advanced techniques, such as Fourier transforms infrared spectroscopy (FT-IR), X-ray spectroscopy (EDS), and electron microscopy (SEM). The mechanism of fiber treatment improving the interface bonding performance was revealed. This work provides a feasible method for the combination of reinforced steel fibers and the UHPFRC matrix.

The corrosion behavior of steel in concrete is an engineering issue that needs to be monitored to ensure structural safety. Novel cement/polymer-based piezoelectric composites were used to fabricate embedded ultrasonic transducers. This study investigated the corrosion development of reinforcement bar in concrete by employing ultrasonic transmission technique. The transmitted ultrasonic wave and acoustic parameters were extracted to reveal the corrosion process of steel. Results show that ultrasonic time- and frequency-domain spectra have weak amplitude attenuation when the corrosion rate of steel is less than 1%. Corrosion then weakens further when the corrosion rate is higher than 5%. A similar variation is observed on the frequency-domain spectrum. Variations in the corrosion rates of steel bars in concrete can be categorized into three periods based on the acoustic parameters, namely, the initial corrosion period when the corrosion rate is less than 1%, rapid corrosion period at a corrosion rate of 1%–6%, and corrosion development period when the corrosion rate is higher than 6%.

In order to evaluate the influence of nano-silica on tricalcium silicate (C3S) hydration, hydration rate, Ca(OH)2 content, quantification of C–S–H, pore size distribution and micrographs of hydration products were measured. Results revealed that with the increase content of nano-silica, the hydration of C3S is accelerated before hydration for 3 d but slows down in the later age (7 d, 28 d and 60 d). C–S–H content increases with the increase content of nano-silica. However, an obvious reduction in C–S–H increment occurs at 7 d, 28 d and 60 d. In addition, the hydration products morphology changes from rod-shape to dense matrix at 28 d after nano-silica modification, and the Ca/Si ratio of C–S–H gel decreases. Besides, the addition of 3 wt.% nano-silica can reduce the total porosity of C3S by 4.04% and optimize the pore structure. Furthermore, the density of paste increases by 4.22%, which finally leads to the more compact structure of hardened paste.

The concrete samples with carbonated steel slag aggregate (CSA), normal aggregate (NA) and crushed steel slag aggregate (SSA) with W/C of 0.42 were prepared. The properties of the resistance on freezing and thawing were investigated and compared by freeze-thaw test with the condition of fresh-water and salt-water. The rapid chloride migration test (RCM) and coulomb electric flux test (CEF) were made in order to test the resistance of the concrete to chloride ion penetration. The ITZ properties and freeze thaw resistance of concrete were investigated by micro hardness tests, SEM, Ca/Si ratios, weight loss and relative dynamic elastic modulus test. The results show that using CSA instead of NA increased the freeze-thaw cycles from about 30 cycles to 175 cycles in salt-water condition and from about 150 cycles to 300 cycles in fresh-water condition. The average value of the hardness of ITZ around CSA is more than 43, which may due to the low W/C ratio caused by high water absorption of the CSA. The range of interfacial transition zone (ITZ) of CSA from SEM and EDS is larger but denser than NA and SSA, and fewer cracks are observed. The resistance to freeze and thaw will be improved by the hydration of steel slag to bond cement matrix. The cambered surfaces of CSA are less likely to occur stress concentration.

The application of self-healing technology in concrete materials was widely investigated in the past decade. Although the micro-capsules and bacteria were considered as promising self-healing agents to realize durability enhancement of concrete, the high cost of micro-capsules and limited bacteria types are still big challenges that limit the widely application of this technology. As a result, it is necessary to develop cost-effective and environmentally friendly materials as self-healing agent in concrete. In this study, carbonated steel slag was used as aggregate to realize the autogenous healing of concrete. The self-healing performance of this concrete was investigated by comparing with the concretes prepared with normal aggregates and crushed steel slag aggregates. In addition, the hydration heat, X-ray diffraction, and scanning electron microscopy/energy dispersive spectroscopy results were analyzed to elucidate the self-healing mechanisms of concrete via using carbonated steel slag as healing agent. It was found that the healing products are mainly composed of CaCO3, Ca(OH)2, calcium–silicate–hydrate, calcium–aluminate–ferrite hydrate as well as amorphous silica. The cracks of aggregate have been healed to a certain extent that maximum healing width is about 20 μm and maximum healing length is about 5 mm.

The combined utilization of mineral admixtures and nano-materials in cement-based composites is an inevitable trend based on the purpose of improving the durability. However, synergistic effect of mineral admixtures with nano-materials and their formation mechanism remains poorly understand. Therein, synergistic effect of nano-silica and fly ash on mechanical property, hydration process, non-evaporative water content, pozzolanic activity were investigated, the pore size distribution of hardened paste were tested and synergetic effect formation mechanism was analyzed. The results indicate that nano-silica improved compressive strength in the early stage and high content of fly ash with moderate fineness contributed to the strength growth in the later stage. In addition, nano-silica promotes cement hydration in early stage and provides favorable hydration conditions for later hydration of fly ash. Synergistic effect of nano-silica and fly ash increases content of non-evaporative water and improves pozzolanic activity. Moreover, synergistic effect of nano-silica and fly ash significantly optimizes pore structure in hardened paste. The suggested synergistic effect mechanism is attributed to the optimization of hydration process, mixing ratio matching and fineness matching among cement, nano-silica and fly ash.

Nano-materials modified cement-based materials have attracted wide attention due to their advantages of improving early strength and durability. However, few studies have been conducted on influence of nano-materials on chemical shrinkage of paste and volume shrinkage of concrete. Therein, the influence rules of nano-silica on chemical shrinkage of paste and volume shrinkage of concrete and its action mechanism have been explored and discussed. The result indicated that added nano-silica aggravated the chemical shrinkage of paste and volume shrinkage of the concrete. The paste chemical shrinkage and concrete volume shrinkage increased with the amount of nano-silica in particular as presented in early stage. The chemical shrinkage of cement at 3d and 28d increased by 93.7% and 57.5% respectively, when adding 1.2% nano-silica. Correspondingly, the 1d, 7d and 60d volume shrinkage of concrete increased by 82.1%, 66.7% and 16.7% respectively. The action mechanism of shrinkage intensification is mainly the pozzolanic effect, nucleation effect and acceleration hydration effect of nano-silica, which accelerated the cement hydration.