•The comprehensive study of using PZT based EMI method to monitor the properties of cement mortar.•Experiments were conducted on mortar with various w/c ratio on different cements.•RMSD is the most ...efficient index to reflect the stiffness growth of cement mortar.•The EMI-RMSD index might be independent from the various w/c ratio of cement mortar.
The use of piezoelectric materials based Electromechanical Impedance (EMI) technique for monitoring the hydration of cementitious materials has caught much attention recently. However, very few literatures have explored the feasibility of using this method on monitoring the stiffness development and compressive strength gain at the very early age properties (4th–8th h) of cementitious materials. This research serves as a comprehensive study to verify the reliability of using lead zirconate titanate (PZT) based EMI method in monitoring the compressive strength gain and elastic modulus of mortar at both very early age (4th–8th h) and early age (1st, 3rd, and 7th day). Extensive experiments and data analysis have been done on ten different mixes with various water-to-cement ratio and Type I & III cement. The EMI signatures are measured for each sample at the period of interest, and post-processed with three statistic model including the root mean square deviation (RMSD), correlation coefficient deviation (CCD), and mean absolute percentage deviation (MAPD) as indices. To examine the correlation and linearity between the compressive strength/elastic modulus obtained via conventional cubic testing using ASTM C109 and the EMI indices, a linear least square regression analysis is performed. As the authors postulated, all the mixes display a good linear correlation of R2. Among all three statistical indices, RMSD index is proved as the most accurate statistical index on strength gain monitoring of cementitious materials. The results indicated the feasibility of using piezoelectric-based EMI method for monitoring the cementitious material’s strength gain at very early age, regardless the concrete mix design.
The retardation of the early-age hydration of cement by steel slag powder severely restricts its use in cementitious materials. This paper reports on the modification of steel slag powder with ...phosphoric acid, which is found to alleviate its inhibition of cement hydration. Comprehensive investigations of solid phase assemblage and elements dissolving behavior in cement pastes reveal that multiple actions combine to determine the effect of phosphoric acid-modified steel slag powder on cement hydration. By using an appropriate amount of phosphoric acid for modification, a noteworthy reduction in C12A7 content and a significant increase in the specific surface area of steel slag powder can be achieved. This consequently diminishes the negative inhibitory effect of steel slag powder on hydration. However, excessive use of phosphoric acid exacerbates the delayed effect of calcium phosphate on mineral dissolution, so that the inhibitory effect of steel slag powder on cement hydration cannot be alleviated.
•Threefold effects determine the effect of phosphoric acid-modified steel slag powder on cement hydration.•Appropriate use of phosphoric acid leads to a decreased inhibitory effect of steel slag powder on hydration.•Excessive use of phosphoric acid cannot alleviate the inhibitory effect of steel slag powder on hydration.
•The recycled sand employed into the mix of 3D printed mortar and the green strength development at early ages was explored.•Recycled sand has significant effect on the early age behavior for those ...mature specimens.•The addition of recycled sand will change the mortar from plastic material to solid material in the later stage.
One of the technical challenges for the extrusion-based 3D printed concrete is that the requirements of green strength development are much different from ordinary concrete, due to the absence of formwork. This study employed the recycled sand into the mix of 3D printed mortar and explored the green strength development at early ages within 2.5 h after extrusion. 25% and 50% natural sand was replaced by recycled sand which was crushed from waste concrete in the mix. Uniaxial unconfined compressive tests were undertaken to obtain the mechanical properties of the printable mortar, while the failure patterns, vertical load–displacement relationship, lateral deformation and stress–strain behavior were recorded and fully analyzed. It is found that the recycled sand had very limited influence for those immature specimens, while exerted significant effect on the early age behavior for those mature specimens. The addition of recycled sand will change the mortar from plastic material to solid material, especially when the rest time reaches 90 min after the extrusion. It is believed that the high cement paste content and large water absorption of recycled sand in the 3D printed mortar affects the early age behavior. Since the incorporation of recycled sand improves the buildability while reduces the open time of the printed material, it is necessary to reasonably adjust the mix ratio and the amount of recycled sand in practical printing.
In concrete, early age pore water evaporation results in volumetric shrinkage that, if restrained, can cause plastic shrinkage cracking (PSC). 3D printed concrete (3DPC) is vulnerable to PSC due to a ...lack of formwork, minimal bleeding water, low aggregate to binder ratio and high quantities of fines in the mixture. A novel experimental method was developed to determine the PSC risk, evaluating the efficacy of PSC prevention measures and understanding the behaviour of PSC in early age 3DPC. This study evaluated the free shrinkage of 3DPC specimens as well as identified and systematically introduced sources of restraint to induce PSC. The free shrinkage results showed a rate of strain gain and peak strain significantly higher than commonly found in ordinary concrete. Severe cracking was observed when the shrinkage was restrained under a moderate evaporation rate. Cracks formed within the first 2 h after printing, earlier than in ordinary concrete. The proposed method was employed to study the fundamental behaviour of crack formation and propagation in 3DPC. The unique filament interlayer plane has a notable effect on the transfer of shrinkage deformation in the specimen. Differential horizontal deformation in consecutive layers and consequential interlayer slip was observed when the shrinkage was restrained. It is reasonable to conclude that interlayer slip caused by early age shrinkage has the potential to reduce the long-term interlayer bond strength and the durability of 3DPC.
•Hydration of OPC is accelerated by S-water, due to the chloride promoting the precipitation.•Hydration rate of C3S is greatly affected by the concentration of chloride in solution.•Compressive ...strength of OPC mortar increased rapidly by the rapid hydration in S-water.•Chloride in S-water promoted the hydration reaction and affects hydrate assemblage.•In later age, mass of C-S-H in OPC paste with S-water is 10% lower than that with D-water.
Using seawater for concrete manufacturing promisingly provides significant economical and environmental benefits. In this study, ordinary Portland cement (OPC) hydration in distilled water and seawater and the corresponding evolution of solid phases was investigated by heat evolution, hydrated phase, hydration kinetics, and microstructure characterization. The results show that seawater can promote the early hydration of tricalcium silicate (C3S) during the hydration acceleration period. The hydrated phase assemblage was affected by the dissolved ions in seawater. Friedel’s salt was detected as a specific hydration phase in seawater, which was formed by chemical combination between the aluminate ferrite monosulfate (AFm) phase and chloride ions. The monocarboaluminate can be converted into a stable phase as Friedel’s salt in the seawater, due to the reaction with chloride ions. Furthermore, the ettringite becomes more stable when coexists with Friedel’s salt than that with monocarboaluminate, and thus ettringite formed in seawater remains 67% higher than that formed in distilled water at the later curing age. Moreover, additional unhydrated cement and less amorphous calcium silicate hydrate (C-S-H) were formed in seawater, which might be responsible for the slightly lower compressive strength of cement mortar prepared by seawater and sea sand. A modeled evolution of the solid phase and pore solution have been established, which agrees well with the characteristics of the dissolution of mineral phase, precipitation of hydration products and changes of pore solution. The related results can provide an insight into the applications of seawater and sea sand concrete for marine infrastructures.
This paper provides a comprehensive review of the utilization of industrial wastes in 3D concrete printing processes. Concrete 3D printing processes are introduced, highlighting the need for unique ...material requirements. The literature on the different wastes that have been used for producing 3D printable mixtures is reviewed, including waste generation, material properties, and the roles of these materials in improving the fresh properties needed for concrete printing. Factors affecting successful utilization of these waste materials in 3D printable concrete are discussed in addition to their environmental impacts. This paper underlines the positive impacts of using industrial wastes for improving the sustainability of 3D printable concrete. It is expected that printable concrete formulations containing high volumes of industrial waste (and chemical admixtures) can be developed to improve the sustainability index of this emerging construction technique.
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oCritical review of 3D printed concrete made with industry waste is presented.oFactors affecting successful utilization and effects of these waste materials are discussed.oSustainability assessment of 3D printable mixes is presented.oThe challenge is to produce high performance 3D printable concrete using high volume of industry waste.
Capillary pressure is frequently measured to evaluate the shrinkage performance of concrete but has been limited to pressures <100 kPa preventing a better understanding of the early age factors ...affecting concrete durability. In this study, high capacity tensiometers (HCTs) were employed for the first time to investigate the behavior of early age concrete. The evolution of capillary pressure in Self-Consolidating Concrete (SCC) with and without shrinkage reducing admixture was evaluated. The results demonstrate that HCTs are capable of measuring capillary pressure beyond 1500 kPa. This transformative new record of capillary pressure behavior has enabled the development of a model for the capillary pressure in early age concrete correlated to water evaporation, self-desiccation, setting time, temperature, and hydration processes. This ability to quantify real-time capillary pressure change in concrete generates important implications for optimizing the commercial durability of SCC and for understanding the link between early age concrete processes and resultant mechanical performance.
•A new four stage model of the capillary pressure in early age concrete is presented.•Bleeding, setting time, evaporation, and hydration govern the length of stages.•A novel technique consisting of the use of HCT is used to measure the capillary pressure of SCC up to 2 MPa.•Evaporation and self-desiccation govern the rate and value of capillary pressure.•The effect of curing time on the capillary pressure of SCC with/out SRA is different.
Developing functional concrete mixtures with less ordinary portland cement (OPC) has been one of the key objectives of the 21st century sustainability movement. While the supplies of many ...alternatives to OPC (such as fly ash or slag) may be limited, those of limestone and silica powders produced by crushing rocks seem virtually endless. The present study examines the chemical and physical influences of these powders on the rheology, hydration, and setting of cement-based materials via experiments and three-dimensional microstructural modeling. It is shown that both limestone and silica particle surfaces are active templates (sites) for the nucleation and growth of cement hydration products, while the limestone itself is also somewhat soluble, leading to the formation of carboaluminate hydration products. Because the filler particles are incorporated as active members of the percolated backbone that constitutes initial setting of a cement-based system, replacements of up to 50% of the OPC by either of these powders on a volumetric basis have minimal impact on the initial setting time, and even a paste with only 5% OPC and 95% limestone powder by volume achieves initial set within 24 h. While their influence on setting is similar, the limestone and silica powders produce pastes with quite different rheological properties, when substituted at the same volume level. When proceeding from setting to later age strength development, one must also consider the dilution of the system due to cement removal, along with the solubility/reactivity of the filler. However, for applications where controlled (prompt) setting is more critical than developing high strengths, such as mortar tile adhesives, grouts, and renderings, significant levels of these powder replacements for cement can serve as sustainable, functional alternatives to the oft-employed 100% OPC products.
Immobilizing heavy metals (HMs) from municipal solid waste incineration fly ash (MSWIFA) using shell coal gasification fly ash (SFA)-based geopolymer can solve the energy and environmental challenges ...simultaneously. In this study, we synthesized a geopolymer with SFA, metakaolin (MK), and steel slag (SS) to solidify and stabilize HMs (Pb, Cr, and Zn) and investigated the early immobilization mechanisms. The results show that the prepared geopolymer possessed high early-age mechanical strength and immobilization efficiency to HMs (>90%), even under the effect of excess HMs. The early immobilization mechanism of the geopolymer for the HMs could be described as follows. (1) Most of HMs were remained in the aluminosilicate. (2) The presence of amorphous zeolite precursor and clay minerals may contribute to restrain the HMs leaching; (3) Pb and Zn were trapped by the gel structure in M-O-Al and M-O-Si forms (M = Pb or Zn), whereas Cr (VI) was reduced to Cr (III). (4) Cr might involve in the geopolymerization of SiO4 and AlO4− units. (5) The immobilization process of Pb and Zn in the geopolymer could be described as crystal growth (NG) - phase boundary reaction (I) - NG - I - diffusion (D), whereas that of Cr is prolonged to NG-I-NG-I-NG-I-D.
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•SFA-based geopolymer was developed to immobilize HMs.•Early-age geopolymer possessed excellent mechanical strength and immobilization efficiency.•Early-immobilization mechanism to HMs among the novel geopolymer.
Recycled powder (RP) that includes recycled paste powder (RPP), recycled mortar powder (RMP), and recycled concrete powder (RCP) is an eco‐friendly alternative binder. This work investigated the ...influence of RP types and fineness on early‐age behavior and mechanical strength of cement‐based materials. The results show that substituting RP for cement causes the reduction of new hydration products in cement‐based materials. The blended RP decreases the fluidity of paste mixture, and the fluidity of paste including fine RPP is lower than that of paste including fine RMP and RCP; moreover, the fluidity increases following the growth of RP particle size. Incorporating fine RPP shortens the setting time of cement‐based materials, while the setting time is prolonged as RMP and RCP incorporate; in addition, the enlarged particle size of RP prolongs the setting time. The drying shrinkage increases following the increasing replacement ratio of RPP. Incorporating less proportion of fine RMP and RCP lessens the drying shrinkage of paste, while the drying shrinkage increases after coarse or high‐volume RMP and RCP substituting. The maximum shrinkage of paste including 30% fine and coarse RCP is 3.4% and 7.3% higher than that of plain paste. Substituting fine RP for less dosage of cement has less impact on the mechanical strength of its newmade mortar, while the mechanical strength markedly decreases after high‐volume or coarse RP substituting. The compression strength of mortar including 30% fine and coarse RCP is 22.2% and 35.1% lower than that of plain mortar.
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