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The problem of potentially toxic elements (PTEs) in farmland is a key issue in global pollution prevention and control and has an important impact on environmental safety, human health, and sustainable agricultural development. Based on the climate background of high-latitude cold regions, this study simulated freeze-thaw cycles through indoor tests. Different initial conditions, such as biochar application rates (0%, 1%, 2%) and different initial soil moisture contents (15%, 20%, 25%), were set to explore the morphological changes in cadmium (Cd) and lead (Pb) in soil and the response relationship to the changes in soil physicochemical properties. The results indicate that soil pH decreases during freeze-thaw cycles, and soil alkalinity increases with increasing biochar content. Freeze-thaw cycles caused the total amount of PTEs to have a U-shaped distribution, and the amount of PTEs in the soluble (SOL) and reducible (RED) fraction increased by 0.28-56.19%. Biochar reduced the amount of Cd and Pb migration in the soil, and an increase in soil moisture content reduced the availability of Cd and Pb in the soil. Freezing and thawing damaged the soil structure, and biochar reduced the fractionation of small particle aggregates by enhancing the stability of soil aggregates, thereby reducing the soil's ability to adsorb Cd and Pb. In summary, for farmland soil remediation and pollution control, the application of biochar has a certain ability to optimize soil properties. Considering the distribution of PTEs in the soil and the physicochemical properties of the soil, the application of 1% biochar to soil with a 20% moisture content is optimal for regulating seasonally frozen soil remediation.

This paper investigated influence of formwork wall effect on fiber orientation and flexural properties (flexural strength and toughness) of ultra-high performance concrete (UHPC) prepared by a direct cast and flow control method. Fiber orientations in samples with different distance from formwork were quantitatively evaluated using image analysis technique. Based on experimental results, the better fiber orientation and higher flexural performance can be obtained near formwork boundaries due to restriction effect on fiber rotation. Considerable variations of flexural strength and toughness with position were induced by the existence of mold sidewalls for directly cast UHPC specimens, reaching up to 54.9% and 49.6%, respectively. This is related to the significant fiber orientation coefficient variation of 21.1%–30.3%. When adopting the flow control method, the variability of fiber orientation coefficient was decreased to the range of 6.0%–12.3%, inducing the reduction of flexural strength and toughness differential to lower than 19.5% and 18.6%. Therefore, the flow control casting method can significantly inhibit the wall effect of formwork and is favorable for improving the homogeneity of UHPC in real structures.

To reduce the consumption of energy and raw materials caused by the production of Portland cement and enhance the carbon dioxide sequestration of building materials, this paper aims to manufacture durable and green magnesium oxysulfate cement based on incorporating mineral admixtures (fly ash (FA) or ground granulated blast-furnace slag (GGBFS)) and CO

This paper investigates the influence of fiber alignment induced during casting on flexural properties of ultra-high performance concrete (UHPC). Straight steel fibers with different lengths of 6, 13, and 20 mm were used at 2% volume ratio. Pullout load and energy of fibers with different lengths were evaluated. Flexural strength and toughness were determined for prismatic samples cast using a flow-induced device that can improve fiber orientation during placement. The horizontal outlet height of the device was adjusted to 10, 20, and 30 mm in order to secure different degrees of fiber orientation. Test results indicate that the longest fiber had approximately 5- and 27-time greater pullout load and pullout energy than those of the shortest fiber. Compared to UHPC made with the shortest fiber, UHPC cast with the longest fiber had approximately 75% and 245% higher flexural strength and toughness, respectively. The fiber dispersion and orientation improved when the horizontal outlet height decreased, resulting in greater flexural strength and toughness. Such enhancement was up to 75% and 100%, respectively, compared to similar UHPC cast using the conventional method that can result in random fiber orientation. Synergistic improvement exceeding 130% in flexural strength was obtained for samples cast with fiber length of 13 mm and horizontal outlet height of 10 mm, compared to samples cast with shorter fiber of 6 mm using the conventional casting method. The smoothed particle hydrodynamics simulation indicated that the ratio between fiber length and horizontal outlet height of 1.1 was more efficient to improve flexural properties of UHPC, compared to the ratios of 0.6 and 1.6 where the highest value led to blockage.

This study evaluates the performance of pervious concrete subjected to rapid freeze–thaw (F-T) cycling, calcium leaching and the combined attack of calcium leaching and F-T cycling. Silica fume, metakaolin and SBR polymer emulsion were incorporated at different levels into pervious concrete mixes to improve strength and durability performances. The results indicated that the addition of 5% fine sand and proper compaction had a positive influence on improving the resistance of pervious concrete to F-T cycling. The increase of supplementary cementitious materials (SCMs) from 5% to 10% significantly improved the resistance to rapid F-T cycling and to the combined attack of calcium leaching and F-T cycling. The optimum content of SCMs was 10% based on the mechanical and durability performance of pervious concrete with acceptable permeability. Calcium leaching in 6 M NH4NO3 solution combined with F-T cycling induced severe surface deterioration and internal damage compared to individual attacks of F-T cycling or leaching. Compared with control and polymer-modified mixes, pervious concrete incorporated SCMs possessed better resistance of calcium leaching and frost. The morphological changes caused by calcium leaching exhibits the decreased volume fraction of solid phases in a cement matrix and consequently increased the porosity, which ultimately degraded strength and durability performance of pervious concrete.

This study focusing on learning about the influence of steam curing on mechanical as well as microscopic performances of metakaolin (MK) contained UHPC matrix. Different levels of MK were introduced into UHPC matrix owning a water to binder ratio (w/b) of 0.20. Consequently, the hydration, mechanical property and microstructures of UHPC matrix were investigated at various steam curing regimes with variable curing temperatures and durations. The compressive and flexural strength tests, chemically bound water, reaction of metakaolin, DTA, XRD analysis, as well as MIP and SEM measurements were performed for investigating relevant properties of UHPC matrix. The test results show that steam curing evidently improves the early age compressive strength of UHPC matrix while restrains the late age strength growth. The higher curing temperature facilitates a higher MK reaction degree in the MK incorporated mixture during early ages but decreases the cement hydration degree. However, increasing the steam curing duration from 24 h to 48 h at both 55 and 90 ℃ does not significantly affect the facilitation of the pozzolanic reaction of MK. Pore structure of MK contained mixture with steam curing is similar to the reference with 20 ℃ water curing at 28 d. The steam curing at 90 ℃ results in a higher porosity for the mixture blended with MK than that at 55 ℃. However, the enhanced porosity does not obviously impair the compressive strength of UHPC matrix. Therefore, a suitable steam curing regime is required for UHPC matrix containing MK to enhance the strength and long-term durability.

This paper investigated the effect of carbonation curing on sulfate attack resistance of cement-coal gangue (PC-CG) pastes. Mechanical strength, volume deformation, and mass change of PC-CG blends were examined after different durations of sulfate solution immersion. On the other hand, the microstructure deterioration mechanism was determined using X-ray diffraction (QXRD/Rietveld), thermogravimetry differential thermal analysis (TG-DTA), mercury intrusion porosimeter (MIP), scanning electron microscope (SEM) measurements. Experimental results showed that accelerated carbonation improved the initial compressive strength before sulfate attack and alleviated the compressive strength loss with exposure to sulfate solution. The final compressive strength of carbonated specimens containing 0% or 10% coal gangue after 6 months of sulfate exposure retained 6.55% or 3.61% higher than those of the corresponding control mixtures before sulfate attack. In addition, the final compressive strength reduction of all carbonated specimens is much lower than those of the control specimens (8.69%∼34.19% VS. 12.93%∼47.52%). Moreover, the accelerated carbonation induced a decreasing expansion for every mixture (carbonated 2.32%∼6.11% VS. control 3.79%∼8.86%) partly due to the reduction of porosity and refinement of pore structure. Therefore, the combination of incorporating coal gangue and carbonation curing provides to a potential for manufacturing cement-based materials with better environmental sustainability and long-term durability in external sulfate attack regions.

Various placement methods can be applied to real ultra-high performance concrete (UHPC) structure elements. When placing fresh mixture at single or several target positions to fill the mold, the fiber orientation in the hardened specimen may be significantly variable with the flow distance. To investigate the influence of various placements on fiber orientation in UHPC beams, three casting methods, e.g., placing fresh mixture from one end, the middle position and two ends of the mold, respectively were simulated by the smoothed particle hydrodynamics (SPH) approach. Experimental investigations were also performed on fiber orientation and flexural properties of UHPC beams prepared by these three placements. Based on the modelling and experimental results, fiber orientation for each placement exhibited a descending tendency along the flow distance and was significantly reduced by the cooccurrence of two opposite flows. The variations of the number of fibers per unit area and fiber orientation coefficient reached 52.4% and 63.2%. Consequently, flexural strength, toughness and deflection at modulus of rupture tended to decrease with the flow distance and were notably aggravated by two opposite flow of mixtures. The variations of these three parameters along the beam specimen reached as high as 32.8%, 51.6% and 40.6%, respectively. Therefore, it is vital to control flow distance and direction of fresh mixture to manufacture UHPC elements with a high homogeneity.

Mortar prisms made with OPC cement plus 30% mass of limestone filler were stored in various sulfate solutions at different temperatures for periods of up to 1 year, the visual appearance was inspected at intervals, and the flexural and compressive strength development with immersion time was measured according to the Chinese standard GB/T17671-1999. Samples were selected from the surface of prisms after 1 year immersion and examined by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), laser-raman spectroscopy and scanning electron microscopy (SEM). The results show that MgSO4 solution is more aggressive than Na2SO4 solution, and Mg2+ ions reinforce the thaumasite sulfate attack on the limestone filler cement mortars. The increase of solution temperature accelerates both magnesium attack and sulfate attack on the limestone filler cement mortar, and leads to more deleterious products including gypsum, ettringite and brucite formed on the surface of mortars after 1 year storage in sulfate solutions. Thaumasite forms in the mortars containing limestone filler after exposure to sulfate solutions at both 5 °C and 20 °C. It reveals that the thaumasite form of sulfate attack is not limited to low-temperature conditions.

A modified testing system characterized by full automation, steady operation and high accuracy of strain and stress measurements was developed to determine the cracking tendency of high strength concrete (HSC) in restrained condition at early ages. The shrinkage stress and the tensile creep behavior of HSC at early ages were investigated. The infl uence of W/C ratio and curing conditions on the early-age shrinkage stress and tensile creep was evaluated. It was found that the lower W/C ratio and drying curing condition resulted in higher shrinkage stress, stress induced tensile creep and greater cracking tendency.