Renhe Yang

Incineration fly ash is a hazardous waste, and thermal treatment achieves its harmless treatment and resource utilization in cement. However, the long-term environmental safety of heavy metals in cement with incineration fly ash has not received much attention. Therefore, the heavy metal safety and mortar performance of cement with incineration fly ash with different sintering modifications in the sulfate environment were evaluated. Results indicated that the mortar mass initially increased and then decreased during sulfate attack. The mass growth of U-IFA (unsintered incineration fly ash) mortar reached 2.81% at 210 cycles, whereas D-IFA (directly sintered incineration fly ash) mortar and C-IFA mortar showed reductions after 120 cycles. C-IFA (cosintered sulphoaluminate cement raw materials and incineration fly ash) mortar exhibited the earliest spalling, compressive strength reduced by more than 22%, which was significantly worse than that of U-IFA and D-IFA. The surface heavy metal concentrations decreased markedly. Under weakly acidic conditions, the leaching of D-IFA mortar and C-IFA mortar increased by 2363.93% and 783.33%, respectively, but remained below the limits. The proportion of labile fractions of Pb and Cr increased, but the overall risk level remained low to medium. The sulfate resistance of C-IFA mortar was weaker than that of U-IFA and D-IFA. Sulfate attack accelerated mortar deterioration and weakened the immobilization of heavy metals; long-term environmental safety assessments of cement with hazardous wastes should be strengthened under complex service conditions.

This study was conducted to valorize fluoride resources from phosphate ore in cement-based materials. Low-alkali and alkali-free liquid accelerators were prepared from two typical fluorine-containing waste acids: fluorosilicic acid and hydrofluoric acid. Their fundamental properties, including setting time and hydration promotion mechanisms, were examined. The findings demonstrated that all acid-derived accelerators effectively shortened the cement setting time, particularly in the alkali-free formulations. Regarding mechanical performance, the low-alkali accelerator prepared with hydrofluoric acid improved the early age compressive strength of the mortar more effectively than the alkali-free version prepared with the same acid. The accelerator synthesized from fluorosilicic acid was found to yield the highest compressive strength. The underlying mechanism was explored by measuring the early hydration heat release via isothermal calorimetry. Furthermore, XRD, TG, and SEM were utilized to analyze the composition and microstructure of the hydration products. A hydration model was proposed to investigate the effect of the fluorine-containing acid type on the accelerator performance.

This article is based on the research background of an early performance of shotcrete under low-temperature environments. Silica fume (SF) (silicon powder) and nano-CSH are used to improve the effect of the low-temperature environment on the performance of alkali-free (AF) liquid accelerators and early strength of shotcrete. The results show that the 10% SF and 3% nano-CSH composite admixture significantly enhances AF accelerators' performance at 5 °C, reducing initial and final setting times to 3.0 and 10 min, respectively, while substantially improving the early age compressive strength of shotcrete. Furthermore, hydration calorimetry, phase analysis, and microstructural characterization reveal that nano-CSH synergistically enhances silica fume nucleation efficiency.

Improving the performance of a high belite clinker (HBC) by selecting appropriate blended materials is of great significance for its large-scale promotion and application. This research attempted to use dihydrate gypsum (DGP), fly ash (FA), and ground granulated blast furnace slag (GGBS) to prepare HBC + DGP (95% HBC + 5% DGP), HBC + FA (82% HBC + 3% DGP + 15% FA), and HBC + GGBS (82% HBC + 3% DGP + 15% GGBS) and explored the influence of different blended materials on various properties of HBC. In addition, the mechanism of the interaction between HBC and different blended materials in early and late stages was also revealed. The experimental results showed that GGBS had the most significant impact on HBC, mainly manifested in optimizing the particle distribution of HBC, increasing the standard consistency water demand of HBC, and shortening the setting time of the HBC paste. The improvement of the mechanical property of HBC by GGBS was its main advantage. Compared with HBC + DGP, the compressive strength of HBC + GGBS mortar increased by 113.9% and 43.5% at 7 and 28d, respectively. It was found that the addition of GGBS could increase the HBC second hydration exothermic peak to 15.4 mW/g. Further analysis proved that in the early stage, strong polarity OH^- could dissociate the Ca-O and Si-O bonds on the glass phase of the GGBS surface, and the released Ca²⁺and SiO₃^2- combined to form CH and C-S-H nanocrystalline nuclei, which could reduce the nucleation barrier of CH crystal and effectively promote C₃S acceleration reaction. In the hydration later stage, the pozzolanic effect was utilized to generate more C-S-H, which could fill the pore of the cement matrix. This further enhanced the compressive strength of the HBC + GGBS mortar.

Diethanolamine (DEA) can be used not only as a cement admixture but also to capture carbon dioxide (CO₂). However, the waste liquid treatment still faces the problems of high energy consumption and increasing environmental burden. The effects of DEA waste liquid (WL-DEA) with multiple cycles of CO₂ absorption and desorption on the setting time, hydration temperature, mechanical strength, and microstructure of cement-based materials were explored. It was found that adding WL-DEA could significantly reduce the setting time and enhance the mechanical strength. This improvement was mainly attributed to two aspects: on the one hand, alcoholamine itself could boost cement hydration, which could accelerate the generation of hydration products such as AFt/AFm, CH, C-A-S-H, and C-S-H, thereby improving the cement early strength and refining the microstructure of hydration products; on the other hand, WL-DEA contained little CO₃^2- and HCO₃^-, which reacted with Ca²⁺ to produce CaCO₃. The above reactions cooperated with the complexation effect of WL-DEA to further promote hydration and optimize densification of the hydration products. The application of WL-DEA in cement-based materials could not only effectively enhance their mechanical strength but also promote the recycling of waste liquid, which provided a new method for promoting the development of a circular economy.

The cement industry has achieved the harmless and resourceful disposal of incineration fly ash, but the environmental safety of heavy metals during its service life is still unknown. Therefore, the evolution of heavy metal leaching during the wet-dry cycle of the incineration of fly ash after different solidification treatments has been studied. We provide a theoretical basis for the long-term safe use of incineration fly ash after resource utilization. The results showed that when incineration fly ash underwent 90 dry-wet cycles, the leaching of Cd exceeded 19.40% of Chinese standard GB 18598-2019. Upon addition of 10% and 30% cement to incineration fly ash, the trend of increasing leaching of heavy metals slowed, with a higher cement content enhancing the solidification capability for heavy metals. When incineration fly ash mixed with 10% cement underwent 180 dry-wet cycles, the leaching of Cd exceeded 40.75% of Chinese standard GB 18598-2019, while that of samples with 30% cement content did not exceed the limit, with Cd leaching at 84.45% of Chinese standard GB 18598-2019. As an admixture, the compressive strength increased with the number of dry-wet cycles. When the incineration fly ash dosage was 20%, the maximum leaching of heavy metals after 180 dry-wet cycles was only 5.19% of Chinese standard GB 18598-2019. The toxicity index of heavy metals also showed a decreasing trend compared with that of incineration fly ash. Therefore, cement admixture with incineration fly ash ensured the safety of heavy metals during its service life.

Incineration of fly ash is a hazardous solid waste, and there are two types of furnaces used for production. The impact of furnace types on the properties and toxicity of incineration fly ash was studied. The resource utilization of incineration fly ash has been tested. The results showed that circulated fluidized bed (CFB) ash and grate (GF) ash were alkaline materials, dioxin content ranged between 98 and 359 ng-TEQ/kg, and heavy metals exceeded 7700 ppm. The leaching of Zn in CFB ash was 110.474 ppm, and that in the GF ash was 5.985 ppm. The chlorine content in GF ash was 27.07%, 2.7 times that of CFB ash. When the content was 5%, GF ash promoted cement grinding and improved the 3-day compressive strength of cement, which was 35.55% higher than the control sample. CFB ash generates hydrogen gas during the hydration process, causing volume cracking. When 20% GF ash was added, the hydration heat release peak appeared 10.5 h ahead of time. According to the Chinese standard GB 175-2023, the maximum dosages of CFB ash and GF ash in cement were 0.22 and 0.57%. There was no risk of excessive leaching of heavy metals. During cement production with CFB and GF ash, attention should be paid to the issues of chlorine and dioxins.

If there was abundant fluorine in shotcrete, it might leach out and pollute the soil or migrate to corrode the reinforcement.Therefore, this research mainly investigated the basic properties of high-fluorine alkali free liquid accelerator (HF-AFA) and its occurrence forms in cement hydration products.The macro-test results showed that with the increase of HF-AFA dosage, it appeared excellent coagulation promoting property. However, when the HF-AFA dosage exceeded 7.0%, the 1d compressive strength of mortar was lower than 7.0 MPa. In addition, by measuring the early hydration heat of cement, C₃A, C₃S, C₂S and C₄AF pastes with and without HF-AFA, and combining XRD and SEM micro-analysis, the occurrence forms of fluorine in different clinker minerals were obtained.The final analysis results indicated that fluorine mainly existed in the form of CaF₂, CaAlF₅ and Ca₂AlF₇ crystals in C₃A and C₃S minerals, while only little CaF₂ crystals appeared in C₂S and C₄AF minerals.

This article mainly tested and evaluated the carbonation resistance of sprayed concrete mixed with different liquid accelerators, and thus attempted to establish a prediction model for the carbonation depth of sprayed concrete. Firstly, the influence of alkaline liquid accelerator based on sodium aluminate (AR), alkaline-free liquid accelerator based on aluminum sulfate (AS), and alkali-free liquid accelerator based on fluoroaluminate (AF) on carbonation resistance of sprayed concrete was explored. In addition, the porosity and pore structure distribution of ordinary sprayed concrete (OSC), sprayed concrete with AR (SC_-AR), sprayed concrete with AS (SC_-AS), and sprayed concrete with AF (SC_-AF) were analyzed by the mercury intrusion porosimetry. The carbonation depth and compressive and splitting tensile strength also were tested. Furthermore, the scanning electron microscopy was employed to observe the micromorphology of hydration products. The experimental results indicated that the carbonation depth order of sprayed concrete was as follows: SC_-AR > SC_-AF > SC_-AS > OSC. After analyzing the disparity of SC_-AR, SC_-AS, and SC_-AF carbonation depth caused by AR, AS, and AF dosage, the influence coefficients of sprayed concrete carbonation depth were introduced. Finally, the carbonation depth prediction models of SC_-AR, SC_-AS, and SC_-AF were preliminarily established.