Effect of Sodium Hydroxide, Liquid Sodium Silicate, Calcium Hydroxide, and Slag on the Mechanical Properties and Mineral Crystal Structure Evolution of Polymer Materials

Research and Development of Red Mud and Slag Alkali Activation Light Filling Materials Preparation by Ultra-High Water Content and Analysis of Microstructure Formation Mechanism

This paper presents the preparation of alkali-activated red mud (RM) light material by an ultra-high liquid-solid ratio (1.98) based on the super water absorption characteristic of RM particles. Compressive strength, dry density, and water absorption are analyzed over time. Besides, the characteristic distributions of porosity and pore size are measured by mercury injection tests, and the microstructure is further analyzed by scanning electron microscopy. The results show that the ultra-high liquid-solid ratio can be used to prepare light samples with superior mechanical properties, low water absorption, reasonable pore distribution, and fine microstructures compared with light samples prepared with a foaming agent. The reason is that the significant increase in the free water does not change the dense microstructure of samples and effectively limits the increase in the detrimental pores. This effectively alleviates the sudden decrease in compressive strength and limits the increase in water absorption.

Preparation of Alkali Activated Cementitious Material by Upgraded Fly Ash from MSW Incineration

Utilization of municipal solid waste incineration fly ash (MSWI-FA) can avoid land occupation and environmental risks of landfill. In this paper, MSWI-FA was used to prepare alkali activated cementitious materials (AACMs) after two-step pretreatment. The ash calcination at 450 °C removed 93% of dioxins. The alkali washing with 0.2 g NaOH/g ash removed 89% of chlorine and retained almost 100% of calcium. The initial setting time of AACMs was too short to detect for 20% of MSWI-FA addition, and the prepared block had extensive cracks and expansion for CaClOH and CaSO₄ inside. Alkaline washing pretreatment increased the initial setting time by longer than 3 min with 30% ash addition and eliminated the cracks and expansion. The significance of the factors for compressive strength followed the modulus of alkali activator > silica fume amount > alkaline washing MSWI fly ash (AW-MSWI-FA) amount. When the activator modulus was 1.2, 1.4 and 1.6, the blocks with 30% of AW-MSWI-FA had a compressive strength of up to 36.73, 32.61 and 16.06 MPa, meeting MU15 grade. The leaching test shows that these AACM blocks were not hazardous waste and almost no Zn, Cu, Cd, Pb, Ba, Ni, Be and Ag were released in the leaching solution.

Coagulation Mechanism and Compressive Strength Characteristics Analysis of High-Strength Alkali-Activated Slag Grouting Material

In deep coal mining, grouting reinforcement and water blockage are the most effective means for reinforcing the rock mass of extremely broken coal. However, traditional cement grouting materials are not suitable for use in complex strata because of their insufficient early mechanical strength and slow setting time. This study innovatively proposes using alkali-activated grouting material to compensate for the shortcomings of traditional grouting materials and strengthen the reinforcement of extremely unstable broken coal and rock mass. The alkali-activated grouting material was prepared using slag as raw material combined with sodium hydroxide and liquid sodium silicate activation. The compressive strength of specimens cured for 1 d, 3 d, and 28 d was regularly measured and the condensation behavior was analyzed. Using X-ray diffraction and scanning electron microscopy, formation behavior of mineral crystals and microstructure characteristics were further analyzed. The results showed that alkali-activated slag grouting material features prompt and high strength and offers the advantages of rapid setting and adjustable setting time. With an increase in sodium hydroxide content, the compressive strength first increased (maximum increase was 21.1%) and then decreased, while the setting time continued to shorten. With an increase in liquid sodium silicate level, the compressive strength increased significantly (and remained unchanged, maximum increase was 35.9%), while the setting time decreased significantly (and remained unchanged). X-ray diffraction analysis identified the formation of aluminosilicate minerals as the main reason for the excellent mechanical properties and accelerated coagulation rate.

Improvement of Mechanical Properties and Condensation Behavior for Alkali-Activated Materials by Sodium Silicate

To further enhance the compressive strength of alkali-activated materials and reveal their condensation behavior, the reactivity of alkali-activated slag materials was enhanced through the addition of different kinds and proportions of sodium silicate. The mechanical properties of the specimens were observed regularly and the condensation behavior was further analyzed. The results showed that both solid and liquid sodium silicate could significantly improve the compressive strength. The maximum increase in compressive strength was 123.7%, while the initial and final setting times were significantly shortened to 9 min. When solid sodium silicate content increased from 5% to 15%, the compressive strength first increased to 34.6 MPa and then decreased to 28.6 MPa, indicating that 10% was the optimum solid sodium silicate content. The large amount of crystallized solid sodium silicate in the specimen led to the decrease in mechanical properties. When liquid sodium silicate content increased from 5% to 15%, the compressive strength first increased to 52.8 MPa and then tended to be stable, implying that 10% was the optimum content. This shows that its reinforcement effect has a maximum limit. The activation effect of liquid sodium silicate was better than that of solid.

Compressive Strength Enhancement in Early Age Acid Activated Mortars: Mechanical Properties and Analysis

To increase the hydration reaction rate and the amount of hydration products, different types and proportions of acids were mixed into mortars. The effects of the acid type and content on the early compressive strength of cement-based materials and the strengthening mechanism were assessed via X-ray diffraction and Fourier-transform infrared spectrum analyses. The addition of acetic acid or hydrochloric acid significantly increased the compressive strength of mortars at 1 d and 3 d, and slightly increased it at 28 d. With increasing acetic acid content, the strengthening effect increased first and then decreased with the best content at 4%. With increasing hydrochloric acid content, the strengthening effect decreased gradually with the best content at 2%.

Preparation and Properties of Municipal Solid Waste Incineration Alkali-Activated Lightweight Materials through Spontaneous Bubbles

A self-foaming alkali-activated lightweight material was prepared by the pretreatment of municipal solid waste incineration bottom ash (BA). The low weight could be achieved without adding a foaming agent by using the low-density and self-foaming expansion characteristics of BA in combination with a strong alkali. The effects of BA, liquid sodium silicate (LSS), and calcium hydroxide (CH) on dry and wet densities, as well as water absorption, are discussed. The results show that increasing the BA content can significantly improve the foaming effect and reduce the dry and wet densities of specimens. However, it also leads to a sudden decrease in compressive strength and a significant increase in water absorption. LSS and CH can significantly improve the ability to seal bubbles by accelerating condensation, and they further reduce dry and wet densities without significantly improving water absorption. It is most effective at BA, LSS, and CH contents of 60, 20, and 2%, respectively.

The Effect of Slag on the Mechanical Properties of Coralline-Activated Materials and the Formation and Transformation of Mineral Crystals

In this study, coralline-activated materials were prepared using ball-milled coral powder as cementitious material and coral sand as fine aggregate. XRF (X-ray fluorescence) and chemical dissolution tests were carried out to determine the content and reactivity of various elements in coral powder. The compressive strength of the developed composites was evaluated at different ages, and the formation and transformation of mineral crystals in coralline-activated samples were further analyzed by XRD (X-ray diffractometer). The results show that the calcium content in coral powder was as high as 89.5% (loss on ignition). However, only 56% of the active calcium could participate in the polymerization reaction. The silicon and aluminum content was too low, and the slag addition could improve the deficiency of silicon and aluminum in coral powder. With the increase in slag content (from 0% to 50%), the compressive strength of the composites increases significantly. Nevertheless, the enhancement is not pronounced when the slag content exceeds 50%. The increase in slag amount can stimulate the transformation of calcium minerals, e.g., aragonite and calcite, into hydrated calcium silicate and calcium aluminosilicate gels, which significantly enhances the resulting compressive strength.