Experimental Study on Solidification and Stabilization of Heavy-Metal-Contaminated Soil Using Cementitious Materials

In order to solve the shortcomings of the traditional curing agent in the treatment of composite heavy-metal-contaminated soil with the solidification and stabilization method, a new type of cementing material A was used as a curing agent, and the Pb, Cd, Cu composite heavy-metal-contaminated soil was artificially prepared to carry out an experimental study on solidification and stabilization (SS) restoration by the mechanical properties test, leaching performance test, and microscopic test. The results show that in the range of test dosage, with the increase in the curing agent content, the unconfined compressive strength of the solidified body increased, and the resistance to deformation was enhanced. From the perspective of leaching characteristics, the new curing agent A had an excellent curing effect on the composite heavy-metal-contaminated soil. To achieve safe disposal, a curing agent content of 10% applies only for the soil heavily contaminated by heavy metals. The curing agent A could significantly reduce the content of acid-extractable heavy metals after solidifying the heavy metal Pb, Cd, and Cu composite contaminated soil and effectively converted it into a residue state. The solidified phase contained hydrated products such as calcium silicate hydrate (CSH) and ettringite (AFt). These hydrated products can inhibit the leaching performance of heavy metal ions through adsorption, encapsulation, and ion exchange. The study provides a feasible method and reference for the solidification, restoration, and resource utilization of heavy-metal-contaminated soil in the subgrade.

Structural and microscopy characterization of an alternative low-energy binder containing Ca(OH)2 as an alkaline activator

The development of potential alternative binders to Portland cement is still becoming a global challenge in housing and infrastructure aspects. That is because cement and concrete become the major materials needed in building constructions. The Ordinary Portland cement can form a solid and hard mass when mixed with water with a certain ratio. This is due to the formation of ettringite and calcium silicate hydrate (CSH) phases that contribute to the strength of the hydrated products about 33–53 MPa. However, the manufacturing temperature of Portland cement can reach up to 1,500 °C in producing clinker. In order to lower the energy consumption and production cost, scientists were trying to utilize pozzolanic materials. The research of pozzolanic materials as alkali-activated cement, such as soil cement or geopolymer cement, is also still conducted. Hence, a better understanding of pozzolanic reaction and its hydration products is needed. In this work, the hydration products of low-energy binders composed of Ca(OH)2-SiO2 and Ca(OH)2-metakaolin-gypsum mixtures were studied. The hydrated products of 41 wt. % Ca(OH)2 – 41 wt. % metakaolin – 18 wt. % gypsum mixtures followed by water immersion curing at 50 °C for 28 days undergone a pozzolanic reaction. XRD characterization showed that the hydrated product is mainly composed of ettringite (60.0 %) and crystalline-CSH (23.4 %). The diffractograms obtained have shown a specific hump indicating the presence of amorphous phases besides the crystalline. To confirm the presence of the non-crystalline or amorphous phases of the hydrated products, a polarizing optical microscope (OM) using a crossed Nicols method was used. The characterization of the phases is the novelty of the present research. The ettringite, crystalline CSH and the amorphous phases act as a strong binder that consequently contribute to its average maximum compressive strength of 22.17 MPa.

Structure, Fractality, Mechanics and Durability of Calcium Silicate Hydrates

Cement-based materials are widely utilized in infrastructure. The main product of hydrated products of cement-based materials is calcium silicate hydrate (C-S-H) gels that are considered as the binding phase of cement paste. C-S-H gels in Portland cement paste account for 60–70% of hydrated products by volume, which has profound influence on the mechanical properties and durability of cement-based materials. The preparation method of C-S-H gels has been well documented, but the quality of the prepared C-S-H affects experimental results; therefore, this review studies the preparation method of C-S-H under different conditions and materials. The progress related to C-S-H microstructure is explored from the theoretical and computational point of view. The fractality of C-S-H is discussed. An evaluation of the mechanical properties of C-S-H has also been included in this review. Finally, there is a discussion of the durability of C-S-H, with special reference to the carbonization and chloride/sulfate attacks.

Usage of supplementary cementitious materials: advantages and limitations

It is well known that cement production is not neutral for natural environment among others due to high CO2 emission. Different strategies of mitigation of negative environmental impact of its production are developed. One of the ways is utilization of supplementary cementitious materials (SCMs) in the manufacture of cement and concrete. Introduction of aluminosilicate SCMs into binding mixture makes that more amount of so-called C–A–S–H phase appears in hydration products, affecting microstructure and properties of final hardened composite. The aim of this work is to discuss the possibilities of utilization of selected SCMs in different binding mixtures including some advantages and limitations. Literature review on the subject was carried out. Some of our own research results were also presented. In the Part I of this review, some information about history of ancient binding materials and the possibilities of inspiring modern engineers with ancient constructions in the aspect of using SCMs in modern concrete were presented. Using pozzolanic aluminosilicate SCMs in relation to their influence on formed products, microstructure and mechanical properties of hardened material were discussed. Some problems with possibilities of study of SCMs reaction degree were identified. Emphasis was put on the usefulness of isothermal calorimetry and thermal analysis for investigations of hydration process and identification of hydrated products as well as evaluation of degree of reaction of SCMs.

Mineralogical Analysis of Portland Cement Pastes Rehydrated

Hydrated products, such as (hydrated) cement pastes, decomposition through physical-chemical alterations when submitted to high temperatures. One of the main factors that lead to microstructural changes during calcination up to 800ºC, is the dehydration phases of hydrated Portland cement. The present study sought to characterize crystalline phases occurring before and after calcination to the produced pastes using X-ray diffraction (XRD) analysis. Cement pastes were produced using Portland cement CP V-ARI RS, similar to type II (ASTM C150-07), with water/cement ratio 0.5. After a 28-day curing period, the pastes were calcinated at 800°C for 60 minutes with a 10°C/min heating rate. Afterwards, the newly produced hydraulic binder was cooled abruptly and reactivated through a rehydration process, and underwent analyses on the 7 and 28 day. During this study it was observed that the originally hydrated products can be rehydrated.

Physical, Mechanical and Chemical Properties of Dewatered Sewage Sludge and Red Gypsum Mix as a Potential Recycling Product

This paper aims to present the characteristic of dewatered sewage sludge (SS) and red gypsum (RG) to determine their recycling potentials into a valuable product. The materials were characterized based on their physical-mechanical-chemical properties and morphology. The results showed that dewatered sewage sludge and red gypsum could complement each other when used together to form a new material the merits of which will be investigated in subsequent studies. RG has a better performance in term of plasticity and strength than SS due to fines content in the former and therefore its ability to retain moisture that contributes to cohesion between particles. Fine prisms and closer packing particles add strength in RG leading to higher resistance in compression. Unlike RG, SS has coarser particle and more voids causing it to lose the moisture content and becomes easily disintegrated. Chemical compositions of RG and SS as determined by XRF demonstrated a significant amount of CaO in the former and SiO2 in the latter; the formation of hydrated products in the form of CSH and CAH gel from hydration or pozzolanic reaction contributes to strength improvement of the SS.

The Influence of Mayenite Employed as a Functional Component on Hydration Properties of Ordinary Portland Cement

Influence of C12A7 (12CaO·7Al2O3) as a functional component on hydration properties of Ordinary Portland Cement is studied using isothermal microcalorimetric technique, X-ray diffraction analysis, and thermodynamic calculation. Meanwhile, hydrate assemblages are simulated by hydrothermal software. C2AH8 (2CaO·Al2O3·8H2O) is generated as a transition phase during the hydration of pure C12A7, while formation of CAH10 (CaO·Al2O3·10H2O) is uncertain. Heat-releasing behavior of Ordinary Portland Cement (OPC) could be noticeably affected by C12A7, especially for the duration of interaction at boundary stage reduces with C12A7 replacement. Correspondingly, all hydration kinetic parameters first increase and then diminish with C12A7 replacement. Simulation results manifest in the main hydration products of OPC being ettringite, C-S-H (Calcium-Silicate-Hydrate) gel, portlandite and brucite. Increasing C12A7 replacement accelerates the consumption rates of gypsum and calcite that are typically included in OPC, and thus the ettringite content is changed and carbonate phases will be produced. Therefore, the microstructure properties of hydrated products of OPC are affected and the compressive strength is influenced. These predications are in good agreement with experimental findings. C12A7 can be used as a functional component to adjust the consumption rate of suphates in OPC, and also components of carbonate phases can be modified in hydrate assemblage.