CEMENT最新文献

Concrete deterioration by Alkali Silica Reactions (ASR) is a severe issue of concrete durability associated with porosity and permeability. Most of the industrial solutions for mitigating ASR rely on controlling the mix design by mineral admixtures or lithium compounds. An innovative approach for mitigating ASR through a secondary role of crystalline waterproofing materials is presented. An aqueous solution of Multi-Crystallization Enhancer (MCE) is intermixed with water or added to the concrete mixture, at a dosage of 2% by weight of cement, then upon curing reduces the permeability under pressure by more than 99%. This study shows that ASR mitigation can be accomplished by incorporating the MCE in the fresh concrete mixture or by prewetting the aggregates. The experiments were made according to the methods of ASTM C1260. The investigated independent experimental variables included water to cement ratio, types of aggregates, method of the MCE addition, and time. The MCE can change the performance of aggregates from reactive to be equivalent to non-reactive. The findings demonstrate that the length expansion from ASR increases with increasing the w/c ratio for all types of aggregates attributed to the increase in the permeability. The MCE addition to mixtures with reactive aggregates enhances the resistivity against ASR by a percentage in the range of 45%-77%. The functionality of the MCE in mitigating the ASR is also confirmed using concrete specimens with long term ASR testing (ASTM C1293).

Class F fly ash-based alkali activated materials (AAMs) have shown promising strength and durability properties for oil- and gas-well cementing applications. Furthermore, they provide a sustainable and cost-effective means to dispose of non-aqueous drilling fluids and replace ordinary Portland cements for cementing purposes. This paper investigates the rheological properties and thickening time behavior of AAMs prepared using four different Class F fly ashes (FA), with varying calcium oxide (CaO) contents, and sodium/potassium silicate activators. The effect of the addition of synthetic based mud (SBM) and polycarboxylate ether-based (PCE) dispersing agent on the rheology of AAMs was also quantified. Potassium silicate-based AAMs demonstrated appropriate viscosity and thickening times (4–9 h) for cementing deeper zones of oil and gas wells. Sodium silicate-based AAMs, although more viscous than potassium silicate-based AAMs, showed suitable thickening times (2–8 h) for shallow cementing jobs. Overall, it was demonstrated that AAMs could be tailored to obtain desirable rheological properties and thickening times by altering the Class F FA, varying the activating solution, and adding PCE and/or SBM.

Achieving the correct sulfate balance in limestone calcined clay cements (LC³) to control aluminate hydration is critical for early hydration and property development, but the role of the calcined kaolin (metakaolin) fraction relative to other compositional variables has not been previously well-explored. In addition, little published research has investigated the influence of water-to-solid ratio (w/s) and superplasticizers in this context. This study assesses the influence and quantifies the relative significance of compositional predictors on the sulfate balance and cumulative heat evolved by 24 h for LC³ through a stepwise regression model. Sulfate balance was defined as the time difference between the sulfate depletion point and the time of maximum of alite peak obtained from a time derivative of data obtained through isothermal calorimetry. A methodology based on Kernel smoothing was used to precisely identify these events. The first 24 h of hydration of some LC³ pastes was also monitored via in-situ X-ray diffraction to develop linkages between LC³ composition and hydrated phase assemblage. The statistical analysis identified the metakaolin fraction as particularly significant for the sulfate balance. The results suggest that the metakaolin fraction influences the sulfate balance of LC³ both directly and through its interactions with other constituent materials such as limestone.

As sustainable construction practices become more popular, researchers are looking into using readily available and inexpensive agricultural waste materials as a supplementary cementitious material. This study investigates the impact of various pretreatment methods on the chemical composition, crystal structure, morphology, and cost of producing pretreated corn stover ash collected from four different sources in the U.S. This study also evaluates the performance of mortars and pastes containing treated corn stover ash through tests such as compressive strength, flow measurement, calorimetry, and thermal analysis. In addition, thermodynamic modeling is used to predict the phase composition, chemical composition of the pore solution, pH, and electrical resistivity of pastes made with selected pretreated corn stover ashes. The results suggest that acid pretreatment is the most effective and economical method for improving the quality of corn stover ash and that it removes a significant amount of alkalis from the raw material. The simulation of the reaction between cement and the pretreated corn stover ash was confirmed by the experimental results, indicating a marked enhancement in the pozzolanic activity and chemical and physical characteristics of the system.

White cement is characterized by higher tricalcium aluminate (C₃A) contents than conventional Portland cement; thus, it can present a reduced setting time, especially under hot weather conditions, limiting the material's application period. An approach to delay these materials' setting time is using retarder admixtures (RA). Nevertheless, there is a knowledge gap regarding the effect of RA on the rheological properties of white cement. This study aimed to evaluate the effect of two types of RA on the mini-slump, rheological parameters, hydration kinetics, and compressive strength of two white cements. Furthermore, the effect of temperatures of 25 and 40 ⁰C on the performance of the admixtures was also assessed. Both RA showed a dispersing effect, reducing cement pastes' dynamic yield stress and plastic viscosity. Moreover, the admixtures extended the induction period of cement hydration by up to 15.4 h without significantly affecting the 72-h cumulative heat at a temperature of 25 ⁰C. At 40 ⁰C, both RA exhibited a lower dispersing and retarding effect. Although RA delayed the initial hydration reactions of the WC, it enhanced the mechanical performance of the mortars after three days of hydration.

Recently calcium sulfoaluminate cements gain increasing attention due to their significant potential to reduce the carbon footprint of cement production compared to Portland cement. However, the conditions applied during its processing play a crucial role for the stability and longevity of the material. Thereby, the temperature has a decisive influence, as it is already known from numerous studies that ettringite structurally changes significantly upon thermal induced dehydration. Within this background, the present study subjects a holistic view of the mechanical, morphological, phase and structural changes of a commercial calcium sulfoaluminate cement related to the dehydration of the contained ettringite upon treatment at drying temperatures from 23 °C to 100 °C for 7 and 28 days. By complementary methods it is shown that with increasing curing temperature, the mechanical stability decreases, the total pore area and porosity increase, while the permeability of the microstructure is lower for samples stored at 100 °C. Removal of water increases the intercolumnar distance within the ettringite lattice, thereby inducing strain which is released upon rehydration. Although during storing at a temperature of 100 °C ettringite is transformed into an X-ray amorphous product, the initial morphology of the crystals embedded in the cementitious matrix is retained.

Rising volume of road dust is a serious concern in global as well as Indian scenario. To find out the possible application, chemical and physical characterizations of road dust of six diversified sites of Delhi, National Capital Region, India were carried out prior to utilize in concrete mix design. The chemical characterizations suggest major and minor components are oxides of silicon, aluminium, iron, sodium, calcium, potassium, sulphur, phosphorous, manganese etc. X-ray diffraction studies depicted major phases of silica, muscovite/ illite, K-Feldspar and albite minerals. Scanning Electron Microscopy studies depicted flacks, fibrous, spherical, irregular, voids and hexagonal morphologies. Further detailed studies of road dust of two sites were carried out in concrete mix design following IS and ASTM test methods to know the effects on compressive strength, flexural strength, water and rapid chloride permeability test after replacing upto 50% of stone sand by road dust. The 7th and 28th days compressive strength test results of two sites concrete showed 16.51%, 8.25% and 4.67%, 2.34 % lesser strength in comparison to control concrete respectively. Similarly, 7th and 28th days flexural strength studies of same sites concrete showed 19.67%, 14.75% and 6.85%, 1.37% lesser strength in comparison to control concrete, respectively. The depth of water penetration test results of two selected sites of concrete showed 13.14% and 10.22% lesser extent of water penetration under 5 bars hydrostatic pressure, when compared to control concrete. The RCPT results of same sites concrete showed 33.47% and 9.48% greater extent of chloride ion permeability, in comparison to control concrete. The results obtained after 7th and 28th days of conventional stone sand concrete and road dust concrete, showed quite comparable results. However, in case of water permeability test the road dust concrete showed lesser extent of water percolation in comparison to conventional concrete, this may be due to presence of more fines in road dust, which may have caused better packing and lesser voids for water to ingress.

Cold Water Extraction (CWE) is a technique used to extract the pore solution of cementitious materials and to study its alkalinity. CWE can be used on paste, mortar or concrete, and requires only standard laboratory equipment. The method is not yet standardised, so several parameters must be arbitrarily selected when conducting the test.

This work investigated the influence of four parameters on the calculated alkali metal concentrations in the pore solution: the method for determining the amount of pore solution (oven-drying at 40 and 105 °C, desiccator with silica gel and solvent exchange), the size fraction of the powdered material, the leaching duration and the liquid-to-solid ratio. A comparison with values obtained by Pore Water Extraction (PWE) on two cement types emphasises and quantifies the crucial impact of the amount of pore solution on CWE results. The results suggest that some bound alkali metals may be released during CWE. A mechanism is proposed, and recommendations are made to limit any effect of this on CWE results.

A review is made of the decisive parameters when predicting moisture conditions in a concrete structure exposed to natural climate. The required material properties such as the desorption isotherm, the scanning curves and the moisture transport coefficient are presented and discussed. The translation of parameters describing a natural climate into boundary conditions at a concrete surface is commented upon and examples are given on the effect of including or neglecting different parameters.

This study focuses on the characterization of white deposit occurring after wet/dry cycles on the surface of cement mortars presenting large amount of AFt phase. Correlative light-electron microscopy (CLEM) was confirmed to be a powerful method in characterizing and identifying microscopic visible white deposit that causes macroscopic visible surface whitening in those cement mortars. It was discovered that the white deposit formed after wet/dry cycles was caused by solids precipitated on the surface of cement mortars during hydration and/or on drying, and those solids were composed of CaCO₃, AFt, AFm or their solid solutions. Due to the resolution limit of the human eye, there is a threshold size 100 μm for the microscopic visible white deposit or its cluster to become macroscopic visible surface whitening. Partial covering of red pigments by the newly formed solids on the surface of the cement mortars further confirmed the relationship between the size and quantity of microscopic visible white deposit and macroscopic visible surface whitening.