CEMENT最新文献

The effects of carbonation on cement-based materials have drawn much attention because of its profound influences on durability performance of concrete structures. Most accelerated carbonation in lab is conducted at RH 50%–70%, which also dries out cement-based materials. The introduced drying action changes pore structure significantly, making the effects of carbonation obscure. To clarify the effects of pure carbonation, water permeability and related micro-structural characteristics are measured on mature mortars, which have been carbonated at water-saturated state. It is found that after carbonation, the porosity of mortars decreases slightly, with finer overall pore structure and lower characteristic pore size. The water permeability also decreases by roughly 40% on average. Based on the pore size distribution curves obtained through the low-field proton nuclear magnetic resonance technique, water permeability is predicted by the Katz-Thompson and Kozeny-Carman theories with satisfactory accuracy. The decrease of water permeability after carbonation agrees well with the reduced characteristic pore length, which quantitatively verifies the observed refinement of nanoscale pore structure due to pure carbonation.

By the year 2050, glass fiber reinforced polymer (GFRP) material from decommissioned wind turbine blades is expected to generate 40 million tons of waste worldwide. Managing GFRP waste is a vexing problem since the materials cannot be easily recycled. One potential waste management solution is to use the glass fiber (GF) component of GFRP as a supplementary cementitious material (SCM) to replace cement in concrete, which has the additional benefit of reducing CO₂ emissions from cement clinkering. The chemical composition of wind turbine GFs is variable, but is predominantly calcium, silicon, aluminum, and iron, with trace amounts of light and heavy metals, making it an attractive candidate for use as SCM. In this study, thermodynamic modeling was used to evaluate the reaction products, pore solution chemistry, and trace metal immobilization potential of three GF compositions (high silica; high calcium; median calcium/median silica) at varying cement replacement levels. These factors influence pore size and structure, which control mechanical properties, freeze-thaw behavior, transport properties, and corrosion potential. For all GF compositions, replacement levels up to 60% produce cementitious materials with higher volumes of C-S-H (and higher alkali and trace metal binding potential) than control mixtures; pore solution pH values appropriate for mixture designs optimized for either ASR or corrosion prevention; and, at replacement levels below 10% and above 40%, reaction of some trace metals to form insoluble precipitates. While further experimental investigation is essential, these models present evidence that the use of wind turbine GF as an SCM is a viable solution for managing this expanding waste stream.

This study uses neutron radiography to evaluate the drying of printed cement paste samples containing cellulose nanocrystals (CNCs). CNCs have previously been used in printed cement paste to decrease the extrusion pressure and increase the degree of hydration (DOH) of the samples. Three different mixtures were prepared consisting of a plain mixture and mixtures containing two different types of CNCs. The influence of the sample surface to volume ratio (S/V) on the drying of cement paste samples and their DOH was examined. Exposing 3D printed samples to drying immediately after preparation can lead to high levels of water evaporation, which can limit the hydration evolution in the system and increase the porosity. The DOH and the drying behavior of cement paste are found to be dependent on the S/V of the element. The DOH decreased with an increase in the S/V of the sample. The addition of the CNCs to the mixture design did not substantially alter the DOH of poorly cured 3D printed samples. Previous work has shown that CNCs addition to the mixture design can lead to an increase in DOH only if water remains in the sample.

Soil organic matters may inhibit the pozzolanic reaction, and thus influence the strength development of soil-employed construction materials. To understand their interaction, the effect of lignosulfonate, here used as model soil organic matter, on the pozzolanic reaction was investigated through batch experiments. Lignosulfonate inhibited the pozzolanic reaction, suppressing calcium silicate hydrate (C-S-H) formation. The suppression did not take place in a continuous way with the addition of lignosulfonate but was triggered at a certain dosage of lignosulfonate. We propose that the inhibition was primarily due to formation of Si-(Ca)-lignosulfonate complex. Such interaction may illustrate the inhibition of the pozzolanic reaction by organic matters in soils at alkaline activation. Below the threshold, lignosulfonate allowed C-S-H formation though modified its structure, which also suggested the possibility of soil organic matters to influence the strength development of construction materials in coexistence of C-S-H formation.

The incorporation of different levels of UOW into Portland clinker raw meals and its effects on the clinker and cement properties were evaluated. Clinkers were produced and characterized by X-ray diffractometry (XRD) and optical microscopy; the cements were produced and physically characterized. Finally, pastes were produced and analyzed using isothermal calorimetry, thermogravimetry, XRD, and compressive strength tests. UOW, when added up to 1.29% in Portland clinker raw meal, acts as a mineralizer, increasing the content of alite by 6.44%. The incorporation of UOW reduces the hydration rate in the first days owing to the increase in the size of the alite crystals and delays the point of sulfate depletion due to the increase in the SO₃ content of the clinkers. Owing to the higher content of alite formed, the cement produced from the raw meal with 1.29% of UOW presents the highest early mechanical strength (up to 7 days).

We use ${}^1$H nuclear magnetic resonance (NMR) methods to show that the relaxation time governing the redistribution of the gel-pore porosity in cement pastes during sorption depends, not surprisingly, on the dry state saturation and also, more surprisingly, on the sample size. The relaxation time is typically in the range 20 to 40 h for cylindrical samples 60 mm long dried to saturations between about 40 and 55%. It increases up to 200 h for samples dried to between 20 and 30% saturation. The times are all very much longer than for 1 mm samples. There is additional evidence to support the idea that the relaxation of hydrate inter-layer sized spaces occurs on at least two timescales, one of which is very much longer (months) than any of those listed above.

This work characterizes the reaction kinetics of supplementary cementitious materials (SCMs) with calcium hydroxide in the modified R³ test. The heat flow curves of 58 SCMs of varying reactivities were studied. Based on the heat flow curves, the SCMs were classified as more reactive, less reactive, and inert. Most of the heat flow curves in the modified R³ test exhibit, after the peak of heat flow, an initial slow decaying power-law regime that transitions into a longer and faster decaying power-law regime. The pre-exponent of the first regime depends on the initial SCM reactivity and correlates well with the 24-hour heat release in the modified R³ test, thus making it a useful metric for rapid classification of SCMs.

Little published data is available to guide engineers in designing calcium sulfoaluminate belite (CSAB) cement mixtures with adequate workability, strength, and durability. This lack of understanding of design factors, especially the effect of varying w/c, represents a significant barrier to widespread CSAB use. In this study hydration, setting time, and strength development of two CSAB cements with w/c from 0.3 – 0.6 were evaluated. CSAB reaction kinetics varied with increased w/c depending on CSAB composition, specifically calcium sulfate content – with higher w/c increasing retardation in higher anhydrite/ye'elimite content cement, but reduced retardation in lower anhydrite/ye'elimite cement. For both cements, greater w/c led to greater total hydration, increased setting times, and reduced compressive strengths in the pastes and mortar samples. Setting time was linked more closely to anhydrite content than w/c, with greater sulfate volumes shortening setting times.

This investigation explored the feasibility of producing strong, good quality, durable concrete in low-clinker systems (less than 50% clinker in some cases). The low-clinker content was achieved by combining interground portland limestone cement (PLC) with high limestone contents and different supplementary cementitious materials (SCMs). Seven cements, with approximate limestone contents between 3% and 31%, from two cement plants were used, in combination with SCMs, in forty-two different mixtures with water-cementitious materials ratios (w/cm) of 0.40 and 0.45. The SCMs included Class F and C fly ashes, Grade 100 slag, and silica fume. Mechanical properties (compressive strength, tensile strength, elastic modulus) and electrical resistivity were measured at 1, 7, 28, and 91 days. Similar compressive strengths were observed for mixtures with equivalent effective w/cm ratios. Although, the combination of PLCs with SCMs for very low-clinker systems resulted in decreased compressive strength, an increase in electrical resistivity was observed. More importantly, strong, good-quality concrete can be produced without sacrificing environmental benefits.

Replacing Portland cement clinker partially with limestone powder offers economic and ecological benefits but may decrease the resistance against carbonation. The diffusivity of carbon dioxide and the moisture conditions in concrete significantly influence the carbonation rate. Thus a test method was developed to determine the effective CO₂ diffusion coefficient (D_CO2). Additionally, the water vapour diffusion coefficients (D_H2O) were analysed. D_CO2 and D_H2O increase with increasing water-to-cement ratios (w/c, related to the CEM I content in the binder). At the same w/c ratio, higher amounts of limestone decrease D_CO2 and D_H2O and increase compressive strength. D_CO2 and D_H2O show a linear correlation for samples with w/c ≥ 0.6 but a non-linear relationship for dense concrete (w/c ≤ 0.5). D_CO2 ranges from 2.6⁻⁹ m²/s to 1.9⁻⁷ m²/s for w/c of 0.5 and 1.25, respectively. D_H2O were between 2.8⁻⁸ m²/s and 4.5⁻⁷ m²/s. A model for estimating D_CO2 in concrete with high limestone contents was derived based on the experimental analysis of the correlations between mix design, compressive strength, and CO₂ diffusion.