Calcium carbonate efflorescence on Portland cement and building materials

Whitish deposits of calcium carbonate, CaCO₃, frequently develop on Portland cement concrete and on masonry units, including brick and tile, which have been bonded with Portland cement. These surface deposits are termed efflorescence and are most frequently encountered in new or recent construction. While efflorescence is not normally damaging, except possibly to decorative coatings, it is aesthetically undesirable. The origin of efflorescence is explained and a physicochemical model is developed to explain and quantify the key features of its formation. Calculations and experiments highlight the important role of soluble alkalis in the formation of efflorescence. Mechanistic interpretations and calculations suggest ways in which efflorescence can be mitigated by interrupting one or more steps of the process in conjunction with improved materials selection.     

Valorization of brick waste by alkali-activation: A study on the possible use for masonry repointing

Alkali-activation of brick waste has recently been proposed as a sustainable route to develop pastes/mortars with tailored mechanical properties and pore system. In this study, the suitability of using pastes from brick waste alkali-activation for repointing existing masonries (i.e., filling the most external part of mortar joints, lost due to deterioration processes) was investigated. Five different formulations (having SiO₂/Al₂O₃ molar ratio ranging from 1.4 to 0.4) and two different curing temperatures (room temperature and 50 °C) were investigated. Open porosity and efflorescence formation were found to decrease for decreasing SiO₂/Al₂O₃ ratio. Curing at high temperature generally favored geopolymerization and reduced efflorescence formation. Pastes with SiO₂/Al₂O₃=0.8 and 0.9 exhibited open porosity and water vapor permeability fairly similar to those of historic lime-based mortars, thus proving to be potentially compatible with them. Further optimization of the mix design seems however necessary to reduce the formation of efflorescence.     

Study of Efflorescence on Experimental Brickwork Piers

Efflorescence on brickwork piers made of three types of brick and six mortars appeared to depend mainly on ( a ) the Portland cement content of the mortar, ( b ) the type of brick, and ( c ) the season of the year. Efflorescence formed in the cold part of the year and was absent in the summer. Portland cement substituted for lime in mortars contributed to efflorescence; however, the amount of efflorescence depended also on the type of brick (probably on the pore structure).     

Physico-chemical characterisation of bricks all through the manufacture process in relation to efflorescence salts

Due to the high competitiveness of the ceramic product market, quality and technical requirement are increasing. The efflorescence problem is a phenomenon that affects the aesthetic quality of brick facades and which is not easily dealt with. The aims of this work are to study the key physical and chemical parameters for this phenomenon in all activities of the brick manufacturing process and to evaluate the influence of the additives employed on the appearance of the efflorescence phenomenon. Porosity and specific surface, mineralogical analysis, total chemical content and soluble salts were determined on green, dried and fired brick, both when no additives were added to the manufacturing process and also when ammonium lignosulphonate and barium carbonate were used as additives. The main mineralogical phases and water soluble salts in the efflorescence phenomenon have been obtained. In some cases, relationships between soluble salts in the end product and soluble salts in the efflorescence have been established. From the obtained results, the use of a mixture of ammonium lignosulphonate and barium carbonate together with the control of the firing process variable is recommended in order to prevent this phenomenon.     

Vanadium staining in fired clay bricks

Abstract

This paper reports on a series of efflorescence tests and instrumental techniques that have been used to determine the nature and mechanism of vanadium staining, and the effects of additions of magnesite, calcite, and fluorspar to the brick mix to eliminate the problem. Scanning electron microscopy, electron probe microanalysis, inductively coupled mass spectrometry (with laser ablation attachment), and X-ray photon spectrometry have been used to more fully understand the elemental composition, oxidation states, and formation of vanadium stains in fired clay bricks. It was found that the stains consist of V, K, Na, Mo, S, and O. Vanadium +4 compounds form initially within the brick and are slowly oxidised to vanadium +5 compounds as the water transports the potassium and sodium salts to the brick surface. X-ray diffraction led to a reasonably successful search match for a KVO(SO₄)₂ phase for the stains. Evolved gas analysis showed loss of vanadium compounds between 700 and 900° C which are most likely VF₃ and VCl₄. Inductively coupled plasma spectrometry laser ablation studies on thin sections of brick showed areas of glassy phases corresponding to the agglomeration of Mg/V/Al/Si and Mg/Al/Si. The increased alkalinity of magnesia and the comparable sizes of V⁴⁺ and Mgions are driving forces for locking vanadium into what is otherwise an acidic glassy phase. The addition of 1 wt-% magnesite to the brick mix led to fired products on a commercial scale that are free from vanadium staining.     

彩色水泥白霜形成机理及抑霜技术研究

用彩色水泥配置的混凝土在使用一段时间后,其表面会产生白霜,即产生水泥的盐析.在介绍白霜的主要成分和形成机理的基础上,进行了彩色水泥抑霜技术的试验研究.试验证明,改变基料、加入添加剂和混凝土表面处理等改性方法,能使水泥盐析现象得到有效遏制,对工程的实施有着重要的现实意义.     

Effect of temperature and mixing conditions on quality and consistency of poly(methyl methacrylate) bone cement

Abstract

The mixing of poly(methyl methacrylate) (PMMA) bone cement has been studied to develop methods for preparing a consistently high quality cement. A novel droplet test experimental procedure was developed that characterised the wetting characteristics involved in bone cement mixing. Using this technique it was established that increased wetting occurred by mixing bone cement at a lower temperature (-28°C) than normal mixing at room temperature.

The effect of temperature on viscosity of the cement mix was also investigated. An increase in viscosity with mixing time was found for all temperatures (owing to dissolution of PMMA in the monomer). However, the rate of increase in viscosity was a function of the initial temperature of the cement components. Cooling of the components initially to -12·6°C resulted in a better mix than room temperature samples, due to the cooled components having more mixing time at a lower viscosity (less than 1000 cP).

Automated mixing of the cement was also investigated. A high speed ‘figure of eight’ mixing machine (Kerr^® Automix^TM computerised mixing dental amalgamator) was used in a comparison with traditional hand held mixing devices. The effect of initial component cooling was also investigated in the high speed unit and cement samples were analysed for porosity and homogeneity of mix (using scanning electron microscopy). Results indicate that the combined effects of low initial temperature and automated mixing produces a bone cement that is more homogeneous and of lower porosity than hand mixed cement.     

FTIR Characterization of Polymorphic Transformation of Ammonium Nitrate

Solid ammonium nitrate (NH ₄ NO ₃ ) exists in five stable polymorphic forms (designated as phases I, II, III, IV, and V) below its melting point at around 170°C. Phase IV is stable in a temperature range of ?17°C ～ 32°C and is the only phase that has been considered by the atmospheric research community until recently. In this study, we examine the IV ? III phase transition of NH ₄ NO ₃ and how relative humidity (RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH ₄ NO ₃ samples, powder produced from grinding commercially produced chemicals and single particles obtained by efflorescence of droplets on PTFE filters, were studied. The powder samples exhibit the IV?III phase transition and the transition temperature depends on the RH while the single particle samples exhibit only the IV?II transition at about 52°C (forward) and 48°C (reverse), bypassing phase III, with transition temperatures independent of the RH. However, grinding of the particles produced through efflorescence results in the IV?III transitions. Differences in crystal structure and moisture content may explain the distinct phase transition behaviors of the two types of samples. These results suggest that solid and pure NH ₄ NO ₃ aerosol particles are stable in phase IV under ambient conditions.     

Behavior of blended cement mortars exposed to sulfate solutions cycling in relative humidity

In recent years, several cases of damage to concrete structures due to sulfate exposure have occurred essentially in the above ground parts of structures. Such distress, often characterized by white efflorescence and surface scaling, is driven by salt crystallization in pores and/or repeated reconversions of certain sulfates between their anhydrous and hydrated forms under cycling temperature and relative humidity (RH). However, the effect of the water/cementitious materials ratio (w/cm), pozzolanic additions, and other parameters on the durability of cement-based materials under such exposure conditions is still misunderstood. In this study, 12 cement mortars having different w/cm (0.30, 0.45, and 0.60) and made with ordinary portland cement (OPC) or OPC incorporating 8% silica fume, 25% class F fly ash, or 25% blast furnace slag were made. Standard bars from each of these mortars were submerged in both 10% magnesium sulfate (MgSO₄) and 10% sodium sulfate (Na₂SO₄) solutions; their expansion and surface degradation was monitored for up to 9 months. In addition, cylinders made from these 12 mortars were partially submerged in 50-mm-deep 10% MgSO₄ and 10% Na₂SO₄ solutions. Half of the cylinders were maintained under constant temperature and RH, whereas the others were subjected to cycling RH. The effect of the w/cm and mineral additions on the classic chemical sulfate attack and development of efflorescence was investigated, and the results are discussed in this article.     

Melt Flow Properties During Capillary Extrusion of Linear Low-Density Polyethylene

Abstract

The melt flow properties of a linear low-density polyethylene (LLDPE) were measured by means of a capillary rheometer under the experimental conditions of temperatures from 220° to 260°C and apparent shear rates varying from 12 to 120 s^?1. The end pressure drop (ΔP _end) was determined by employing the Bagley's plotting method. The results showed that ΔP _end increased nonlinearly with increasing shear stress. The end pressure fluctuation phenomenon was observed at lower shear stress level, and several plateau regions were generated in the end pressure drop-shear stress curves, suggesting onset of the wall-slip phenomenon during die extrusion of the resin melt. The critical shear stress with onset end pressure fluctuation phenomenon increased with a rise of temperature. Furthermore, the melt shear flow did not strictly obey the power law. The melt shear viscosity decreased nonlinearly with increasing shear stress and with a rise of temperature, whereas the dependence of the melt shear viscosity on the test temperature accorded with a formula similar to the Arrhenius expression.     

Orimulsion fly ash in clay bricks—part 3: chemical stability of ash-bearing products

The chemical stability of clay bricks containing Orimulsion ash (up to 6 wt.%) was assessed with a laboratory simulation of the brickmaking process. The development of efflorescence, the amount of water soluble salts and their elution, the fraction of S, V, Ni, Mg, Ca, Na and K immobilized in the ceramic matrix, and the amount of volatile elements potentially released during firing were determined by ICP–OES, XRPD, TGA, SEM and EMP analyses. The stabilizing mechanism acts through the capture of metals into the crystalline structure of silicates formed at high temperature. However, the firing conditions adopted in the brick industry (about 900 °C) do not permit the complete reaction of sulphates; there is considerable efflorescence and soluble salts are formed, even with 1.5% of ash added, producing a risk of sulphate attack to the mortars. On the other hand, the decompostion of sulphates during firing could bring about remarkable SO_x emissions, particularly in carbonate-free bodies. In conclusion, the disposal of Orimulsion ash in clay bricks must be practiced with caution and an amount of ash below 1 wt.% weight is strongly recommended.     

Study of Desorption and Adsorption During Grain Aeration

ABSTRACT

One requirement in designing and operating and operating system is to minimize the moisture change in stored grain. Moisture transfer during the aeration process is generally believed to be caused by the partial pressure difference between water in grain and water in air (P₃?P₄). A mathematical model was developed for rough rice, which predicts the partial pressure difference under various grain and ambient conditions. Observations of moisture change over 10 aeration tests confirmed the good correlation between sorption phenomena and partial pressure differences. Studies indicated that air relative humidity, air temperature, grain temperature and grain moisture are significant factors influencing the partial pressure differences and, thus, affecting     

以玄武岩废渣为集料研制胶粘防水砂浆的研究

玄武岩板材是我国近年来开发的新型建筑板材制品,主要用于内外墙体贴面,其胶粘砂浆以水泥砂浆为主。由于玄武岩内部有孔洞会在在板材外墙装饰后局部产生泛碱现象,为此研制出了专用砂浆以提高粘结强度和防水性能,满足玄武岩板材良好的装饰功能。     

Cement-based mixes: Shearing properties and pore pressure

This study presents original results on the rheological measurement of concrete mixes. It focuses on how to determine their mechanical and physical behavior under shearing stress. More specifically, the influence of aggregate content on shearing properties is studied. A vane rheometer was developed to characterize fresh cement-based materials. In addition to the conventional concrete rheometer, a special hydraulic pressure transducer was fitted to the container to monitor the pore water pressure variation while shearing the material. Experiments on cement paste, mortar, and concrete bring a new approach to help us understand the behavior of fresh-state mixes. The results show 1) a correlation between water pore pressure and torque applied on the vane; 2) a critical sand volume fraction, ?_c, as a limit between colloidal interaction behavior and frictional behavior in mortars; beyond this critical fraction, a leap in yield stress and a drop in pore pressure due to granular dilatancy are noticed; 3) the granular content clearly influences the increase in yield stress of the cement mixes: above ?_c, this increase becomes negligible.     

水泥添加量对电解锰渣免烧砖性能的影响

控制骨料掺入量为20％,成型压力为2 MPa,混料含水率为25％的条件下,系统研究了水泥添加量对电解锰渣免烧砖性能的影响.结果表明:当水泥添加量为20％时,电解锰渣免烧砖7 d抗压、抗折强度分别为10.21 MPa和2.35 MPa,28 d抗压、抗折强度分别为13.76 MPa和2.74 MPa,体积密度为1.36 g/cm3,线收缩率为1.13％,吸水率为33.32％,饱和系数为1.47,有轻微泛霜现象产生,强度达到国家烧结普通砖的标准要求,其他性能满足国标一级品的要求.     

Heterogeneous porosity distribution in Portland cement exposed to CO2-rich fluids

Efficient and safe storage of injected supercritical carbon dioxide (CO₂) underground is now one potential solution for reducing CO₂ emissions in the atmosphere. Preventing any CO₂ leakage through a wellbore annulus after injection is a key to maintaining long-term wellbore integrity. Most wells in depleted oil and gas fields may be re-used to inject CO₂. These wells were mostly cemented with conventional Portland cement. It is thus crucial to study how such cement behaves at depth in CO₂-rich fluids.Set cement samples are exposed to CO₂ fluids under pressure and temperature to simulate downhole conditions. The degraded cement exhibits significant mineralogical changes and heterogeneous porosity distribution. The bulk porosity evolution, as well as local porosity gradients through the samples, is quantified using combined mercury porosimetry and back-scattered electron image analysis. Both techniques show an initial sealing stage related to calcium carbonate precipitation plugging the porosity, followed by a dissolution stage marked by a significant increase of porosity.     

Use of porous lavas as supports of photocatalysts

Porous lavas, more precisely pumice stone, are promising supports for TiO₂ used as a photocatalyst. TiO₂ deeply penetrates into pores that favours its retention. Its deposition is convenient and facile and the photocatalytic activity is not significantly affected by the erosion of the surface. The immobilization of TiO₂ on pumice stone gave better results for the photocatalytic degradation of 3-nitrobenzenesulfonic acid than conventional sol–gel dip-coating on cement and red brick. A layer of pumice stone as pellets, fixed on a cement layer and impregnated with TiO₂ is used in a thin film fixed bed reactor, for the photocatalytic treatment of water.     

Temperature Changes during Microwave-Vacuum Drying of Sliced Carrots

Abstract

The temperature changes during microwave-vacuum drying of sliced carrots were investigated. Sliced samples were dried to 7–10% moisture content (wet basis) at a wide range of microwave power and vacuum pressure levels. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However, for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. The experiments also showed that, with the decrease of moisture content X _w (dry basis), for samples with thickness ≤ 8 mm, the drying process of sliced carrots experienced three distinct periods: a warming-up period (X _w  = 7.68) without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation temperature of water in the food at the vacuum pressure; a constant temperature period (2 ≤ X _w  < 7.68) in which most of moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period (X _w  < 2) in which the drying rate decreased and sample temperature increased rapidly. The mathematical models for predicting sliced sample temperature were also developed based on the energy conservation and regression of the experimental date.     

MECHANISM OF TWO-DIMENSIONAL HEAT AND MASS TRANSFER DURING CONVECTTVE DRYING OF POROUS MXDIA UNDER DIFFERENT DRYING CONDITIONS

ABSTRACT

A numerical investigation was conducted to study two-dimensional heat and mass transfer during convective drying of clay brick. The set of macroscopic equations takes into account the effect of gaseous pressure. The established numerical code has allowed us to determine effects of the surrounding air conditions (temperature, pressure and vapor concentration) on drying Kinetic and on space-time evolution of the state variables (temperature, gaseous pressure, and liquid saturation).     

Corrosion resistance of Al–Cr-slag containing chromium–corundum refractories to slags with different basicity

Al–Cr slag is the solid waste generated by the smelting of Cr metal. It presents a range of environmental hazards. This study addressed the corrosion resistance of Al–Cr slag containing chromium–corundum refractories to slags with different basicity. Herein, we provide suggestions for the use of Cr–corundum of different basicity in kilns. Al–Cr slag, brown fused Al₂O₃, and chrome green were used as the raw materials, with pure calcium aluminate cement being used as a binder. The brick samples, prepared using different blends of chrome green and corundum, were fired at 1600?°C, and subsequently subjected to a slag corrosion test. After corrosion by slag of different basicity, the phase composition and microstructure of the sample were analyzed by X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy. There were two major findings. First, Cr–corundum brick made from Al–Cr slag has a better slag corrosion resistance than that made from Cr₂O₃ and brown fused Al₂O₃. Second, Cr–corundum brick made from Al–Cr slag has superior corrosion resistance to slag with a CaO:SiO₂ ratio of 2:1.