Utilization of borogypsum as set retarder in Portland cement production

Boron ores are used in the production of various boron compounds such as boric acid, borax and boron oxide. Boric acid is produced by reacting colemanite(2CaO·3B₂O₃·5H₂O) with sulphuric acid and a large quantity of borogypsum is formed during this production. This waste causes various environmental problems when discharged directly to the environment. Portland cement is the most important material in the building industry. This material is produced by adding about 3-5% gypsum (CaSO₄·2H₂O) to clinker as a set retarder. The aim of this study was to stabilize borogypsum, and to produce cements by adding borogypsum instead of natural gypsum to clinker. Concrete using cement produced with borogypsum was tested to find the mechanical properties and the test values were compared with those of concrete from cement with natural gypsum. Compressive strength of concrete from cement produced with borogypsum was found to be higher than that of natural gypsum. Also, the setting time of cement with borogypsum was longer than that of the Portland cement.     

Setting and hardening of borogypsum-Portland cement clinker-fly ash blends. Studies on effects of molasses on properties of mortar containing borogypsum

The present work describes a study of setting and hardening of blends of borogypsum, fly ash, and Portland cement clinker (PC). The possibility of using borogypsum instead of natural gypsum in fly ash-cement matrix has been investigated through several tests. In addition, the effects of molasses on the setting times of cement and strength of the mortar were also studied. The setting times of the cement were retarded when the natural gypsum was replaced by borogypsum. Molasses exhibited a rather significant retarding effect when used in combination with borogypsum in cement. The inclusion of molasses to the system at a level of 0.1% resulted in a reduction in early strength of the mortar. However, it significantly enhanced the strength of the mortar after 7 days of curing age. In general, the cement prepared with borogypsum was found to have similar strength properties to those obtained with natural gypsum, and inclusion of molasses into the system significantly increased the strength of the sample after 7 days of curing age.     

Sulfate attack on alkali-activated slag concrete

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete in sulfate environment. Two tests were used to determine resistance of AAS concrete to sulfate attack. These tests involved immersion in 5% magnesium sulfate and 5% sodium sulfate solutions. The main parameters studied were evolution of compressive strength, products of degradation, and microstructural changes. After 12 months of exposure to the sodium sulfate solution, the strength decrease was up to 17% for AAS concrete and up to 25% for ordinary Portland cement (OPC) concrete. After the same period of exposure to the magnesium sulfate solution, the compressive strength decrease was more substantial, up to 37% for OPC and 23% for AAS. The main products of degradation were ettringite and gypsum in the case of Portland cement and gypsum in AAS. OPC samples had significant expansion, cracking, and loss of concrete, while AAS samples were not expanded but cracked in the test. During experiments with the sodium sulfate solution, some increase in strength of AAS concrete was recorded, likely due to continuing hydration.     

Characteristics of pastes from a Portland cement containing different amounts of natural pozzolan

This work is concerned with assessing the influence of natural pozzolan on the physical, mechanical and durability properties of blended Portland cement pastes. The results indicate that final setting times of natural pozzolan blended Portland cement pastes range from 4 to about 5 h. Naphthalene-type superplasticizer tends to retard the hydration process of plain and natural pozzolan blended Portland cement pastes. These blends show slightly higher setting times than those without superplasticizer. The use of superplasticizer is found to have a significant influence on the workability. At a lower level of Portland cement replacement by natural pozzolan, the addition of 1% superplasticizer by weight of blended Portland cement leads to a significant decrease in the water to Portland cement plus natural pozzolan ratio for a given workability. However, for the blended Portland cement with a high proportion of natural pozzolan, the increase in water content causes the porosity to increase with an accompanying decrease in compressive strengths. The variations in composition and cure time are found to provide significant changes in compressive strength. Depending on these parameters, the variation in compressive strength can be estimated by using the equation, σ=σ₀/[1+exp(a+bp+cp²)]ⁿ, where σ is the compressive strength of natural pozzolan blended Portland cement paste at a given cure time and natural pozzolan replacement level (MPa); σ₀ is the compressive strength of plain Portland cement pastes with or without superplasticizer at a given cure time (MPa); p is the natural pozzolan replacement level (%); a, b, c, n are the empirical constants to be determined. The blend with a composition of 80% Portland cement and 20% natural pozzolan and 1% superplasticizer provides superior strength and durability characteristics in comparison to the counterparts without superplasticizer and to the blends with a high proportion of natural pozzolan. At high contents of natural pozzolan, the resistance to freezing and thawing is found to be impaired. Moreover, these blended cements do not provide high durability performance against sulfate attack.     

Studies on high-performance blended/multiblended cements and their durability characteristics

Number of blends were prepared by intergrinding clinker, gypsum, fly ash, calcined clay, microsilica and limestone in laboratory ball mill in varying percentages, and their physical properties such as fineness, consistency, setting time and compressive strength have been determined. The durability tests on selected compositions were also conducted by exposing the mortar cubes separately in 5% Na₂SO₄ and 5% NaCl solutions till the age of 90 and 180 days. The performance was observed by compressive strength development criteria after various length of exposure. Results have been discussed and found that the durability of blended cement is higher than the ordinary Portland cement.     

Reactive pozzolana from Indian clays—their use in cement mortars

Studies were undertaken to produce reactive pozzolana i.e. metakaolin from four kaolinitic clays collected from different sources in India. The metakaolin produced from these clays at 700-800 °C show lime reactivity in between 10.5 to 11.5 N/mm² which is equivalent to commercially available calcined clay Metacem-85. The microstructure of the metakaolin has been reported. The effect of addition of metakaolin up to 25% in the Portland cement mortars was investigated. An increase in compressive strength and decrease of porosity and pore diameter of cement mortars containing metakaolin (10%) was noted over the cement mortars without metakaolin. The hydration of metakaolin blended cement mortars was investigated by differential thermal analysis (DTA) and scanning electron microscopy (SEM). The major hydraulic products like C-S-H and C₄AH₁₃ have been identified. Durability of the cement mortars with and without metakaolin was examined in different sulphate solutions. Data show better strength achievement in cement mortars containing 10% MK than the OPC mortars alone.     

Utilization of weathered phosphogypsum as set retarder in Portland cement

In this study, usability of weathered phosphogypsum (PG) from residue areas as set retarder in Portland cement was investigated. The effects on the setting and mechanical properties of PG added in ratios 1, 3, 5, 7, 10, and 12.5 wt.% to Portland cements were studied and compared with a Portland cement containing natural gypsum (NG). It was found that PG can be used in place of NG for Portland cement according to Turkish standards. The highest 28-day compressive strength was found in the sample with 3 wt.% PG.     

Alternative calcium-sulfate-bearing materials as cement retarders: Part II. FGD gypsum

The aim of this paper is to investigate the possible displacement of natural gypsum (CaSO₄·2H₂O) in cement with an alternative setting retarder, such as the industrial by-product derived from flue gas desulfurization process called FGD gypsum. These calcium-sulfate-bearing materials (CSBM), alone or in mixtures, were ground with clinker both in laboratory and industrial scale to examine their influence on the physical and mechanical properties of cement, as well as on the industrial production line of cement. From the present work, it is extracted that the use of mixtures of sulfate-bearing materials with gypsum seems to be advantageous for the actual control of setting time. The addition of FGD gypsum increases setting time without affecting compressive strength profile. During the industrial trial, the formation of hemihydrate form of calcium sulfate dihydrate has a profound regulatory effect on the setting and strength performance of the cement partially replaced with FGD gypsum.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

The permeability of Portland limestone cement concrete

The effect of limestone addition on the air permeability, water permeability, sorptivity, and porosity of limestone cement concrete has been investigated. Six Portland limestone cements (PLCs) with different limestone content (10-35% w/w) were produced by intergrinding clinker, gypsum, and limestone. A water-to-cement ratio (w/c) of 0.70-0.62—depending on the cement strength class—was used to prepare concrete of the compressive strength class C20/25 of EN 206-1. A modified commercial triaxial cell for 100-mm-diameter samples was used for the determination of the gas (N₂) and the water permeability of concretes. In addition, the sorptivity and porosity of the samples were measured, while thin sections of the concrete specimens were examined by means of optical microscopy. It is concluded that the PLC concrete indicates competitive properties with the ordinary Portland cement (OPC) concrete. Furthermore, the limestone addition has a positive effect on the water permeability and the sorptivity of concrete.     

Examination of mortar bars containing varying percentages of coarsely crystalline gypsum as aggregate

Contamination of aggregate sources by coarsely crystalline gypsum occurs frequently in the Middle East. Mortar bars were made which contained up to 5% gypsum (by weight of aggregate) in the form of aggregate pieces. The bars were made using three different cements of varying C₃A content and were stored at 20°C and 38°C. The results show that significant expansions do not occur within mortar bars if their total sulphate content lies below the present British Standard limit of 4% SO₃ by weight of cement. Sulphate Resisting Portland Cement (SRPC) can tolerate a higher level of contaminant gypsum than Ordinary Portland Cement (OPC). Temperature also effects the degree of expansion, especially in the case of high C₃A cement.     

Physico-chemical properties of by-product gypsum

Four samples of by-product gypsum were analysed by chemical and physical methods and subjected to comparable degrees of calcination in a special rotary furnace. The hydration characteristics of the resulting calcium sulphate hemihydrates were determined in terms of three basic parameters viz. α₀, a measure of heterogeneous nucleation in the system; k, a measure of self-acceleration of the reaction; and θ, the period of induction. Measurements were made of the size distribution of particles, pH, water requirement, the colour and compressive strength of cast gypsums made from the hemihydrates, and comparisons were made with two samples of calcined natural gypsum. Additions of calcium oxide were also made to the by-product gypsums and the same procedures followed. Hydration of the calcined products resulted in considerable differences in the kinetic parameters. Both differences in nucleation and transport of calcium sulphate from the grains of hemihydrate to the growing crystals of the dihydrate seemed to contribute to variations in reactivity. The by-product gypsums calcined alone, or in the presence of calcium oxide, yielded hemihydrates with water requirements greater than those of the calcined natural gypsums. The compressive strengths of cast gypsums made from these hemihydrates were somewhat less than those made from the calcined natural gypsums. The colour co-ordinates were about the same. After treatment with calcium oxide, the by-product gypsum appears to be suitable for making plaster of Paris.     

超硫酸盐水泥净浆的酸性侵蚀劣化机制

本文对比研究了超硫酸盐水泥(SSC)与硅酸盐水泥在盐酸侵蚀条件下的力学性能变化规律,探讨了SSC的酸性侵蚀劣化机制。测试了SSC净浆试件正常养护后在盐酸溶液以及清水中抗压强度的变化,并计算了抗压强度保持率;采用X射线衍射(XRD)、扫描电子显微镜(SEM)和热重分析(TG-DTG)等微观测试方法分析了SSC在盐酸侵蚀下水化产物种类和数量的变化,并探究了其微观结构的演化过程,分析了劣化机理。结果表明:不同于硅酸盐体系,SSC体系主要水化产物为钙矾石和水化硅酸钙;与硅酸盐水泥相比,SSC在酸中具有较高的抗压强度保持率和更好的抗酸性侵蚀性能;盐酸侵蚀SSC后,SSC主要水化产物均被分解,体系中存在二水石膏和大量的二氧化硅胶体,而硅酸盐水泥体系中则存在大量的二氧化硅胶体。     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Utilization of secondary lead slag as construction material

Secondary lead slag, a waste product from battery smelting using CaCO₃ as flux, has been investigated for its use as an admixture and/or aggregate in the production of concrete blocks. The slag was added as partial replacements of cement and/or aggregate. The results revealed that the oxide components of the slag were similar to those of ordinary Portland cement (OPC). The CaO content in the slag is 6.2 times less than that in OPC, while its iron content, as FeO, is 15.1 times higher. Interestingly, it also possesses magnetic property. All samples exhibited higher compressive strengths than that of the sample without slag (STD) which increased with increasing the slag contents and ages. The highest compressive strength was of the sample containing 20% slag as cement substituent and 100% slag as aggregate replacement owing to 259% of that of the STD at 60 days. All samples showed higher water absorption than that of the STD. The higher the slag contents, the more the water absorption. The absorption was, as expected, decreased with ages. Magnetic property of the slag plays an important role in the properties of the concrete blocks. For environmental concern, leachability of lead (Pb) from all samples was also carried out.     

Comparison of solidification/stabilization effects of calcite between Australian and South Korean cements

The differences in the effect of calcite on the strength and stability of Pb-rich wastes solidified and stabilized using Australian and South Korean ordinary Portland cements are examined in this study. Pb-rich waste stabilized using Australian OPC has been shown to possess both substantially higher unconfined compressive strength and lead immobilization ability than South Korean OPC as a result of its higher C₃S content and the associated enhanced degree of precipitation of lead on the surfaces of silicate phases present. Calcite addition is observed to have an accelerating effect on the OPC-induced solidification/stabilization of Pb-rich wastes as gauged by the unconfined compressive strength and leachability of the solids formed. This effect is observed to be far more dramatic for South Korean OPC than for Australian OPC. Using scanning electron microscopy, waste stabilized with cement and calcite was observed to develop significantly greater proportions of hydrated crystals than wastes stabilized with cement alone. The results of X-ray diffraction studies have shown that the presence of calcite in South Korean OPC results in greater acceleration in the formation of portlandite than is the case for Australian OPC.     

Superplasticized portland cement: production and compressive strength of mortars and concrete

This paper deals with the effect of intergrinding different percentages of a naphthalene-based superplasticizer with Portland cement clinker and gypsum on the fineness of the product, and on the water requirement and the compressive strength of the mortars made with the superplasticized cement. The properties of the fresh and hardened concrete made with the superplasticized cements were also investigated. The results showed that the intergrinding of a given amount of a naphthalene-based superplasticizer with Portland clinker and gypsum reduced the grinding time required for obtaining the same Blaine fineness as that of the control Portland cement without the superplasticizer. The water requirement of the mortars made with the superplasticized cements was similar to that of the mortars made with the control Portland cements when the same amount of the superplasticizer was added at the mortar mixer; for a given grinding time and a Blaine fineness of 4500 cm²/g, the mortars made with the superplasticized cement had higher compressive strength than those made with the control Portland cement. For a given grinding time or Blaine fineness of cement ≥5000 cm²/g, the slump loss, air content stability, bleeding, autogenous temperature rise, setting times, and compressive strength of the concrete made with the superplasticized cements were generally comparable to those of the concrete made with the control Portland cements when the superplasticizer was added at the concrete mixer.     

Investigation into relations among technological properties, hydration kinetics and early age hydration of self-leveling underlayments

Technological properties such as flow value, setting times, compressive strength and early age dimensional stability as linear shrinkage and expansion have been studied for two types of self-leveling underlayments (SLU) in which the kind of calcium sulfates was varied. The influence on hydration kinetics has been measured by isothermal heat flow calorimetry. The results obtained for the technological properties change significantly when different kinds of calcium sulfates are used. The basic trend for the results changes when the composition changes from a calcium aluminate cement system to a Portland cement system. Additionally, there was an interesting relationship to final dimensional stability and shape of heat evolution curve. Moreover the time to reach plateau of dimensional stability was related to the development of compressive strength. In the meanwhile, using hemihydrate in Portland cement systems caused low compressive strength and significant expansion. On the other hand, in the results of XRD measurements, the first genesis of Ettringite corresponded to the first shrinkage of SLU. Interesting results related with technological properties and hydration kinetics or the results of XRD other than these above results were obtained.     

Physicochemical, mineralogical, and morphological characteristics of concrete exposed to elevated temperatures

Concrete cubes prepared from ordinary Portland cement (OPC) of known chemical, mineralogical, and physical performance characteristics and fired to various temperature regimes up to 1000 °C in steps of 100 °C for a constant period of 5 h have been studied by using X-ray diffraction (XRD) and DTA/TGA to establish the effect of elevated temperatures on the mineralogical changes occurring in the hydrated phases of concrete. The changes in physical state of concrete were studied by measuring ultrasonic pulse velocity (UPV) and consequent deterioration in the compressive strength with increase in temperature. Scanning electron microscopy (SEM) studies showed distinct morphological changes corresponding to deterioration of concrete exposed to higher temperatures.     

铝土矿选尾矿制备硅铝聚合材料的反应产物及抗化学侵蚀性能(英文)

以煅烧铝土矿选尾矿为硅铝质原料,以矿渣微粉为促硬剂,以水玻璃为激发剂,制备得到了硅铝聚合材料。运用X射线衍射、扫描电镜、热分析研究了硬化浆体的反应产物及其微观形貌、热性质。通过观察3%硫酸钠溶液、3%硫酸镁溶液、5%硫酸溶液、5%盐酸溶液对砂浆试样外观、质量、强度的影响,研究了其抗化学侵蚀性能,并比较了其与铝酸盐水泥、快硬早强硫铝酸盐水泥、中抗硫酸盐硅酸盐水泥及矿渣硅酸盐水泥的区别。结果表明:反应并不生成晶体物质,而是无定形态的铝硅酸盐;硬化体中呈现片层状显微形貌的物质能够吸附水分,从而在灼烧过程中表现为脱水吸热及质量损失;该片层状物质随着龄期的延长而变得愈发细小和复杂,进而在宏观上表现为强度增长及脱水温度升高;硅铝聚合砂浆分别经3%硫酸钠溶液、3%硫酸镁溶液浸泡28d后,与各水泥砂浆试样比较,其不仅外观完整,而且强度并没有下降,反而具有几乎相同的强度增长,即说明其具有更优异的抗硫酸侵蚀性能;硅铝聚合砂浆与各水泥砂浆经稀酸溶液浸泡28d后,前者不仅能保持原始外观,而且表现为更低的质量及强度损失。