采用双掺法配制沙漠砂混凝土性能试验研究

运用同时掺加机制砂和高性能减水剂的"双掺法"制备了不同强度等级的沙漠砂混凝土,分析了高性能减水剂对沙漠砂混凝土性能的影响及不同强度等级下采用双掺法配制的沙漠砂混凝土与普通中砂混凝土坍落度及强度等性能上的区别。结果表明,高性能减水剂不仅能改善沙漠砂混凝土的和易性,且具有早强效果,并对最终强度的提高有一定帮助。双掺法制备的沙漠砂混凝土与同配合比的普通中砂混凝土相比,坍落度相差不大,在强度方面当所配混凝土强度等级较低时,二者的抗压强度相近;随着混凝土强度等级的提高,沙漠砂混凝土的抗压强度逐渐低于普通中砂混凝土。此外,与按照常规方法制备的沙漠砂混凝土相比,双掺法制备的沙漠砂混凝土的坍落度有很大提高。     

Compressive strength and preliminary in vitro evaluation of gypsum and gypsum–polymer composites in protein-free SBF at 37 °C

Gypsum is a bioresorbable material that has been used in many applications such as tissue regeneration. Mechanical properties of gypsum have limited its applications to non-load bearing sites. The current study aimed at studying the compressive strength and behaviour of gypsum–polymer composites in protein-free simulated body fluids (SBF). Polymers studied were poly(vinyl alcohol) (PVA) and its copolymers with vinyl acetate and itaconic acid in addition to vinyl acetate and vinyl chloride. Composites with the highest compressive strength results were chosen for the preliminary in vitro evaluation in protein-free SBF solutions. Changes in the concentrations of Ca²⁺ and PO₄³⁻ ions, weight loss and morphology of the solid samples were monitored after soaking them in SBF and 1.5 SBF solutions. Results showed resorption of gypsum, concurrently with deposition of apatite in all composites, including polymer-free gypsum. Mechanical integrities of all samples were maintained, suggesting their stabilities when used as bone cements.     

High-temperature elasticity of fibrous ceramics with a bird's nest structure

New fibrous ceramics with polycrystalline mullite fibers as the matrix and silica–boron sols as the high temperature binder, which was inspired by the bird's nest structure in nature, were synthesized. The most important structure characteristic of this fibrous material is that the silica–boron binder only fixed the fibers at the crossing points rather than filled the pores among the fibers. The elastic behavior was investigated, both at room temperature and elevated temperature. Compared to conventional ceramic matrix composites, the samples show a much higher degree of elasticity because of the bending of the fibers. The rebound resilience decreased slowly with the increase of the temperature, but it still remained 86% of that at ambient temperature at 1000 °C. The sample exhibits good elasticity performance, relatively high strength (2.25 MPa) and high porosity (83%) indicating it is a potential high-temperature seal material.     

The effect of particle size distribution on the properties of blended cements incorporating GGBFS and natural pozzolan (NP)

This paper investigates the effect of particle size distribution on the properties of blended cements incorporating ground granulated blast-furnace slag (GGBFS) and natural pozzolan (NP). Pure Portland cement (PPC), NP and GGBFS were used to obtain blended cements that contain 10, 20, 30% additives. The cements were produced by intergrinding and separate grinding and then blending. Each group had two different Blaine fineness of 280 m²/g and 480 m²/g. According to the particle size distribution (PSD) curves, 46% of the coarser specimens and 69% of the finer specimens passed through the 20 μm sieve. It was observed that the separately ground specimens were relatively finer than the interground ones and had higher compressive strength and sulfate resistance. The separately ground coarser specimens had the lowest heat of hydration. The separately ground finer specimens, which had the highest compressive strength and sulfate resistance, had the highest percent passing for each sieve size. For these specimens 34, 69, 81 and 99% passed through 5, 20, 30 and 55 μm sieves, respectively. For the interground specimens, which had the same fineness, the respective values for the same sieves were 32, 68, 75 and 94%.     

An investigation on the use of tincal ore waste, fly ash, and coal bottom ash as Portland cement replacement materials

The possibility of using tincal ore waste (TW), coal bottom ash (BA), and fly ash (FA) as partial replacement in concrete was examined through a number of tests. The properties examined include setting time, compressive strength, mortar expansion, water consistency of mortar, and microstructure. The results showed that compressive strength of all specimens containing 1 wt.% of TW was higher than that of the control at the 28th day of curing. At 90 days, the contribution to strength by BA+TW and FA+TW was higher than in the concrete-prepared equivalent TW beyond 3 wt.% of Portland cement (PC) replacement. With the replacement of 3-5 wt.% of PC by TW, the compressive strength of the concrete decreased compared to control concrete. However, the values obtained are within the limit of Turkish Standards (TS). Adding BA or FA with TW improved the performance relative to TW replacement only. Increasing replacement of TW gives rise to a higher setting time. As a result, TW, BA, and FA samples may be used as cementitious materials.     

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.     

The optimization of a gypsum-based composite material

Contemporary requirements for gypsum-based composite materials (GBCM) for rendering or plastering include controlled setting time, good workability, sag resistance, high compressive and flexural strength, perfect bond to concrete or brick, water resistance, and improved heat and noise insulation. The application of a number of chemical admixtures and mineral additives was found to be necessary to provide the required performance for gypsum-based materials. Among the necessary chemical admixtures are the following: a retarding admixture, a water-soluble polymer (MC), an air-entraining admixture (AE), and a superplasticizer (SP). This paper describes the effect of the different admixtures on the consistency, setting time, and the compressive strength of GBCM. It also discusses the application of the stepwise optimization (SWO) method for the evaluation of the GBCM composition.     

Strain distribution on the surface of specimen in superplastic compression

Strain distribution on the surface of specimens during superplastic flow in compression is determined. It is shown to be distinctly different from what has been reported for tensile superplasticity. The origins of these differences are discussed.     

Use of zeolite, coal bottom ash and fly ash as replacement materials in cement production

In this research, the effects of zeolite, coal bottom ash and fly ash as Portland cement replacement materials on the properties of cement are investigated through three different combinations of tests. These materials are substituted for Portland cement in different proportions, and physical properties such as setting time, volume expansion, compressive strength and water consistency of the mortar are determined. Then, these physical properties are compared with those of PC 42.5. The results showed that replacement materials have some effects on the mechanical properties of the cement. The inclusion of zeolite up to the level of 15% resulted in an increase in compressive strength at early ages, but resulted in a decrease in compressive strength when used in combination with fly ash. Also, setting time was decreased when zeolite was substituted. The results obtained were compared with Turkish Standards (TS), and it was found that they are above the minimum requirements.     

Wear resistance of polymer-impregnated mortars and concrete

The wear resistance of mortar and concrete samples untreated and impregnated with solutions of epoxide resin was tested. The compressive strength has a decisive influence on the abrasion of untreated mortar and concrete samples. The idealized diameter area of the maximal grain of the gravel in the sample has less influence on it. The wear resistance of impregnated samples with low compressive strength markedly increases. The hardener used influences the polymerization rate of the activated resin in the structure of samples, with time, and their abrasion is influenced also in this way.