Showing papers on "Cement published in 2014"

Geopolymers and Related Alkali-Activated Materials

Abstract: The development of new, sustainable, low-CO2 construction materials is essential if the global construction industry is to reduce the environmental footprint of its activities, which is incurred particularly through the production of Portland cement. One type of non-Portland cement that is attracting particular attention is based on alkali-aluminosilicate chemistry, including the class of binders that have become known as geopolymers. These materials offer technical properties comparable to those of Portland cement, but with a much lower CO2 footprint and with the potential for performance advantages over traditional cements in certain niche applications. This review discusses the synthesis of alkali-activated binders from blast furnace slag, calcined clay (metakaolin), and fly ash, including analysis of the chemical reaction mechanisms and binder phase assemblages that control the early-age and hardened properties of these materials, in particular initial setting and long-term durability. Perspectives fo...

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

Abstract: The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Influence of limestone and anhydrite on the hydration of Portland cements

Abstract: The addition of CaCO 3 and CaSO 4 to Portland cement clinker influences the hydration and the strength development An increase of the CaSO 4 content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength The late compressive strength is decreased in Portland cements containing higher quantities of CaSO 4 The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored Monocarbonate is present only after 1 week and longer independent of the quantity of calcite available and the content of sulphate in the cement

Waste glass in the production of cement and concrete – A review

Abstract: Cement and glass industries are facing a lot of challenges due to the high greenhouse gases emissions, the intensive use of energy and the intensive use of the earth’s natural resources. The current situation of discarding waste glass to landfills is also not offering an environmental friendly management for the waste glass, due to the nonbiodegradable form of the waste glass. However, the chemical composition and the pozzolanic properties of waste glass are encouraging for the use of this waste in the cement and concrete industries and to provide an environmental friendly solution for the glass and cement industries. This paper reviews the different uses of waste glass in cement and concrete and the effect of the glass properties on the performance and durability of the produce cement and concrete.

Effect of activator type and content on properties of alkali-activated slag mortars

Abstract: Investigation of alkali activation of Turkish slag (AAS) was carried out using sodium silicate and sodium hydroxide activators within the scope of this study. The objective of the present work is to determine the SiO 2 /Na 2 O ratios ( M S ) and Na 2 O contents of the solutions on the development of workability, setting times, mechanical properties, drying shrinkage, water absorption characteristics and microstructure of alkali activated slag cement binders. Test results showed that M S and Na 2 O contents of activator solution are of great importance on the properties of AAS. Portland cement free high performance composite with compressive strength values about 100 MPa can easily be achieved by activation of slag without heat curing. Moreover, in case of activation by optimum M S ratio, sodium silicate activated AAS mortars present higher compressive strength, lower water absorption, higher workability, lower porosity and a wide range of setting times compared to NaOH activated AAS mortars and Portland cement mortar. Consequently, it can be said that this new binder is likely to have enormous potential to become an alternative to Portland cement.

Preparation and Mechanical Properties of Graphene Oxide: Cement Nanocomposites

Abstract: We investigate the performance of graphene oxide (GO) in improving mechanical properties of cement composites. A polycarboxylate superplasticizer was used to improve the dispersion of GO flakes in the cement. The mechanical strength of graphene-cement nanocomposites containing 0.1–2 wt% GO and 0.5 wt% superplasticizer was measured and compared with that of cement prepared without GO. We found that the tensile strength of the cement mortar increased with GO content, reaching 1.5%, a 48% increase in tensile strength. Ultra high-resolution field emission scanning electron microscopy (FE-SEM) used to observe the fracture surface of samples containing 1.5 wt% GO indicated that the nano-GO flakes were well dispersed in the matrix, and no aggregates were observed. FE-SEM observation also revealed good bonding between the GO surfaces and the surrounding cement matrix. In addition, XRD diffraction data showed growth of the calcium silicate hydrates (C-S-H) gels in GO cement mortar compared with the normal cement mortar.

High-volume natural volcanic pozzolan and limestone powder as partial replacements for portland cement in self-compacting and sustainable concrete

Abstract: A laboratory study demonstrates that high volume, 45% by mass replacement of portland cement (OPC) with 30% finely-ground basaltic ash from Saudi Arabia (NP) and 15% limestone powder (LS) produces concrete with good workability, high 28-day compressive strength (39 MPa), excellent one year strength (57 MPa), and very high resistance to chloride penetration. Conventional OPC is produced by intergrinding 95% portland clinker and 5% gypsum, and its clinker factor (CF) thus equals 0.95. With 30% NP and 15% LS portland clinker replacement, the CF of the blended ternary PC equals 0.52 so that 48% CO2 emissions could be avoided, while enhancing strength development and durability in the resulting self-compacting concrete (SCC). Petrographic and scanning electron microscopy (SEM) investigations of the crushed NP and finely-ground NP in the concretes provide new insights into the heterogeneous fine-scale cementitious hydration products associated with basaltic ash-portland cement reactions.

Resistance of concrete and mortar against combined attack of chloride and sodium sulphate

Abstract: Marine environments are typically aggressive to concrete structures, since sea water contains high concentrations of chlorides and sulphates. To improve predictions of concrete durability within such environments, it is important to understand the attack mechanisms of these ions in combination. In this research, the reciprocal influence of Cl− and SO42− was investigated for four mixtures, namely with Ordinary Portland Cement, High Sulphate Resistant cement, and with Blast-Furnace Slag (50% and 70% cement replacement). Chloride penetration depths and diffusion coefficients were measured to investigate the influence of SO42− on Cl− attack. Besides, length and mass change measurements were performed to examine the influence of Cl− on SO42− attack. Since the formation of ettringite, gypsum and Friedel’s salt plays an important role, XRD-analyses were done additionally. It can be concluded that chloride penetration increases when the sulphate content increases at short immersion periods, except for HSR concrete. Concerning the sulphate attack, the presence of chlorides has a mitigating effect.

A hydration study of various calcium sulfoaluminate cements

Abstract: The present work studies the hydration process and microstructural features of five calcium sulfoaluminate (CSA) cements and a ternary mixture including also ordinary Portland cement (OPC). The pastes were studied with simultaneous differential thermal-thermogravimetric (DTA-TG) analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and expansion/shrinkage tests. The DTA-TG analysis confirmed the role of the hydration reactions involving the main CSA clinker constituent, tetracalcium trialuminate sulfate, which produced (i) ettringite when combined with lime and calcium sulfate, (ii) ettringite and aluminum hydroxide in the presence of calcium sulfate alone, and (iii) monosulfate and aluminum hydroxide in the absence of both lime and calcium sulfate. The MIP and SEM were able to discriminate between expansive (ternary mixture and CSA cement containing 50% gypsum) and non-expansive cements. Expansive cement pastes had (i) a nearly unimodal pore size distribution shifted toward higher radii and (ii) ettringite crystals smaller in size during the first day of curing. In a SEM image of a hardened paste of the CSA cement containing 50% gypsum, a stellate ettringite cluster was observed.