T. Vázquez

Alkali-activated fly ash/slag cements: Strength behaviour and hydration products

Abstract The activation of fly ash/slag pastes with NaOH solutions have been studied. The parameters of the process studied are: activator concentration (NaOH 2 and 10 M), curing temperature (25°C and 65°C), and fly ash/slag ratios (100/0, 70/30, 50/50, 30/70, and 0/100). The equations of the models describing the mechanical behaviour of these pastes have been established as a function of the factors and levels considered. The ratio of fly ash/slag and the activator concentration always result to be significative factors. The influence of curing temperature in the development of the strength of the pastes is lower than the contribution due to other factors. At 28 days of reaction, the mixture 50% fly ash/50% slag activated with 10 M NaOH and cured at 25°C, develop compressive mechanical strengths of about 50 MPa. The nature of the reaction products in these pastes has been studied by insoluble residue in HCl acid, XRD, FTIR and MAS NMR. It has been verified that slag reacts almost completely. It has also been determined that the fly ash is partially dissolved and participates in the reactive process, even in pastes activated at ambient temperature. The main reaction product in these pastes is a hydrated calcium silicate, like CSH gel, with high amounts of tetracoordinated Al in its structure, as well as Na ions in the interlayer spaces. No hydrated alkaline alumino-silicates with three-dimensional structure characteristics of the alkaline activation of fly ashes were formed.

Chemical stability of cementitious materials based on metakaolin

The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.

MECHANICAL AND DURABLE BEHAVIOUR OF ALKALINE CEMENT MORTARS REINFORCED WITH POLYPROPYLENE FIBRES

The development of new binders, alternative to traditional cements and concretes obtained by the alkaline activation of different industrial by-products (blast furnace slags and/or fly ashes), is an ongoing study and research topic of the scientific community. The mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres has been the object of the present investigation. Three different alkaline matrices were used: (a) granulated blast furnace slag activated with waterglass (Na2SiO3+NaOH) with a concentration of 4% Na2O by mass of slag and cured at room temperature, (b) aluminosilicate fly ash activated with 8M NaOH and cured at 85 °C during the first 24 h and (c) 50% fly ash+50% slag activated with 8M NaOH solution at room temperature. In the mechanical tests (flexural and compressive strengths), two different dosages of fibres were used: 0.5% and 1% by mortar volume. Shrinkage tests according to ASTM C 806-87 standard with (1%) and without fibres were also carried out. The durability tests carried out were freeze/thaw and wet/dry cycles. In these tests, the dosage of fibre was 0.5% by mortar volume. The results obtained show that the nature of the matrix is the most important factor to strength development, more than fibre presence and content amount.

Procedure of synthesis of thaumasite

Abstract In this work, synthesis procedures to obtain thaumasite were revised. Synthesis of thaumasite applying Strubles method (mix of two cold sugary solutions containing CaO and Na 2 SiO 3 +Na 2 SO 4 +Na 2 CO 3 , respectively, in stoichiometric proportions) was carried out. Amorphous calcium carbonates and calcium silicate hydrates (intermediate products) were obtained at the very beginning stages of the reaction, which evolved with time forming thaumasite. Thaumasite polluted by calcite and calcium silicate hydrate was obtained after 2 years of reaction. Pure thaumasite was obtained when initial solutions contained less sodium carbonate and sodium silicate than the stoichiometric proportions. The synthesized thaumasite was characterized by X-ray diffraction (XRD), FTIR, SEM/EDX, and NMR.

Behaviour of cement mortars containing an industrial waste from aluminium refining: Stability in Ca(OH)2 solutions

Abstract The physical and chemical interaction between a solid industrial waste from aluminium refining and saturated Ca(OH)2 solution, as well as the effects of substituting siliceous sand for the waste on the physical and mechanical properties of mortars were studied. The waste is a solid that contains reactive alumina capable of combining with the calcium hydroxide. These reactions result in stable and insoluble compounds. This alumina, together with the halite (also present in the waste composition), chemically react with a saturated solution of Ca(OH)2, giving as a main reaction product the so-called Friedels salt (Ca4Al2Cl2O6 · 10H2O). Stratlingite and Si-hydrogarnets were among other products detected. The waste has a high specific surface area. The cement/waste mixtures therefore require a higher quantity of mixing water than cement/sand mixtures. The result is a decrease of the mechanical strengths and an increase of the total porosity. However, a decrease of the average size of the pores occurs, which can have a positive effect on the durability of the final material.

Stable Ca3SiO5 solid solution containing manganese and phosphorus

Abstract The effect of simultaneous addition of varying concentrations of manganese and phosphorus oxides on a C S ≈ 3 composition has been investigated. Mineralogical changes on synthesized powders were identified by X-ray diffraction, Infra-red spectroscopy and DTA. M3 polymorphs alite formation with Mn and P inclusions less than 0.69 (wt%) Mn2O3 and 0.62 (wt%) P2O5 is shown. The Electron Paramagnetic Resonance investigations of synthesized monoclinic C3S show that Mn ions are in octahedral site as Mn2+ oxidation state. The formed solid solutions could be described with the following cationic vacancies formula: (Ca3−5X Mn4XX)(Si1−2X p2x) O5, with X ≤ 0.005.

A study of a new liquid phase to obtain low-energy cements

Abstract The composition of the melt phase, found at approximately 1180°C in CaOSiO 2 Al 2 O 3 Fe 2 O 3 CaSO 4 CaF 2 ystems has been studied. We verified its usefulness in obtaining clinkers of high alite content at 1300°C, with low energy consumption.

Influence of KOH solution on the hydration and carbonation of high alumina cement mortars

The influence of KOH presence on the evolution of hydration and carbonation of high alumina cement mortars at two different curing temperatures (4 and 40 °C) has been studied. It has been confirmed that hydration reactions at both temperatures are accelerated with KOH presence and it has a great influence on hydrated and carbonated species. The massive deposition of carbonation products leads, in all cases to a decrease in mortar porosity which leads to an increase of mechanical strengths.

Effect of fluorspar on the formation of clinker phases

Abstract The evolution of the phases of a commercial clinker in relation to the time and temperature on the clinkerization has been studied. The influence on this evolution by the addition of a small quantity of fluorspar (/t 0.2%) has been determined. The work was basically supported by a study carried out by IR spectroscopy.

Modelling of the burnability of white cement raw mixes made with CaF2 and CaSO4

Abstract Burnability of white cement raw mixes made using CaF2 and CaSO4 as fluxes/mineralizers is studied and modelled. A methodology based on a statistical method called experimental design is used to establish the model. An equation that explains the burnability of these new raw mixes is established. Through this equation, it is deduced that limestone particle size, fluorite and gypsum content have a major influence on burnability, while silica modulus has little effect.