J.H. Sharp

Effect of temperature on the hydration of the main clinker phases in portland cements: part ii, blended cements

The hydration of two Mexican Portland cements has been investigated at five temperatures in the range from 10 to 60°C. Samples were tested after eight periods of hydration during which they were immersed in water for between 1 day and 360 days, using quantitative X-ray diffraction, thermogravimetry, compressive strength determination, and scanning electron microscopy. Increased temperature initially accelerated the hydration of the four major anhydrous phases present in both cements. In the longer term, however, a reduced degree of hydration was observed for the alite and ferrite phases, accompanied by decreased compressive strength and increased apparent porosity.

The microstructure and mechanical properties of blended cements hydrated at various temperatures

The development of microstructure and compressive strength of three blended cement pastes hydrated at temperatures ranging from 10°C to 60°C is described. The replacement materials were pulverised fuel ash (PFA), volcanic ash (VA), and ground, granulated blast furnace slag (GGBFS), and the blended cements had the same compositions as those reported in previous studies. The cement pastes were cured under water and tested for compressive strength at various time intervals over a period of 1 year. The only blended cement paste that had substantially improved strength compared with the neat cement paste was that containing blast furnace slag, especially at 60°C. Selected samples were examined by backscattered electrons in scanning electron microscopy (SEM). Generally, the microstructures of the pastes cured at 60°C showed greater apparent porosity than those cured at 10°C. The mechanism of hydration of the various blended cements is discussed.

Thaumasite formation in Portland-limestone cement pastes

Small cylinders (10-mm diameter × 10-mm height) made at a water:solids ratio of 0.5 from Portland cement with 0, 5, 15, and 30% limestone additions were cured in water at room temperature for 28 days. They were subsequently stored in various solutions at 5°C for periods of up to 420 days. The pastes were inspected visually and examined by X-ray diffraction every 28 days. Selected samples were also examined by thermal analysis and scanning electron microscopy. Pastes containing fine limestone additions were susceptible to formation of thaumasite after only a few months of exposure to sulfate solutions. The extent of thaumasite formation was greater with increasing limestone additions and when magnesium sulfate was present in the solution. Thaumasite formation was then accompanied by formation of brucite and secondary gypsum. Calcium hydroxide was a reactant rather than a reaction product and C-S-H gel was also consumed.

Industrial trial production of low energy belite cement

Abstract The Portland cement industry consumes large amounts of energy and produces huge quantities of carbon dioxide, which contribute to global warming, the so-called “Greenhouse Effect”. Industrial trials are reported for the production of belite cements (≈3000 t) at lower temperatures and with lower lime saturation factors than for ordinary Portland cement. Belite cements with reasonably good properties have been made on an industrial scale from limestone, burnt clay, volcanic ash, pyrite ash and gypsum. A rapid rate of cooling improves the hydraulic activity, and also the physical–mechanical properties by stabilising reactive forms of belite. Such low energy cements provide a cheap alternative to Portland cement with properties that are acceptable for many applications and the additional benefit of possible improved durability.

The chemical composition of mortars made from magnesia-phosphate cement

Abstract Mortars made from magnesia-phosphate cement were observed to set within 15 minutes at 22°C and to harden within 1 hour. The major hydrate formed was struvite, NH4MgPO4·6H2O, usually accompanied by schertelite, (NH42Mg(HPO4)2·4H2O, at least initially. Some hydration products also contained dittmarite, NH4MgPO4·H2O, and/or stercorite, NaNH4HPO4·4H2O, but these were present only as minor constituents.

The microstructure and mechanical properties of mortars made from magnesia-phosphate cement

Mortars made from magnesia-phosphate cement at a water:solids ratio of 1:16 were of high early strength and low porosity. The hydration products were crystalline, often with striking morphology. An increase in the w:s ratio increased the porosity and decreased the compressive strength of the mortars. It also affected the morphology of the struvite crystals, as did the incorporation of sodium tripolyphosphate. At the lower w:s ratio struvite was observed by SEM in the form of needles, while schertelite had a platey morphology. The dramatic crystallinity of these hydrates suggests a through-solution mechanism for their formation. (Author/TRRL)

The mineralogy and microstructure of three composite cements with high replacement levels

The hydration products of three high replacement, composite cement pastes, i.e. Portland cement mixed with 75% and 90% ground, granulated blast furnace slag (BFS) and 75% pulverised fuel ash (PFA), are reported and compared with those from a 100% ordinary Portland cement (OPC) paste. The samples were cured in air under the same temperature and humidity conditions and tested at various times for up to six months. The hydration products were identified by means of X-ray diffraction (XRD) and their microstructure by scanning electron microscopy (SEM). Although the observed hydration products were mostly as expected, due to the high replacement levels, the degree to which these phases was present was unusual. In particular the calcium hydroxide initially formed in the BFS-cement systems was totally consumed within six months, indicating the important pozzolanic behaviour of BFS at such high replacement levels.

Long term durability of Portland-limestone cement mortars exposed to magnesium sulfate attack

Mortar prisms made with Portland-limestone cement have been stored in air and in 1.8% magnesium sulfate solution at 5 °C and have been examined over a period of 5 years. This paper is primarily concerned with the results obtained at the end of this period. The limestone content in the samples varied from 0% to 35%, but the water to cement plus limestone powder ratio was kept constant. The status of the samples after storage for 5 years is reported based on visual examination and a thorough characterisation using X-ray diffraction, infra-red spectroscopy and scanning electron microscopy. The prisms stored in magnesium sulfate solution were all showing clear signs of deterioration, increasing in intensity with limestone content. The mortar prism with 5% limestone replacement was, however, seriously degraded in comparison with the ordinary Portland cement control prism, and it is shown that this was due to the thaumasite form of sulfate attack.

The thaumasite form of sulfate attack in Portland-limestone cement mortars stored in magnesium sulfate solution

Abstract Mortar prisms (40×40×160 mm 3 ) made at a water:solids ratio of 0.5 and a cement:aggregate ratio of 1:2.5 from Portland cement with 0%, 5%, 15% and 35% limestone additions were cured in water at 20 °C for 28 days. They were subsequently stored in air and submerged in 1.8% magnesium sulfate solution at 5 and 20 °C for a year. The prisms were inspected visually every 28 days, the solution was changed every 84 days, and selected samples were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) after specified intervals. The thaumasite form of sulfate attack was readily identified in the ordinary Portland cement (OPC) – 35% limestone mortar after 126 days storage in magnesium sulfate solution. The surface layer of the prism had spalled and was mushy, while the core was still solid and sound. Gypsum, thaumasite and brucite were identified in the surface layer. Secondary electron images of polished sections of deteriorated Portland-limestone cement mortars revealed the microstructure of the cement to be suffering from the thaumasite form of sulfate attack. The extent of this attack was greater at 5 °C than at 20 °C, although some thaumasite was formed even at the higher temperature.

Delayed ettringite formation in heat-cured Portland cement mortars

Abstract A Portland cement mortar was subjected to elevated temperature curing at 100°C for 12 h and then stored under water at room temperature. Expansions, attributable to delayed ettringite formation, were found to develop over a period of 1 year. Sulfate ions released to the pore fluid at elevated temperatures, and partly sorbed by C-S-H gel, evidently formed ettringite in the outer products and the paste–aggregate transition zones during subsequent water storage at room temperature. The results of X-ray microanalyses implied that a potential for ettringite band formation had been established in the mortar. Investigations of microstructural features by backscattered electron imaging indicated that the expansion was caused by generation and extension of these ettringite bands. No evidence in support of an alternative mechanism based on a homogeneous expansion of the cement paste could be found.