X-ray diffraction is one of the most important analytical tools for the investigation of cements and clinkers and their hydration products. Powder diffraction provides essential information about the phase composition of cements and Rietveld refinements enable quantitative analysis.
A new important field of investigation s in situ X-ray diffraction of hydrating cement pastes. Here the aim is to analyze hydration kinetics and observe phase transitions during the setting and hardening process of cementitious systems. X-ray analysis can be applied from the beginning of the reaction (within a few seconds of mixing the cement with water) through to reaction times of several days. Consequently, an XRD system for in situ investigations must meet a variety of different needs. The most important being: rapid sample preparation, high data quality and very high detection speed.
The Malvern Panalytical transmission X-ray geometry fulfills all these requirements and offers a new approach for time- resolved in situ measurements of both kinetic systems and standard powder samples. Advantages and disadvantages of the new setup applied to hydrating cement pastes are discussed, especially in comparison with conventional reflection systems.
The four main clinker phases of ordinary Portland cements (OPCs) are alite (3CaO•SiO2, abbreviation C3S), belite (2CaO•SiO2, C2S), aluminate (3CaO•Al2O3, C3A) and ferrite (4CaO•Al2O3•Fe2O3, C4AF). Each of the four phases shows ionic substitutions with different elements, and also polymorphism. Additional minor phases in OPC clinkers are free lime (CaO), periclase (MgO) and potentially several sulfates, eg arcanite (K2SO4). In general it is necessary to add sulfate- containing compounds such as gypsum (CaSO4•2H2O) or anhydrite (CaSO4) to avoid fast and uncontrolled setting. Normally, ettringite (C3A•3CaSO4•32H2O) is the first temporary hydration product of OPCs. After just 30 seconds, evidence of the first ettringite crystals can be seen using Cryo-Transfer Scanning electron microscopy. The early hydration process (up to around 3 h reaction time) is dominated by the strong formation of these short prismatic, hexagonal crystals. Compounds such as monosulfate (C3A•CaSO4•12…14H2O) and semicarbonate (C3A•½Ca(OH)2•½CaCO3•11.5H2O) may also be formed in lower quantities. The silica-based reaction begins after 2 to 4 h hydration. Portlandite (Ca(OH)2) and calcium silicate hydrate (C-S-H-phases, 2CaO•SiO2•1…3H2O) are the products of this reaction. Because of the small crystal sizes, or the semi-crystalline or amorphous character of C-S-H, these can be difficult to detect with X-ray analysis. As a result, the presence of portlandite is used to indicate C-S-H formation.
In situ measurements of cement kinetics
Carbonation is a process that takes place on cement paste and cement stone surfaces in contact with CO2. There is sufficient CO2 in air to induce this reaction. Phases such as monocarbonate (C3A•CaCO3•11H2O) or calcium carbonate (CaCO3) are formed. It is necessary to suppress these surface carbonation effects when making kinetic measurements in order to obtain realistic bulk material results. In every case, in situ measurement of cement paste requires encapsulation of the hydrating sample. This entails using specialized sample chambers for XRD systems that guarantee high humidity and the exclusion of CO2; or sample holders that allow water- and air proof preparation.
Samples and preparation
All samples were CEM I 52.5 R Portland cements. Water and cements were mixed in ratios of 0.35 to 1.0 (w/c). The transmission sample holder enabled perfect slurry film preparation of cement pastes between two pieces of transparent foils which were stretched from two plastic rings.
Preliminary tests showed that w/c ratios of 0.4 to 0.6, and defined cement paste initial weights of 0.1 to 0.3 g, deliver adequate and consistent results. W/c 0.5 and an initial weight of 0.15 g were taken as baseline conditions for the experiments.
Further experiments were performed by putting the samples in glass capillaries with diameters of 0.1, 0.3 and 0.7 mm mounted on the capillary spinner stage. The results obtained with 0.1 and 0.3 mm capillaries were also usable, but did not achieve the high data quality and ease of use of the sample preparation between the two foils.
All tests were conducted at room temperature from 22 °C to 25 °C.
Figure 1: Cryo-SEM image of OPC after 3 h hydration, showing clinker grains covered with ettringite crystals and the first C-S-H phases