Determination of the selectivity coefficient of a chloride ion selective electrode in alkaline media simulating the cement paste pore solution.

The measurement of the free chloride concentration in the pore solution of cement paste or concrete is of interest for assessing the probability of corrosion of the steel reinforcements embedded in concrete. This determination is performed through leaching or pore-pressing methods or through embedded potentiometric sensors into the hardened cementitious material. The potentiometric determination of chloride in cement paste pore solutions is limited by hydroxide ion interference, due to the high alkalinity of such media. The potentiometric selectivity coefficients, kCl-,OH-, are determined for a chloride ionic selective electrode in alkaline solutions simulating the electrolyte present within the pore network of cement paste and concrete. This is done using a fixed interference methodology, with least-squares non-linear curve fitting for obtaining the selectivity coefficients together with other relevant electrode parameters. The limit of detection of the Cl− ISE, due to OH− interference, varies with pH. For the pore solution corresponding to an ordinary Portland cement paste or concrete, this limit can be set between 3 × 10–3 m, and 7 × 10–3 m, i.e., approximately below a chloride concentration value of 1 × 10–2 molal. Taking into account these limits of detection the free Cl− concentrations able to depassivate the steel, can be adequately determined by potentiometric measurements with calibrated ISEs in the expressed pore solutions of Portland cement concretes, without significant influence of OH− interference. Nevertheless, this effect can prevent the adequate measurement of low free Cl− concentrations, below the corresponding limit of detection valu

Influence of limestone on the hydration of ternary slag cement

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cements have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was followed by the implementation of QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different w/s ratios. Thus, the method was used to analyse hydrated cements without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify to the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus lowering of the aluminium concentration in the pore solution led to the improved slag hydration

Hydration of water- and alkali-activated white Portland cement pastes and blends with low-calcium pulverized fuel ash

Pastes of white Portland cement (wPc) and wPc-pulverized fuel ash (pfa) blends were studied up to 13 years. The reaction of wPc with water was initially retarded in the presence of pfa particles but accelerated at intermediate ages. Reaction with KOH solution was rapid with or without pfa. A universal compositional relationship exists for the C-A-S-H in blends of Pc with aluminosilicate-rich SCMs. The average length of aluminosilicate anions increased with age and increasing Al/Ca and Si/Ca; greater lengthening in the blends was due to additional Al3+ at bridging sites. The morphology of outer product C-A-S-H was always foil-like with KOH solution, regardless of chemical composition, but with water it had fibrillar morphology at high Ca/(Si+Al) ratios and foil-like morphology started to appear at Ca/(Si+Al) ≈1.2-1.3, which from the literature appears to coincide with changes in the pore solution. Foil-like morphology cannot be associated with entirely T-based structure

ALKALI-SILICA REACTIONS IN PORTLAND CEMENT CONCRETE

The traditional method of studying alkali-silica expansion reactions has been to cast rectangular prisms or cylinders of mortar or concrete, expose them to various regimens of temperature and moisture, and observe length changes of the specimens. While this approach yields information on expansion potential, it reveals nothing about the chemical reactions taking place in the mortar or concrete. In order to obtain samples of the pore solution in reacting mortars a pore fluid expression die was designed and built to operate at pressures up to 80,000 psi (552 MPa). These expressed solutions were then analyzed by atomic absorption spectroanalysis and microchemical methods. It was found that major chemical changes could take place in the pore solutions prior to the initiation of expansion and these changes frequently went to completion days or even months before expansion approached completion. Studies with pozzolans revealed that pore solution chemical changes associated with their reactions were similar to those associated with expansive reactions, but were generally much more rapid. Studies of the amount of water chemically bound by a unit weight of portland cement revealed that water chemically bound by portland cement is apparently reduced as a linear function of the ratio of pozzolan to cement. Data from a study of the effect of the presence of available calcium on the diffusion of potassium in opal suggests that, in the presence of calcium, the amount of potassium found in the outer layers of the opal may be nearly ten times as great as that in the absence of calcium

Physics and chemistry of alkali-silica reactions

The philosophy underlying recent research on alkali-silica reactions is reviewed and illustrations of recent results are provided. It has been possible to follow the kinetics of the chemical reaction between dissolved alkalis and opal in mortars by monitoring the rate at which alkalis are removed from the pore solutions of reacting mortars. Studies of the expansion behavior of synthetic alkali silica gels under controlled conditions were carried out and show no obvious correlation to chemical composition. The alkali reaction in mortars was found to produce changes in the appearance of opal grains documentable by the use of a scanning electron microscope