Mechanism of sulfate attack on portland cement concrete — Another look

Sulfate-generated deteriorations in normal portlant cement concretes include expansion, cracking, loss of strength and stiffness, and sometimes disintegration. The chemical phenomenon of ettringite formation as a result of reaction between sulfate water and hydration products of portland cement does not adequately explain all the physical manifestations of the sulfate attack. Furthermore, ettringite which causes expansions in some cases is apparently responsible for high strength in other cases. The published literature does not contain satisfactory explanations for this anomalous behavior of ettringite. In this paper, the author has attempted to provide answers to some of the questions.     

Reactions between methanol and portland cement paste

The paper concerns the suitability of methanol replacement as a method for drying cement paste specimens prior to microstructural examination. Thermogravimetric tests on both cement paste and calcium hydroxide which were soaked in methanol for various periods show that methanol alters sample composition by reacting to form a carbonate-like product. Also, some of the methanol is found to remain with the solid at temperatures above 300°C. It is concluded that until more is known about the interaction between methanol and the cement paste constituents, the correct interpretation of test results of methanol treated specimens is likely to be very difficult.     

The stability of bound chlorides in cement paste with sulfate attack

This paper presents an experimental investigation on the stability of bound chlorides in chloride-contaminated cement pastes with and without FA/GGBS when subjected to Na₂SO₄ and MgSO₄ attack. It is shown that bound chlorides were released in the chloride-contaminated pastes when exposed to Na₂SO₄ or MgSO₄ solution. This is mainly attributed to the decomposition of Friedel's salt (FS), where Cl⁻ bound in FS is replaced by SO₄²⁻. However there were fewer released chlorides found in the pastes exposed to MgSO₄ solution than in those exposed to Na₂SO₄ solution. This is partly due to the low pH in the pore solution and partly due to the blocking effect of brucite on ionic transport caused by MgSO₄. The inclusion of FA/GGBS in concrete can increase the decomposition of FS and thus the release of bound chlorides. However, it also resists the penetration of Na₂SO₄ and thus reduces the attack of Na₂SO₄.     

Microstructure of calcium hydroxide depleted portland cement paste I: Density and helium flow measurements

Hydrated portland cement contains both free and combined Ca(OH)₂ together with C-S-H. Removal of the Ca(OH)₂ in controlled amounts, first by leaching with an aqueous solution of Ca(OH)₂ and then with water, allows the role of C-S-H to be studied. Helium pycnometric and inflow measurements were made on unleached, leached, and overleached material. Leaching of the free Ca(OH)₂ did not alter the C-S-H material and did not change any physical properties. Possible engulfing of C-S-H by Ca(OH)₂ is not effective in preventing helium or water flow. Overleaching to the extent that 50% of the CaO was removed from the cement broke down CaO₂ sheets, causing ready access of helium to collapsed interlayer positions. Evidence of a change in the orientation of interlayer water was observed.     

Properties of portland cement paste reinforced with mica flakes

Results of an investigation of the use of high-aspect-ratio mica flakes in hardened portland cement matrices are reported. Increases in flexural strength by a factor of 2 and in fracture toughness by a factor of 4, depending on matrix porosity, are obtained by the addition of small amounts of mica. The dependence of these properties on matrix porosity is determined and the effect of mica flake addition on matrix characteristics discussed, e.g., porosity, surface area, non-evaporable water content.     

X-ray microtomography (microCT) of the progression of sulfate attack of cement paste

High-resolution X-ray computed tomography (i.e., microCT or microtomography) was used to study the sulfate attack of cylinders of Type I cement paste cast with water-cement (w/c) ratios of 0.45, 0.50 and 0.60. Damage levels in samples exposed to a Na₂SO₄ solution with 10,000 ppm sulfate ion concentration were qualitatively rated from 0 (no damage) to 4 (extreme damage) based upon visual examination of the samples' exteriors and microtomography of the samples' interiors. The greater the w/c ratio, the more rapid the onset of sulfate damage. The corners of the cylinders appeared to be particularly susceptible to spalling, and damage may have continued into the cement paste by formation of subsurface cracks.     

The effect of admixtures on the strength-porosity relationship of portland cement paste

The effect of four types of admixtures (flyash, water-reducing agents, an air-entraining agent and limestone mineral powder) on the strength-porosity relationship of portland cement paste was studied. The volume concentration of hydrate substance, and hence porosity, appears to be the most important strength parameter regardless of the presence of admixtures.     

Comparison of mechanical properties of compacted calcium hydroxide and portland cement paste systems

Mechanical properties of Ca(OH)₂ are compared with those of portland cement paste at different porosities. Intrinsic values of modulus of elasticity (E) and microhardness (H) for Ca(OH)₂ compacts were obtained by extrapolating to zero porosity log E and log H versus porosity curves. The values for the Ca(OH)₂ and portland cement paste systems are of similar magnitude. Flexural strength values for both systems were also determined to be of similar magnitude for the porosity range studied. Fracture terms (K_c, J_c, G_c, Y_T) were determined. K_c and Y_T versus porosity plots for both systems revealed that these fracture terms are of similar magnitude in the porosity range common to both.     

Helium flow characteristics of rewetted specimens of dried hydrated portland cement paste

In an effort to determine how and when water re-enters interlayer spaces the helium flow technique is applied to the dried hydrated portland cement system during re-exposure to water vapour. Two sets of samples at four water-cement ratios were exposed consecutively, to 11, 32, 42, 66, 84 and 100 per cent RH. After each exposure they were reconditioned to 11 per cent RH before measurements were made. Measurements were also taken following second drying.

Results show that water re-enters the interlayer structure after exposure to the lowest humidity. Measurements following second drying show that rewetting regenerates the process that occurred on first drying.     

Low-temperature scanning electron microscope analysis of the portland cement paste early hydration

The use of the frozen hydrated scanning electron microscopy (FHSEM) in the study of cement paste is described. This technique permits analysis of the fractured surface of cement paste in a fully hydrated state with water present as ice in a low temperature scanning electron microscope. At 110 K the paste has a substantial increase in mechanical strength, because water is converted from liquid to a solid state, and this permits the use of bulk specimens at very early hydration. Some preliminary results for 1 hour hydration are presented and future applications of this technique are discussed.     

Drying shrinkage of portland cement pastes I. Microcracking during drying

The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness <2mm) was suddenly exposed to low relative humidity (～50%), the cracks eventually closed up. It was concluded that no matter whether or not this microcracking happened, the shrinkage of the specimen after reaching equilibrium was unrestrained.     

The contact zone between portland cement paste and glass “aggregate” surfaces

The morphology of the contact zone developed between Portland cement paste and glass slide “aggregates” has been explored using SEM and other techniques. A duplex film of about 1 μm total thickness is rapidly deposited on the glass surface. This is a continuous film of Ca(OH)₂ overlain by a parallel array of rod-shaped CSH gel particles projecting normal to the interface. The nearby cement paste exhibits high porosity, but after a few days becomes partly filled with a secondary deposit of stacked platelets of relatively pure Ca(OH)₂. Cement particles near the interface hydrate in a peculiar manner. A hydration product shell is quickly formed, but the encapsulated cement particles dissolve away to leave partly or completely empty shells. This behavior occurs with various Portland cement types and presumably occurs near aggregate surfaces in concrete.     

Effect of water and other dielectrics on subcritical crack growth in portland cement paste

In studying the effect of water and a series of aliphatic alcohols on subcritical crack growth in cement paste, log crack velocity-stress intensity factor curves were obtained by means of a double-torsion technique. The relative position of the curves was found to be dependent on the dielectric constant of the test media. Flexural strength of cement paste saturated in organic fluids is also dependent on the dielectric constant. Stress corrosion processes involving chemical attack of Si---O bonds in cement paste appear to be operative.     

The dispersion model for hydration of portland cement I. General concepts

This paper introduces an analytical mathematical model for development of the hydration of Portland cement: t₀ + t₁A + t₂A² = t t is time, and t₀, t₁ and t₂ are time-constants depending on temperature, cement type, admixtures etc. A is the hydration ratio, i.e. the ratio of hydrated to unhydrated cement. The dispersion model has been derived on basis of the typical particle size distribution for tube-milled products. Earlier findings by us have proven the particle size distribution to be a dominant factor in the correct modelling of cement hydration. The parameterization of hydration curves, which is easily performed with this dispersion model, gives a sufficient characterization of these for purposes of practical interpretations of the effects of factors decisive for the development of hydration.