D.D. Double

The effect of lead nitrate on the early hydration of portland cement

Abstract Solution analysis, calorimetry and electron microscopy have been used to study the retarding effect of Pb(NO 3 ) 2 admixtures on the early stages of hydration of Portland cement. Analyses of cement filtrates show rapid precipitation of basic lead compounds incorporating nitrate and sulphate. The precipitation is accompanied by an increase in the early heat liberation followed by a longer term retardation. Microscopy shows that the precipitate is largely in colloidal gelatinous form and coats the surfaces of the cement grains. The protective effect of these coatings is clearly responsible for the inhibition of hydration.

Studies of the growth of “silicate gardens” and related phenomena

Various growth morphologies obtained in “silicate gardens” are described and the influence of salt type and silicate concentration are examined. It is demonstrated that these growths develop by an osmotic mechanism based on the semipermeable properties of the silicate gel membrane precipitated between the metal ion and the silicate. The osmotic effect is not confined to silicates but also includes aluminates and ferrocyanides — and in general, it probably applies to systems where a continuous colloidal gel membrane is precipitated between two aqueous solutions of differing compositions. The implications in relation to certain practical problems (i.e. the hydration of Portland cement, the corrosion of metals in an aqueous environment) are considered.

The chemistry of hydration of high alumina cement in the presence of accelerating and retarding admixtures

Abstract The effect of accelerators, and in particular lithium salts and citric acid solutions, on the setting time of high alumina cement has been studied using calorimetry, solution analysis and X-ray diffraction techniques. Results are discussed with respect to the ternary CaOAl 2 O 3 H 2 O solubility diagram. It appears that there is a nucleation barrier to the precipitation of the main products of hydration, CAH 10 and C 2 AH 8 and that lithium salts function as accelerators by precipitation of a lithium aluminate hydrate which acts as a heterogenous nucleation substrate. It is suggested that retardation by citric acid is due to the precipitation of protective gel coatings around the cement grains which impede hydrolysis or inhibit growth of the hydration products.

A classification of inorganic and organic admixtures by conduction calorimetry

Abstract A wide range of inorganic and organic admixtures have been investigated using isothermal conduction calorimetry. The results are presented as a series of maximum rate of heat evolution (Q max ) versus reciprocal to peak height (1/t max ) plots. Using equivalent molar concentrations of admixture under controlled conditions a comparative ranking system of inorganic and organic admixtures has been devised. Some general conclusions are drawn about the mechanism of action of accelerating and retarding admixtures.

High strength cement pastes: Part 2 Reactions during setting

A study of the chemical reactions occurring in high strength cements is presented with particular reference to the cement-polymer interaction. The addition of small amounts (1.6%) of polyvinyl (alcohol/acetate) to the high alumina cement Secar 71 significantly retards the normal hydration reactions. At much larger doses (10% PVA) the crystalline hydrates are completely suppressed and the polymer reacts with the cement solution to form calcium acetate and a crosslinked polymeric product. In the case of high strength OPC/polyacrylamide pastes, the high pH of the cement solution converts the polymer to polyacrylic acid which reacts with the cations from the hydrated cement to form a crosslinked metal-polyacrylate. This latter system is analogous to reactions which occur in certain polyacrylic acid-based dental cements.

Calcium and silicon concentrations in solution during the early hydration of portland cement and tricalcium silicate

Abstract The solution chemistry of Portland cement and C 3 S pastes has been studied with particular attention to the concentrations of calcium and silicon in the aqueous phase during early stages of hydration. Results are discussed in relation to solubility data available from studies of OPC and C 3 S pastes and from the CaOSiO 2 H 2 O system. It is shown that under normal conditions the concentration of silicon in solution is extremely low ( 2 . However, retarding admixtures such as oxalic acid and EDTA, which are strong calcium binding agents, release a flush of silicon into solution within the first few minutes of hydration. The results lend support to the osmotic membrane model of cement hydration and also provide insight into the mechanisms by which accelerating and retarding admixtures function.

Some general considerations of a membrane/osmosis model for portland cement hydration

Abstract The proposed analogy between the process of membrane formation and osmosis in “silicate gardens” and Portland cement hydration is discussed. The major similarity is considered to be the formation of solids at a fluid interface at which the local supersaturation is high so that colloidal solids having no long-range order are produced. A membrane/osmosis model is then applied in explanation of various features of cement hydration, including the dormant period, the action of gypsum and morphological features such as “Hadley grains”.

Physical properties of high strength cement pastes

Abstract Recent developments in methods of processing ordinary Portland cement have shown that above average strengths (in flexure) are easily obtained without expending energy on high pressure compaction techniques. This paper reports strengths and other physical properties of both ordinary pastes and modified (macro defect free) pastes and attempts to compare and contrast the two types. The apparently greater notch sensitivity of the modified pastes is explained in terms of a reduced inherent flaw size. Optical microscopy shows that large pores are absent in the modified paste but transmission electron microscopy (TEM) shows that the fine scale microstructures of ordinary pastes and high strength pastes are very similar.

An investigation on the nature of porosity in hardened cement pastes using small angle neutron scattering

The porosity and pore-size distribution in hardened cement paste have been measured by several techniques which often give conflicting results. Small angle neutron scattering is a new technique in the area of cement technology, which enables information to be obtained at the fine end of the pore distribution (less than approximately 30 nm diameter using this machine) without any specimen drying or pretreatment. The technique indicates a bi-modal distribution of spherical or near spherical pores with diameters of approximately 5 and 10 nm, accounting for less than about 2% of the total paste volume. The pores seem relatively unaffected by the water to cement ratios investigated; however, drastic effects do seem to occur on drying at 105° C.

The hydration of Portland cement, C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C3Sand β-C2S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C3S and β-C2S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.