Interactions between portland cement and sulphonated melamine formaldehyde superplasticizer and inorganic glass fibre
In Part I of the thesis the effect oif sulphonated melamine formaldehyde (SMF) superplasticizer on the hydration, microstructure and internal chemistry of ordinary Portland cement, as well as on individual pure cement clinker components such as tricalcium aluminate (C3A) was investigated. Conduction calorimetry, thermal analysis, X-ray diffractometry, scanning electron microscopy and pore fluid extraction techniques were used. In order to determine SMF content of the cement pore fluids, it was necessary to develop a new analytical method for SMF. Because of high pH of cement pore fluids the conventional U.V. method was found to be inapplicable. Therefore a new method based on spectrofluorometry was developed; it can be used up to a. pH of about 10, and neutralization can be made using HN03 solution. The role of SMF during cement hydration was further assessed using the method. The results showed that C3A phase of Portland cement adsorbed large amounts of SMF superplasticizers from the aqueous phase of C3A-gypsum mixes in the first few minutes of hydration. In Part II of the thesis, chemical interactions between alkali-resistant (AR) glass fibres and cement matrices was studied. The effect of silica fume additions on the interactions were also investigated. The results of the present work suggest that there are a number of points to be clarified, concerning the use of superplasticizers and AR-glass fibres in cement. The future work to be carried out on superplasticizers may be divided into several groups such as alkali-silica reaction, corrosion of reinforcing steel and durability aspects. Superplasticizers are normally employed as sodium salts. It is evident that addition of superplasticizers into ordinary Portland cement increases the Na+ content of pore solution as well as its pH. Therefore, the rate of alkali-silica reaction may be enhanced due to the increasing pH of cement matrices. Superplasticizers are currently used for the placement of concrete in heavily steel-reinforced structures. The alkaline nature of concrete matrices provides excellent protection for steel reinforcement. However, carbonation of the concrete lowers the pH of the matrix. On the other hand, penetration of aggressive species such as Cl- ions from the sea or other sources accelerates the corrosion of steel reinforcement. There is as yet no detailed study on the chemical behaviour of superplasticizers in chloride- bearing concretes, especially, in relation to their effect on steel reinforcement corrosion kinetics. Superplasticizers greatly influence the morphology of cement hydration products, mainly calcium hydroxide. There is controversy as to whether calcium hydroxide crystals formed in cement paste make a positive or negative contribution to the strength development of cement pastes. In addition, the effect of the presence of thin calcium hydroxide crystals on the durability and long-term properties of cement pastes is not yet known. Zr02-containing commercial AR-glass fibres show poor durability characteristics in a Portland cement matrix, cured in a humid environment, although the deterioration of zirconia glass is much slower than that of ordinary glass. Attack on the fibres is attributed to both the high alkalinity of Portland cement matrices and the growth of hydration products, mainly calcium hydroxide. One solution to these problems seems to be to reduce both the alkalinity and the calcium hydroxide content of cement pastes. Pozzolanic materials such as silica fume, fly ash, slag etc. are very effective in reducing both the pH and the calcium hydroxide content of cement pastes. From the results presented in this thesis, the highly brittle nature of cement pastes with silica fume leads to a decrease in flexural strengths of AR-GFRC composites, although it improves the durability of composites to some extent. The use of other pozzolans, for example, fly ash and slag have not received much attention. Another possibility is the use of less alkaline cement matrices to decrease the chemical attack on AR-glass fibres. In addition to changing composition of cement matrices, the covering of the surface of original glass fibres by alkali- resistant organic or inorganic materials may protect the glass from chemical attack and slow down the corrosion process. Investigations on more alkali-resistant glass compositions may,- also result in the manufacturing of new types of glass fibres.