David Bonen

Magnesium sulfate attack on portland cement paste-I. Microstructural analysis

Abstract The mechanism of magnesium sulfate attack in portland cement and portland cement with silica fume paste was studied by investigating the microstructure and composition of specimens immersed for one year in magnesium sulfate solution. Results are presented in two parts. Part I, presents the microstructure of the pastes. Part II, presents the chemical and mineralogical variations across the specimen together with a proposed mechanism of magnesium sulfate attack. The magnesium sulfate attack resulted in the formation of the “surface double-layer” which was composed of brucite and gypsum layers. In addition, there was a sequential formation of inner gypsum layers resembling the occurence of lisegang bands found in rocks. The surface double-layer was composed of brucite, about 40 to 120 μm thick, and followed by a contiguous layer of gypsum, about 20 to 70 μm thick. Subsequently, up to four internal gypsum layers were deposited at depths of up to 1200 μm from the surface. These layers were parallel to the beam surfaces ranging in thicknesses from 15 to 60 μm. In portland cement with silica fume paste there were almost no inner gypsum layers, instead, massive deposition of dispersed gypsum crystals appeared up to at a distance of about 800 μm from the surface. The attack had apparently influenced the local degree of hydration of the cement constituents; near the surface only residual ferrite and belite crystals were observed, but in the middle, unreacted alite and belite were predominant.

The superplasticizer adsorption capacity of cement pastes, pore solution composition, and parameters affecting flow loss

Abstract Studies are reported on the interaction of sodium salt of polynaphthalene sulfonate superplasticizer (PNS) with different cement types, its effects on the flow loss, and the chemistry of the pore solution at early time of hydration from mixing to presetting. Six commercial cements displaying a wide compositional range and fineness were selected. Results show that most of the superplasticizer is removed from the pore solution immediately after mixing. The adsorption capacity of the paste is mainly determined by the PNS molecular weight, cement fineness, and C 3 A content In turn, the rate of flow loss appears to be strongly governed by the ionic strength of the pore solution and marginally to C 3 A content and ettringite formation. Oversulfating the cements by addition of up to 7% of gypsum, reduces the flow loss but within the range of data available, does not enhance the uptake rate of the superplasticizer by the cement constituents.

Magnesium sulfate attack on portland cement paste — II. Chemical and mineralogical analyses

Abstract Many of the chemical and mineralogical changes in pastes made with portland cement (PC) and with portland cement plus silica fume (PC-SF) subjected to magnesium sulfate attack occurred in a zone which was referred to as “transition zone”. This zone extended from the surface double-layer inwards where only few changes could be detected. The width of the transition zone varied according to the character of the paste, being greater for the PC paste. Among the characteristic features of the transition zone were the continuous increases in (1) the Ca/Si ratio of the CSH gel, and (2) the bulk CaO/SiO2 ratio of the paste from the surface inwards. In addition, the transition zone had a smaller amount of unhydrated cement particles and exhibited a strong differentiation in the relative amount of the residual phases. The mechanism of the magnesium sulfate attack involved the formation of two counter diffusion patterns. Hydroxide ions, (OH)−, diffused outwards from the paste, whereas SO42− ions diffused inwards from the surface. Due to the lower Ca/Si ratio of the CSH gel in the PC-SF paste and its alteration products, the PC-SF paste was more susceptible to the magnesium sulfate attack than the PC paste. A schematic model is presented which is based on the microstructure (presented in Part I) and the spatial chemical and mineralogical variations. The model attempts to provide explanation and trace the progression of magnesium sulfate attack.

A microstructural study of the effect produced by magnesium sulfate on plain and silica fume-bearing Portland cement mortars

Abstract A chemical and microstructural study has been carried out on Portland cement mortars with and without silica fume subjected to magnesium sulfate attack. The mortars appeared to be less durable than their counterpart pastes as indicated by greater surface deterioration, greater depth of sulfate ingress, more extensive gypsum deposition, and a higher degree of decalcification of the CSH gel. The plain (non-silica fume) mortar was distinguished by massive deposition of gypsum. Preferred sites of deposition were around aggregates, in air voids and other cavities. In the silica fume bearing mortar less gypsum was deposited. Nevertheless, the CSH of the silica fume bearing mortar experienced a higher degree of decalcification than the CSH of the plain mortar. Based on occurrence of surface deterioration and cracking pattern, it appears that silica fume addition did not improve durability. It is concluded therefore that the decalcification of the CSH plays a major role in the deterioration process.

Occurrence of large silica fume-derived paticles in hydrated cement paste

Abstract In an examination of a 1-year old hydrated silica fume bearing cement paste a number of large (35–80 μm) rounded siliceous particles were found that had apparently been derived from the coarse fraction of the silica fume. Calcium had diffused inward from the periphery of the grains, and in most cases had reached the centet. The Ca:Si mole ratio near the outer zone was almost the same as that in the surrounding matrix CSH gel, but decreased continuously with distance toward the center. Potassium and sodium were also found within the particles, but were distributed inversely to the calcium, i.e. increasing toward the center. The reaction product generated here appears to be CSH and not potentially expansive alkali silica reaction product gel, although such gel might be produced with high alkali cements.

The Evolution of Cementitious Materials Through History

The use of cementitious materials dates back to the beginning of the Epipaleolithic period. Examples for ancient cementitious materials from Israel, Egypt, Turkey, and Italy are numerous. Prior to Aspdins patent of portland cement at the first half of the 19th century, cementitious materials were composed of earth, mixture of earth and limestone, calcium sulfates, and slaked lime with and without pozzolans. The latter comprises pozzolanic materials from volcanic and sedimentary origin, crushed burnt clay brick, and dust brick. Frequently, organic fibers were incorporated for reinforcement. This paper describes the evolution of the cementitious materials through time and highlights the durability of ancient cementitious materials as compares to that of portland cement concrete. Although modem concrete is characterized by its high strength and low permeability, it often faces durability problems. In turn, ancient concretes examined exhibit low strength but have proved to be durable materials. Microstructural examination reveals that the groundmass of the latter has been carbonated and is highly porous. Nevertheless, no specific cracking pattern could be observed. The outstanding performance of ancient concrete structures implies that thermodynamic stability rather that mechanical strength is a key point for a long-term durability.