Harald Justnes

Thaumasite formed by sulfate attack on mortar with limestone filler

Abstract Early evidence of thaumasite formation in mortar with limestone filler exposed to sulfate containing tunnel water in Norway is reviewed. The problem is discussed in light of the new European cement standard allowing cements containing up to 35% limestone (e.g. CEM II/B-L) rendering them prone to detrimental sulfate attack. Experiments are performed where mortars with 20% limestone or quartz filler, respectively, are stored in 5% sodium sulfate solution saturated with gypsum at 5 °C. Length change, flexural strength and compressive strength are measured periodically for a year. The microstructure of the mortars is inspected by scanning electron microscopy and energy dispersive analyser of X-rays documenting the formation of sulfate containing species including ettringite and thaumasite.

Technical calcium nitrate as set accelerator for cement at low temperatures

Abstract The objective was to test the efficiency of technical calcium nitrate (CN) as a set accelerator for different cements at low temperatures (5–7 °C). The applied dosages were 0.00, 1.55, 2.32, 3.10, 3.86 and 7.73 % CN of the cement weight, corresponding to 0.00, 1.00, 1.50, 2.00, 2.50 and 5.00 % NO 3 , respectively. Five different portland cements were chosen covering a wide range in C 3 A contents, since it was initially believed that the set regulating mechanism involved C 3 A (i.e. the efficiency depend on the C 3 A content). The setting characteristics of the cement pastes with w c = 0.40 were determined by an automatic Vicat-needle apparatus in a room with an ambient temperature of 5–7 dgC. The results revealed that the set accelerating efficiency of CN depended very much on the cement type. However, no correlation with the C 3 A content was found. On the other hand, the set accelerator efficiency of CN seems to increase with increasing belite content according to the Bogue analyses of the cements, or other cement characteristica promoting belite formation in the clinker process.

THE CHEMICAL SHRINKAGE OF POZZOLANIC REACTION PRODUCTS

The total chemical shrinkage of silica fume and Class F fly ash, both as pozzolanic materials reacting with lime and as mineral additives replacing portland cement, was studied. By increasing pH, the rate related to the pozzolanic reaction decreased for silica fume and increased for fly ash. Although the presence of alkalis are catalytically necessary for a rapid pozzolanic reaction of silica fume, the pH increase reduces the solubility of calcium hydroxide (CH) due to the common ion effect. This may explain why the reaction rate decreases if dissolution of CH, followed by precipitation of CSH, is the rate-limiting step. The increased reactivity of fly ash, caused by a pH increase, indicates that the dissolution of the glassy aluminosilicate phase by alkalis was determining the overall rate of the process. The total chemical shrinkage was crudely estimated to be 8.8 ml/100 g of reacted silica fume and 10.0 ml/100 g of reacted fly ash, as compared with 6.3 ml/100 g of portland cement. The measured shrinkage for silica fume could be higher than the above value since minor amounts of silicon metal in the silica fume could produce an expansion due to evolved hydrogen gas.

Kinetics of Reaction in Cementitious Pastes Containing Silica Fume as Studied by 29Si MAS NMR

The present paper summarizes studies of cementitious systems containing silica fume by 29Si MAS-NMR, often in combination with DTA/TG enabling calculation of the empirical formula for the CSH-gel and a balanced general hydration reaction for the silicious cement minerals with or without silica fume. The paper is divided into the systems lime/silica fume, calcium silicate/silica fume and portland cement/silica fume with subdivision into effect of w/(c+s) and curing temperature. The preceding findings clearly demonstrate that a cementitious paste is a “dynamic” system where the CSH-gel may vary its composition within a wide range, dependending on the mixing and reaction conditions.

Hardening Retarders for Massive Concrete

The heat of hydration in massive concrete structures can raise the temperature to a level where thermal cracks can pose a problem. Hardening retarders are admixtures that lower the rate of hydration, distribute the heat release over time, and lower maximum temperature in concrete. Such admixtures will inherently lead to lower early strength, but should lead to comparable 28-day strength to reference concrete. Relative large amounts of urea works, in particular when the additional retardation of setting (not hardening) is counteracted by the set accelerator calcium nitrate. However, these dosages are high, and urea will also slowly decompose to ammonia that may limit the urea application to outdoor use, if any. The latest potential admixtures are combinations of minor amounts of strong setting retarders like organic acids (0.1-0.3 %) with the setting accelerator calcium nitrate (1-3%), where a true synergy between the two leads to hardening retardation.

Feasibility of Calcined Marl as an Alternative Pozzolanic Material

Calcareous clay rich in smectite was calcined at temperatures of 600–1000 °C using a pilot- and industrial scale rotary kiln. Compressive strength of mortars was tested between 1–365 days, when 20–65 % of OPC was replaced by calcined clay at equal w/c-ratios. With respect to reactivity as a pozzolan, the optimum calcination temperature was around 800 °C. With a replacement level of 50 % the 1-day strength was reduced but high enough for demoulding concrete infield practice, while after 28 days almost the same strength as with no replacement could be obtained. The raw and reactive calcined state of the clay was characterised using different methods like XRD, TG/DTG, SEM, FTIR, Al27-NMR and Mossbauer Spectroscopy. At the optimum calcination temperature calcium carbonate from the clay is only partly decomposed. The main calcium carbonate source is coccoliths which enabled the formation of a reactive Ca enriched glass phase together with the decomposing clay minerals. Oxidation of Fe2+ to Fe3+ resulted in a structural disordering increasing the reactivity of the calcined clay. Pozzolanic activity was tested in pastes of calcined clay and calcium hydroxide.

The use of blast furnace slag in North Sea cementing applications

This paper discusses further the application of Blast Furnace Slag (BFS) in the cementing of oil and gas wells, with particular emphasis being placed on the suitability of BFS for offshore operations in the North Sea. The paper outlines the chemical reactions which occur during curing of BFS and discusses effects of different BFS sources and testing requirements. The application of BFS as a drilling fluid additive to improve cement bonding by solidification of the filter cake is discussed with respect to the effects of BFS on drilling fluid rheology and fluid loss. BFS is found suitable for low volume operations such as plug cementing, however wider use BFS is seen to be limited by logistics and occupational safety aspects for offshore North Sea applications. The environmental benefits to be gained by use of BFS is limited.

Reactivity and Microstructure of Calcined Marl as Supplementary Cementitious Material

The reactivity and microstructure of cement paste and mortar where cement is replaced by up to 65 vol.% calcined marl are discussed. It was found that the compressive strength evolution of mortar is following the same linear relation with amount of hydrate water at early ages for different cement replacements (35-65 vol.%), but that this deviates and give higher strength than predicted by the bound water at higher ages. Strength increases on a long term in spite of depleted calcium hydroxide at earlier ages and are discussed in terms of changes in CSH and CAH. XRD does not reveal any unusual crystalline products, but ettringite and hemi-/mono-carboaluminate hydrate. SEM with WDS in a 2 year old mortar found pure CAH in a pore with atomic Ca/Al = 1.6 and some Si that might be a hydrogarnet.

INFLUENCE OF SETTING ACCELERATORS ON CHEMICAL SHRINKAGE OF PORTLAND CEMENT

The influence of a number of calcium salts on the total chemical shrinkage (used as a measure of cement hydration) of different portland cement pastes was followed during the first 48 hours. All calcium salts (acetate, chloride, formate, nitrate and nitrite) were added in an equimolar dosage of Ca2+ corresponding to 1.5% calcium nitrate by cement weight. An automatic Vicat-apparatus was used to monitor the setting time of the cement pastes. Experiments conducted at 5 degrees C, 13 degrees C and 23 degrees C revealed that calcium nitrate was the most effective set accelerator at lower temperatures and even more effective than calcium chloride at the lowest temperature. The anions of the different calcium salts were also found to influence the setting and the efficiency of each accelerator strongly depended on the cement types.

Changes in the Microstructure of Cement Paste and Concrete due to Calcium Nitrate Addition

Calcium nitrate (CN) has been increasingly used as a chloride free set accelerator in later years. Mature cement pastes and concrete have been subjected to microstructure investigations in order to identify possible and long term changes when a high dosage of CN was added to the fresh mixes. The following changes ere found for 2 year old cement pastes (w/c = 0.50) based on ordinary Portland cement (OPC) and sulphate resistant Portland cement (SRPC) when 5.26% CN was added: (1) The degree of hydration was only marginally lower. (2) The amount of calcium hydroxide (CH) was significantly lowered (= 10%) in the case of OPC and unaltered for SRPC. (3) The amount of chemically bound water in both pastes was increased. (4) The average length of the polysilicate anions in the amorphous CSH-gel was prolonged (17% for OPC and 5% for SRPC). (5) The porosity of the OPC paste was increased (+7%) and inhomogeneously distributed, while it was decreased (-2%) for the SRPC paste. (6) The morphology of calcium hydroxide (CH) in the pastes was changed from being evenly distributed to be gathered in larger (=50um diameter) clusters. This phenomenon may be explained by restricted diffusion due to the high Ca2+ concentration supplied by CN. This latter effect was also observed for a plain concrete (w/c = 0.57) based on high strength Portland cement (HSPC) and 3.86% CN>