Jamal M. Khatib

Relative strength, pozzolanic activity and cement hydration in superplasticised metakaolin concrete

Abstract The role of metakaolin (MK) in enhancing the strength of concrete is reviewed and the principal mechanisms identified. Metakaolin concretes with a range of MK contents (0–30%) have been cured for periods of 1 to 90 days. The change in relative strength with both curing time and metakaolin content is discussed in relation to the ‘filler effect’, acceleration in OPC hydration and the pozzolanic reaction. The observed results establish that there is an optimum OPC replacement level of 20 wt% MK and that the contribution which MK makes to strength is restricted beyond 14 days.

PORE SIZE DISTRIBUTION OF METAKAOLIN PASTE

The porosity and pore size distribution of cured OPC-metakaolin paste has been investigated. Pastes containing 0,5,10 and 15% metakaolin were prepared at a constant water/binder (wb) ratio of 0.55. Specimens were moist cured for periods from 3 to 365 days. The intruded pore volume and the pore structure were determined by mercury intrusion porosimetry. The proportion of large pores (radius > 0.02) in the paste decreases with both increase in metakaolin content and increase in curing time. Although the total intruded pore volume generally decreases with increase in curing time it is found to increase with increase in metakaolin content. Also an increase in pore volume is observed between the curing times of 14 and 28 days for pastes containing metakaolin. This latter observation is explained in terms of two possible mechanisms.

Portlandite consumption in metakaolin cement pastes and mortars

Cement mortars and pastes containing 0, 5, 10 and 15% replacement of cement with metakaolin and with a waterbinder (wb) ratio of 0.55 have been moist cured for periods from 3 to 365 days. The Portlandite content at different ages has been determined by thermogravimetric analysis and this has been related to changes in relative strength. Ca(OH)2 in mortars was found to be less than that in pastes. In the metakaolin mortars and pastes removal of Portlandite by pozzolanic reaction reached a maximum at about 14 days, corresponding with a maximum in relative strength. Beyond 14 days severe retardation of the pozzolanic reaction was observed and relative strength rapidly declined.

Sulphate Resistance of Metakaolin Mortar

The partial substitution of cement (both high C3A and intermediate C3A) with metakaolin (MK) is investigated in terms of resistance of MK mortar to sodium sulphate (Na2SO4) solution. Results on strength, porosity, pore size distribution, and calcium hydroxide (CH) contents are also reported. The sulphate expansion results demonstrate that the sulphate resistance is increased as the replacement level of cement with MK increases, up to at least 25% replacement. Refinement in pore structure and reduction in CH content increase with increase in MK content (for moist-cured mortar), and these two factors are considered to be the principal factors affecting improvement in sulphate resistance. Also, although after prolonged periods of exposure to Na2SO4 solution, there is (relative to water-cured specimens) significant strength loss of PC mortars and mortars with low levels of MK (5 and 10%); for mortars with high levels of MK (15, 20 and 25%), there is consistent strength gain.

Absorption characteristics of metakaolin concrete

Abstract The water absorption (WA) by total immersion and by capillary rise of concrete containing metakaolin (MK) is investigated. Cement was partially replaced with up to 20% MK. The results show that the presence of MK is greatly beneficial in reducing the WA by capillary action. There is a systematic reduction in absorption by capillary action with the increase in MK content in concrete. This reduction is further supported by visual examination of specimens. The absorption by total immersion, however, tends to increase slightly with the increase in MK content. Between 14 and 28 days curing, there is a slight increase in absorption by total immersion and by capillary rise for all MK concretes. Correlation between the absorption characteristics, dynamic modulus of elasticity (Ed), strength and pore size distribution was conducted.

Factors influencing strength development of concrete containing silica fume

Existing data on the relationships between temperature, pozzolanic activity and cement hydration are reviewed with particular emphasis on condensed silica fume (CSF)-ordinary Portland cement blends. CSF concrete with a range of fume contents has been cured at two temperatures (20 °C and 50 °C) for periods up to 91 days. Strength development and relative strength are considered in relation to temperature, cement hydration and pozzolanic action. The observed results establish that relative strength varies directly with CSF content and that the strength enhancement at early curing periods, which is achieved by increase in curing temperature, is a result of increased reaction rate between Ca(OH)2 and CSF.

Absorption characteristics of concrete as a function of location relative to casting position

Abstract Three different concrete mixes were prepared, the control mix in which no cement replacement material was added, and mixes where 22% and 9% by weight of cement was replaced with fly ash and silica fume respectively. Mixes were cast in 100mm cube moulds and cured at 20 °C and 45 °C using a variety of different curing regimes with respect to relative humidity and curing time to simulate concretes in hot and temperate climates. After curing, sliced samples were taken from various locations (faces) of the cube to determine their absorption. Two methods were used to study the absorption characteristics, the shallow immersion and the capillary rise. A large variation in absorption values existed between the upper surface during casting of the concrete cubes, the base and the sides. The absorption value of the complete unsectioned cube (100 mm) is similar to that of sectioned side face. A near-linear relationship exists between the two absorption methods.

Influence of superplasticizer and curing on porosity and pore structure of cement paste

Abstract Most concrete produced today contains admixtures. Superplasticizers (SP) are used for the purpose of improving workability and reducing the water to cement ratio; therefore producing more durable concrete. SP cause better dispersion even at high water to cement ratio. Although SP improves the dispersion of particles, it is not quite clear how the addition of SP affect the porosity and pore size distribution of cement paste. The purpose of this study was to examine the influence of one type of SP on porosity and pore size distribution under different curing regimes. Paste specimens with and without SP were prepared at constant water to cement ratio of 0.45. Specimens were cured for 28 days and some for six months. Specimens were exposed to high temperature (45°C) and normal temperature curing (20°C) and also subjected to different relative humidities (∼100%, 55% and 25%). Curing at high temperature was carried out to simulate temperature in hot climates. Tests on porosity and pore size distribution were conducted using mercury intrusion porosimetry. The results show that the inclusion of SP decreases the total intruded pore volume of paste. The dominant pore diameter, however, does not seem to be affected and the percentage of pores smaller than 100 nm increases in the presence of SP.

Sulphate resistance of mortar, containing ground brick clay calcined at different temperatures

Abstract The sulphate resistance of mortar containing ground calcined brick clay (GCBC) calcined at different tempertaures (600–1100 °C) and ground sand (GS) as cement replacement materials is investigated. Also the porosity, pore size distribution and strength of a selection of these mortars are determined. Mortars containing GCBC calcined at a temperature higher than 900 °C show superior sulphate resistance to those containing GCBC calcined at temperatures below 900 °C. Although the intruded pore volume is higher at early ages of curing, the inclusion of GCBC in the mortars leads to refinement of pore structure and its contribution to strength is significant after a curing period of 90 days. The influence of the incorporation of GCBC (calcined at different temperatures) on the sulphate resistance of mortar, is discussed in terms of fundemental mechanisms.

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.