Effect of gypsum content on the compatibility of cement with alkali-free accelerators

The setting times and early strength of shotcrete are greatly affected by the gypsum content of the cement used. In this work, the setting times, compressive strength and microscopic analysis (X-ray diffraction quantitative analysis, thermogravimetric analysis and scanning electron microscopy) were used to determine the effects of changes in gypsum content on the hydration and mechanical properties of cement mixed with an accelerator at different temperatures. The results showed that an increase in the gypsum content of the cement promoted the formation of ettringite (AFt) and decreased the setting times of the cement pastes at 0°C, 20°C and 40°C. Mortar compressive strength tests showed that, after curing at 0°C, 20°C and 40°C, the highest compressive strength was exhibited with a final molar aluminate-to-sulfate ratio (C3A/SO3; henceforth called the MASR) of 0.84, 0.84 and 1.18, respectively. The mechanical properties of the hardened cement pastes were adversely affected by MASRs that were too high or too low. When the MASR was too high, the conversion of AFt to monosulfate (AFm) was facilitated and C3S hydration was slowed. Conversely, when the MASR was too low, this enabled large amounts of AFt to occupy the space where calcium silicate hydrate (C-S-H) gels were formed, resulting in decalcification of the C-S-H gels.

The objective of the study is to evaluate the influence of gypsum content on certain properties of blast furnace slag cement (with 40% blast furnace slag) at gypsum levels ranging from 2% to 5%. The properties of the cement investigated include water of consistency, initial and final setting times, compressive strength at ages of 1, 3, 7, and 28 days, and heat of hydration of the cement paste within the first 24 hours. Experimental results indicate that increasing the gypsum content has minimal impact on the water of consistency but prolongs the setting time, while still meeting the technical requirements according to TCVN 6260:2020. The optimal gypsum content, in terms of strength, for blast furnace slag cement is 4%, providing compressive strength comparable to the control sample of ordinary Portland cement at 3 and 28 days of age.

Gypsum is an indispensable component of ordinary Portland cement. The main function of gypsum is to regulate the setting time of cement. Gypsum has a great influence on water consumption of standard consistence and strength simultaneously. In this article three cementitious systems are prepared by three main mixed materials: limestone, fly ash and slag. The effect of gypsum content and the relation between gypsum quantity and the fluidity of cement with Marsh cone method are studied. Experimental results show that when the gypsum was added in the three kinds of cementitious system, water consumption of standard consistence decreased obviously, and with the quantity of gypsum increased, water demand for normal consistency reduced first, up to a certain extent tended smoothly. With the increase of gypsum quantity, initial and final setting time had different degrees of extension. While a saturation quantity is achieved, setting time will keep steady. The flexural strength and the compressive strength increased gradually and tended to be stable with the increase of gypsum quantity. When gypsum content exceeded a certain range, compressive and flexural strength decreased gradually. But gypsum content and the mobility of systems had little relativity with Marsh cone method.

Effect of aluminum sulfate on the hydration of Portland cement, tricalcium silicate and tricalcium aluminate

The aim of this paper is to examine the effect of a blending dose of calcium sulfoaluminate (synthetic ye'elimite, C4A3$) and gypsum on the hydration and hardening properties of a new ye'elimite (C4A3$) blended PII 52.5 cement. By using X-ray diffractometry, mercury intrusion porosimetry, thermogravimetry and scanning electron microscopy, it was revealed that the hydration of tricalcium silicate (C3S) is significantly retarded by the lack of gypsum; the renewed hydration of tricalcium aluminate (C3A) was retarded in cement pastes containing a high amount of C4A3$. The dosage of C4A3$ and the ratio of gypsum/C4A3$ both have a notable influence on the hydration heat, hydration products and the compressive strength and linear expansion. The cement paste containing 3% C4A3$ at gypsum/C4A3$ ratio of 2 exhibited the best performance, in terms of both high early and long-term compressive strength and acceptable linear expansion (0·04% on 300 d); the cement can therefore be adopted in industrial structures.

Comparison of the Performance of the Steel Fiber Shotcrete and Cast Concrete considering Accelerator Types and Core Sizes from a Tunnel Site

The effect of fly ash fineness on heat of hydration, microstructure, flow and compressive strength of blended cement pastes

ABSTRACTThe relative impact and interaction of two components, gypsum and water, in high ye’elimite containing calcium sulphoaluminate-belite cement pastes at an age of 7 days was studied to find the optimum conditions for compressive strength. Using statistical analysis of 149 compressive strength results having a range of water and gypsum contents, the statistical significance of the variables was investigated. X-ray diffraction analysis demonstrated the different hydration processes occurring within samples based on water and gypsum contents, while mercury intrusion porosimetry comparison confirmed the importance of pore structure towards determining ultimate paste strength. Thermodynamic modelling was also used to contribute to the understanding of the microstructural development. The compressive strengths of cement pastes with varying gypsum and water content were shown to be influenced by both factors, with a non-linear dependence on gypsum content observed.

In this study, the effects of soaking time and confining water pressure on the compressive and flexural strengths of rock due to the dissolution of gypsum content in rock samples with 96% gypsum (CaSO4·2H2O) content collected from northern Iraq (Kurdistan Region) were investigated. Experiments on compressive strength and flexural strength were performed on gypsum rock samples in normal and pre-saturated conditions. The pre-saturated samples were submerged in distilled water for 35, 70 and 105 d and subjected to confined pressures of 0–0·5 MPa. Thermogravimetric analyses showed a notable reduction in the weight of gypsum rock between 25 and 120°C. Also, the total weight loss at 800°C for the field rock sample was 7·53%. The weight loss of the gypsum rock increased with increasing soaking time and confining water pressure. The gypsum content decreased by 11% under a soaking time of 105 d and a confining water pressure of 0·5 MPa with respect to the natural samples. The compressive strength of the normal gypsum rock was 19·6 MPa, and it decreased by 68 and 90% after 105 d of soaking under confining water pressures of 0 and 0·5 MPa, respectively. The flexural strength of the normal gypsum rock was 10·8 MPa, and it decreased by 65 and 91% after 105 d of soaking under confining water pressures of 0 and 0·5 MPa, respectively. Two non-linear constitutive models were used to simulate the experimental stress–strain relationships in various conditions. The constitutive model parameters were sensitive to the gypsum content. Based on the methods of the coefficients of determination (R2) and root mean square errors, the non-linear constitutive model (Vipulanandan p–q model) predicted the stress–strain behaviour for the gypsum rock very well.

This paper selected two kinds of alkali-free liquid concrete accelerators and tested their compatibility with ordinary Portland cement, Portland cement and moderate heat Portland cement by measuring the setting times, compressive strength and compressive strength ratio of samples. The results showed that the compatibility is good between alkali-free accelerators and two types of cement: ordinary Portland cement and moderate heat Portland cement. However, the compatibility of two accelerators and Portland cement are quite different, the compatibility of AFA-2 accelerator is excellent, but AFA-1 accelerator is very poor. The setting times of alkali-free accelerators is mainly influenced by the mixing materials content, gypsum content, C3A and C3S content. In order to ensure the mechanical properties and durability of concrete, the setting times of new concrete accelerator is not the shorter the better, the appropriate initial and final setting times are 1min30s~5min and 4min~ 12min respectively.

Due to environmental damage caused by the Portland cement manufacturing process, industry and academia have been required to reduce greenhouse gas emissions from the production process. The massive use of supplementary cimenticious materials (SCM’s) is one of the most viable options for reducing clinker factor and pollutant emission. LC3 cement is a type of cement that uses high additions contents, counting on the synergistic reaction between calcined clay and limestone to obtain mechanical strengths similar to those of traditional cements. This research intends to investigate the influence of the gypsum content on the compressive strength of a LC3 cement. For the accomplishment of this study six mortars were produced. In three of them the cement used was produced with 3, 5, 7% gypsum, without SCM’s. In three mortars, a 45% replacement cement was used (30% calcined clay and 15% limestone). The proportion of gypsum in these cements was 3, 5 and 7% in relation to the total mass. For the hardened state analysis, the compressive strengths were compared at the ages of 1, 3, 7, 28 and 91 days. A statistical analysis was performed on the results. From the results obtained it was possible to verify that the gypsum content influences the compressive strength of the LC3 cement. The highest mechanical strengths were obtained with the use of larger quantities of gypsum.

Gypsiferous soils have specific feature under its suction studying. These soils contain salt solution that affects on the suction components at high water content levels. This paper investigates soil suction of three types of gypsiferous soils namely: Al-Muhallabeia, Al-Jarin, and Al-Slmanii using filter paper technique. The investigated soils were located in Nineveh province (400 Km north Baghdad capital) and found to have a gypsum content of about 35, 23, and 6%, respectively. This study includes some factors (water content, gypsum content, and clay content) that affect the soil suction for gypsiforous soils. The effect of gypsum content on slope of the relationship between soil suction with pF unit and water content was examined. The results revealed that there is a linear relationship between soil suction and its water content for three studied soils. Both total and matric suction values increase as soil water content decrease. On the other hand, the difference between total and matric suction values increase with soil samples water content. The soil clay content has more effect on gypsiferous soil suction than its gypsum content. Key words: Gypsiferous soil, Soil suction, Total suction, Matric suction, Gypsum content

Based on aluminum sulfate, a fluorine-free and alkali-free liquid accelerator (FF-AF-A) was prepared in this study. The setting time and compressive strength of three cement types with different FF-AF-A dosages were fully investigated. The compatibility of the FF-AF-A with the superplasticizers were also investigated, and the early hydration behavior and morphology of the hydration products of reference cement paste with the FF-AF-A were explored by hydration heat, X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Test results indicated that adding the FF-AF-A at 8 wt% of the cement weight resulted in 2 min 35 s initial setting time and 6 min 30 s final setting time. The 1-day compressive strength of the cement mortar with 8 wt% of FF-AF-A reached 13.5 MPa, which represents an increase of 35% as compared to the strength of cement mortar without the FF-AF-A, and the 28-day compressive strength ratio was 119%. In addition, the FF-AF-A also showed good compatibility with different superplasticizer dosages. The results show that, when the FF-AF-A was added to the cement paste, it promoted the formation of ettringite crystals due to the aluminum ions (Al3+) and sulfate ions (SO4 2-) reacted with gypsum in the cement, as well as promoted the hydration of tricalcium aluminate (C3A) and tricalcium silicate (C3S) leading to the overall structure becomes more compact. As a consequence, the hydration heat rate of the cement sharply increased, the cement paste setting time is shortened, and the compressive strength of cement mortar is improved.

이 연구에서는 숏크리트에 적용하기 위한 고분말도 시멘트의 기본 특성을 검토하고자 하였다. 고분말 시멘트의 주요특성을 연구하기 위하여 입도분포, 응결시간, 압축강도를 측정하였으며, SEM 관찰, DSC 열분석, X선 회절분석을 실시하였다. 고분말도 시멘트는 일반 포틀랜드 시멘트와 비교하여 응결시간이 크게 단축되었으며 압축강도가 개선되었다. 기기분석 결과 고분말도 시멘트는 초기수화물이 보다 미세하고 광범위하게 분포하는 것으로 확인되었으며, 이러한 결과는 시멘트의 분말도가 상승함에 따라 시멘트 입자와 물과의 접촉면이 증가하기 때문이다. SEM 관찰, DSC 열분석, X선 회절분석 결과 알루미네이트계 급결제는 칼슘알루미늄 수화물의 생성을 촉진하고, 알칼리프리계 급결제는 에트린자이트와 모노설페이트의 생성을 증진하는 것으로 나타났다. 페이스트 응결시간 측정으로부터 알루미네이트계 급결제와 알칼리프리계 급결제가 응결시간을 단축시키는 것으로 확인되었으며, 고분말도 시멘트는 압축강도가 크게 향상되는 것으로 나타났다. 특히 알루미네이트계 급결제는 응결시간 단축에 효과적이며, 알칼리프리계 급결제는 7일 이후의 강도증진에 효과적인 것으로 확인되었다.

Sulfates in fine aggregate are a major problem when it exists in excessive amount especially in the Middle East and Iraq. Most of sulfate salts in fine aggregate are composed of calcium, magnesium, potassium and sodium sulfates. Calcium sulfates is the most common salt present in fine aggregate. It is usually finding as gypsum. It is difficult to obtain the specific sulfates content in fine aggregate within standard specifications. This research was conducted to investigate the effect of adding different contents of gypsum to fine aggregate as a replacement by weight on some properties of two types of concrete {self-compacted concrete (SCC) and high strength concrete (HSC)}. In these work three bases mixes of each type of concrete are used: mixes with different contents of metakaolin, mixes with different contents of gypsum and mixes incorporating different contents of metakaolin and gypsum. This study is devoted to determine the allowable content of sulfates in fine aggregate. Three levels of gypsum were tested (0.5, 1, 1.5) % by weight of fine aggregate and three levels of metakaolin were tested (5, 10, 15) % by the weight of cement. The experimental program is devoted to produce concrete with different levels of metakaolin and gypsum and determine its mechanical properties such as compressive strength and splitting tensile strength. The results arrived from this work show that the optimum gypsum content was 1.5% by weight of fine aggregates for mixes of SCC which gives increases in compressive strength and tensile strength, and 1% gypsum for mixes of HSC, results showed also that the metakaolin improved the properties of the two types of concrete and increased the loss which caused by sulfates. The best mix ever in SCC is 1% gypsum with 5% metakaolin, and 1% gypsum with 10% metakaolin for HSC.