In the work, an approach towards a carbon-neutral cement industry using CO2 mineralization of recycled concrete paste is investigated. It focuses on the effect of temperature on the enforced carbonation of cement paste. The enforced carbonation is a rapid process at ambient temperature, which is further accelerated at elevated temperatures. Moreover, the extend of the reaction is increased when the temperature rises. The carbonation reaction is divided into the following two kinetic stages: During the first stage, the carbonation kinetics is controlled by the availability of CO2 that increases at higher temperatures. During the second stage, the reaction kinetics is controlled by the dissolution of the hydrates. Increased temperature accelerates this process by increasing undersaturation level of the dissolving phases. The main carbonation products are calcium carbonate and an alumina-silica gel. The increasing temperature has limited impact on these phases, while the differences come mainly from the different degrees of carbonation.

The effect of early hydration behaviour on the long-term performance of cement is profound, but such studies are lacking. The early hydration behaviour and mechanical properties of basic magnesium sulfate (BMS) cement were investigated using an electrodeless resistivity test combined with compressive strength measurement, X-ray diffraction analysis, scanning electron microscopy and mercury intrusion porosimetry. According to the resistivity variation curve, the early hydration process of BMS cement can be divided into three stages (induction period, acceleration period and deceleration period). A linear correlation between resistivity and setting time was established. The initial and final setting times of BMS cement could thus be estimated using two points on the differential resistivity curve – the time when the resistivity growth rate starts to increase and the time of maximum resistivity, respectively. A linear fitting equation between resistivity at 24 h and compressive strength of the BMS cement after curing for 28 days was determined. The correlation coefficient was high (0.9979). Using the equation, the long-term strength (28 days) of BMS cement could be precisely predicted from the measured resistivity at 24 h. This study provides a feasible, accurate and in situ method for understanding the early hydration behaviour and quality monitoring of BMS cement.

The effect of sulfate attack (SA) on the strength of ultrahigh-performance concrete (UHPC) made with different replacements (0%, 50% and 100% by weight) of natural fine aggregate with recycled fine aggregate (RFA) was investigated. The UHPC samples were soaked in 10% sodium sulfate solution for 0–180 days (SA duration). The sulfate ion concentration, flexural strength and compressive strength of the UHPCs were investigated. The microstructure of the UHPCs before and after SA was analysed using a multi-technique approach (analysis of interfacial transition zones (ITZs), X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry). The results showed that, with an increase in SA duration, the compressive strength of the UHPC made with RFA increased. The generated gypsum and ettringite refined the microstructure by converting harmful pores (>20 nm) into harmless pores (<20 nm) in the UHPC matrix. For the same SA duration, the sulfate ion concentration of UHPC increased with an increase in RFA content due to more ITZs, pores and microcracks introduced by the RFA. Strength development models for UHPC with different RFA contents under SA were developed.

Coal gasification ash (CGA) is a solid waste produced by coal gasification. It can be used to produce autoclaved building materials because tobermorite, the main hydration product in such materials, can be formed from CGA and calcium oxide (CaO). In this study, tobermorite was prepared through the hydrothermal treatment of CGA. Given that residual carbon is usually present to some degree in CGA, its effects on the formation of tobermorite were specifically studied using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and nitrogen adsorption–desorption tests. The results showed that tobermorite can be prepared from a CGA/calcium oxide mixture at 160°C without additional alkaline material. However, residual carbon can retard the transformation of calcium silicate hydrate into tobermorite and thus increase the required formation temperature while prolonging the formation time. With an increase in the residual carbon content, the morphology of the resulting tobermorite changes from needle-like and plate-like to irregular, which may also have a detrimental effect on the mechanical properties of the material.

This aim of this work was to develop a low-cost, high-performance cement-based composite using straw cotton straw fibre (CSF) and expanded polystyrene (EPS). To this end, cement mortar specimens with different proportions of CSF (5%, 10% and 15%) and EPS (1%, 2% and 3%) were prepared. A control group (CG), without CSF or EPS was also tested. The compressive strengths, anti-splitting and flexural strengths of the specimens with different mix proportions were tested at for 7 days and 28 days. The thermal conductivity and water absorption of the specimens at 28 days were also measured. It was found that, for a constant content of CSF or EPS, the compressive strength and axial compressive strength of the samples at 28 days decreased with an increase of EPS or CSF content. The maximum compressive and axial compressive strengths of the CSF–EPS specimens were 37% and 42.6% lower than those of the CG, respectively, whereas the maximum flexural and anti-splitting strengths were 33% and 94% more than those of the CG. With an increase in the content of CSF and EPS, the water absorption of the specimens increased, whereas the thermal conductivity decreased. The cementitious composite made with 5–10% CSF and 1–2% EPS met the requirements for the mechanical properties and thermal insulation performance of self-insulating blocks.

This study uses conventional 87Sr/86Sr and 143Nd/144Nd isotope and interelement ratios of Ca, Sr, K, Mn, Mg and Ti as fingerprints for provenancing ordinary Portland cements (OPC). Herein, the first database of Sr and Nd isotope ratios investigated in OPCs, stemming from 29 cement plants located worldwide, was created. The results show that the Sr isotope ratios of OPCs are higher than those of seawater from the observed geological period. The spread of 143Nd/144Nd in OPCs is not as large as the spread for 87Sr/86Sr isotope ratios. However, the combination of both Sr and Nd isotope ratios provides the potential for distinguishing between cements of different production sites. Most of the OPCs investigated have measurable differences in their 87Sr/86Sr and 143Nd/144Nd isotope ratios, which can be employed as a valuable analytical fingerprinting tool. In the case of equivocal results, divisive hierarchical clustering was employed to help overcome this issue. The construction of geochemical profiles allowed the computing of suitably defined distances between cements and clustering them according to their chemical similarity. By applying this methodology, successful fingerprinting was achieved in 27 out of the 29 ordinary Portland cements that were analysed.

Fly ash-based geopolymer paste with expanded vermiculite (EV) powder addition was synthesized and its microstructure, compressive strength, setting time, moisture control, efflorescence extent and thermal conductivity were studied. The results showed that EV addition resulted in the increase of standard consistency water consumption and setting time. As a consequence, its excessive addition caused a larger amount of harmful pores, which was detrimental for compressive strength of geopolymer paste. However, geopolymer pastes with an appropriate amount of EV addition (2-7 wt%) presented a slight increase of compressive strength because of the filler effect. Mg2+ and Fe2+diffused from EV interlayer through ions exchange between EV and geoploymer solution participated in geopolymerization. This was reflected by the formation of N-(M)-A-(F)-S-H evidenced through SEM-EDS and FITR analyis. In addition, Na2+/Mg2+or Na2+/Fe2+ ions exchange reduced the mobility of Na2+and therefore decreased the efflorescence extent. Moreover, EV addition favored the improvement of moisture control and thermal conductivity properties of geopolymer paste.

Concrete technologists use different types of additives such as fly ash, slag, natural pozzolans and nanomaterials toenhance concrete performance and durability. However, a detailed explanation of the early-age hydration process and microstructural modification of concrete in the presence of nanomaterials remains to be presented and extensive research is required for strategic modification of cementitious systems. This study focused on the precise monitoring of early-age hydration with the incorporation of nanoalumina (nAl) in tricalcium silicate (C3S) and Portland cement paste and mortar. The dosage of nAl was varied from 1 to 5% (by weight) in C3S and from 0.1 to 1.0% in Portland cement, with a water/cement ratio of 0.4. The hydration studies showed that the nAl increased the cross-linkage in calcium silicate hydrate gel through substitution of aluminium by silicon, which was responsible for the enhancement of the modulus of elasticity (by 40%) with 1.0% nAl) after 7 days of hydration. In summary, the incorporation of nAl modified the concrete microstructure in the initial days of hydration, leading to higher concrete performance and longer service lives of concrete structures.

Wet stored fly ash is increasingly being considered as a cement component in concrete. However, the effect of these conditions on the material's reactivity is uncertain. The research described here investigated this property for wet laboratory-stored (10% moisture) and site stockpile fly ashes, using lime consumption (BS EN 196-5, Frattini) and activity index (BS EN 450-1) tests. Progressive reactivity losses occurred with laboratory storage up to 730 days. This was influenced by dry fly ash fineness and holding period, suggesting that the formation of agglomerates/products (assessed by scanning electron microscopy) affects lime's access to particle surfaces, with similar type behaviour for stockpile materials. Compressive (cube) strength reductions were also found between dry and wet stored fly ash concretes. Stockpile fly ash reactivity following laboratory- (drying/ball milling) and pilot-scale (flash drying/de-agglomerating, air classifying, micronising and carbon removal) processing was then investigated. Exposure of reactive material using these methods appears to be important, with greater improvements generally noted as the fly ash particle size is reduced and at later test ages. To meet activity index requirements, fly ash sub-10 μm contents, with the Portland cement used, needed to exceed about 30%, irrespective of the storage conditions/processing used. Minor benefits to concrete strength were obtained with increasing sub-10 μm contents, particularly beyond 28 days.

A self-expanding filling material for mine-sealing walls (SEFM-MSW) was developed to solve the problems of the current main filling materials of MSW, such as high consumption of materials, high labour intensity and poor sealing effects. The material composition and structure were examined in this work and it was found that the performance of the SEFM-MSW was optimised with the addition of an expansion agent at a dosage of 0.4–1.0%, an admixture (dosage of 1.0–1.5%), fibre (dosage of 0.1%) and a water/cement ratio of 0.45–0.50. The relationship between the 28-day compressive strength and porosity of the SEFM-MSW could be expressed by an exponential function or a power function. The SEFM-MSW compressive strength decreased with an increase in porosity, which is consistent with the Ryshkewitch and Balshin models. The results also showed that when the expansion ratio of the SEFM-MSW was higher, the average pore size was larger, the pore size distribution was wider and the compressive strength was lower. The results of this study have realistic impacts for practical engineering applications.

Cement–sodium silicate grout (CSG) is widely used to control water inrush disasters, and its apparent viscosity considerably impacts its water-plugging effect. However, traditional grouting materials and methods are inappropriate for high-temperature environments as high temperatures can affect the grout's viscosity. To thicken grout and increase its apparent viscosity, viscosity-modifying admixtures can be used. In this work, two types of bentonite (calcium bentonite (Ca-B) and sodium bentonite (Na-B)) and hydroxyethyl methyl cellulose (HEMC) were used to modify CSG and laboratory tests were performed to evaluate the fluidity, gelation time and rheology. The results showed that both bentonites and the HEMC decreased fluidity and prolonged the gelation time. HEMC, Ca-B and Na-B decreased the fluidity by 46.8–60.4%, 12.5–31.5% and 17.7–39.1%, respectively, at different temperatures. HEMC, Ca-B and Na-B increased the gelation time by 23.8–50.1%, 23.3–71.4% and 20%–57.1%, respectively. Bentonite can partially resist high temperatures and improve the apparent viscosity of grout owing to its water-absorption capacity. Conversely, HEMC has a negative effect on apparent viscosity, which is attributed to the formation of a complex microstructure resulting from intermolecular cross-linking between the cement particles and HEMC, preventing the connection of sodium silicate.

The behaviour of cement composites containing rice husk ash, which have previously undergone high-temperature thermal curing, was experimentally investigated. A specific initial thermal curing up to 85°C was designed to trigger delayed ettringite formation, as was a specific exposure environment, by water immersion at 38°C, over the course of 1 year. Expansion measurements and microstructural analyses were performed to evaluate the level of attack and the integrity of mortars and concretes. To complement the study, the mechanical properties of concretes were assessed to detect the level of damage by expansion from delayed ettringite formation. Mortar performed differently from concrete, bringing risks of mistaken conclusions about admixture performance. Tests on concrete have indicated that the addition of rice husk ash can reduce the level of expansion; however, the amounts used were not sufficient to mitigate the expansion completely in order to avoid delayed ettringite formation and, consequently, the associated damage.

In this paper, a comprehensive semi-empirical model is developed to predict the drying shrinkage strain of concrete reinforced by graphene oxide nanosheets (GONs). The model involves various internal and external parameters. The effects of three factors,including the age when drying starts, the GON content and aspect ratio on the drying shrinkage strain of concrete reinforced with GONs are investigated. It is found that the presented results are in good agreement with the experimental data compared to the results obtained from the other models, including CABR, CEB-78, CEB-90, ACI209(92), B3, and GL2000 for plain concrete, and modified ACI209(92) and B3 models for concrete with GONs. Moreover, several sensitivity analyses are done for the drying shrinkage strain of concrete with 0.02, 0.05 and 0.08wt% as functions of the relative environmental humidity, temperature, compressive strength, cement content, aggregate-cement ratio, size and dimensions of the sample, gravel-sand ratio, sand-cement ratio, and water-cement ratio. It is noted that the developed model not only evaluates the drying shrinkage behavior of concrete containing GONs well but it can also be applied to predict the drying shrinkage strain of cement that is reinforced by GONs.

Air-entrained wet shotcrete improves the pipeline transportation efficiency of concrete. The characteristics of the fine aggregate in the mix have different effects on the fluidity and mechanical properties of air-entrained wet shotcrete. The effects of four different properties of fine aggregates (mud content, sand ratio, water content and particle size) on the fluidity and mechanical properties of air-entrained wet shotcrete were studied, with particular attention paid to the sand ratio and mud content. The fluidity and mechanical properties of wet shotcrete were optimum when the sand ratio of the fine aggregate was 0.56 and the water content was 3%. The fine aggregate should have a reasonable particle size as far as possible. Montmorillonite in the mud inhibited the fluidity and mechanical properties, so mud content should be kept as low as possible. The results of this work provide a reference for the practical application of air-entrained wet shotcrete, so as to better deal with pipe blockage.

The use of high-temperature X-ray diffraction (HT-XRD) to study the mass transfer of raw meal constituents towards forming clinker phases and the occurrence of free lime (calcium oxide), also known as burnability, was assessed. A measuring strategy with temperature ranging from 1000°C to 1450°C was developed and compared with a conventional burnability method. The free lime determined by the methods showed that HT-XRD produced good results for the evaluation of burnability. In addition, HT-XRD revealed the formation of intermediate phases, providing insight into early reactions in a cement kiln. The particle size of quartz was found to affect crystal expansion of the phase at a high temperature, subsequently affecting the formation of silica polymorphs. The different raw meals used in this study also indicate that the formation of different silica polymorphs affects the formation of C2S. The lack of knowledge regarding the influence of β-quartz on the reduction of free lime is highlighted.

Magnesium oxysulfate cement has low strength and poor water resistance, so an additive is essential for the preparation of basic magnesium sulfate cement (BMSC). In this paper, the effects and the mechanisms of three organic weak acids on the water resistance in relation to flexural strength of BMSC are investigated. The results show that malic acid fails to ameliorate the water resistance in relation to flexural strength, but citric acid and tartaric acid are successful. The ionisation constant and molecular formulas of different weak organic acids can affect the performance, such as fluidity, strength and water resistance. Therefore, the effect of citric acid is better than tartaric acid, and the optimal dosage of citric acid is 1.0%.

This study examined the efficacy of waste glass cullet (WGC) as a substitute for natural fine aggregate in alkali-activated composites when exposed to sulfuric acid and hydrochloric acid solutions for 1 year. The physical appearance, surface alkalinity, mass, mechanical strength and microstructure of hardened samples before and after immersion in acid solutions were investigated. The findings this work indicated that physical, mechanical and microstructural damage of the specimens due to acid attack increased with an increase in the percentage of WGC. This was attributed to the smooth surface texture and angularity of the WGC, which affected the bond with the damaged paste matrix at the interfacial transition zone and increased the porosity. However, the acid resistance of the mortars containing up to 50% WGC was found to be satisfactory when compared with the mortar without WGC. Therefore, the use of WGC as a partial replacement (up to 50%) for natural sand is feasible for alkali-activated systems under acid exposure.

Scanning transmission X-ray microscopy (STXM) and calcium L2,3-edge near-edge X-ray absorption fine structure (Nexafs) spectra were used to investigate the morphology and chemical information of cement-based materials. The interaction between calcium chloride (CaCl2) and monosulfoaluminate (Ms) was investigated and the intermediate phases were identified. With an increase in the concentration of the calcium chloride solution, calcium Ms was converted into ettringite, Kuzel's salt (Ks) and Friedel's salt (Fs) to varying degrees, while SO42− ions in the ettringite and Ks were continuously replaced by Cl− ions in the solution, eventually forming Fs and gypsum. Based on the calcium L2,3-edge Nexafs absorption spectra, ettringite, Ms, Ks and Ms were greatly distinguished by the number and shape of the leading peaks in the spectra due to the difference in SO42− content among the interlayer structures of Ms, Ks and Fs. The present work is based on the references for the STXM study of the calcium (sulfo)aluminates, for demonstrating a more complete understanding of the phase evolution under chloride environments.

Geopolymers prepared with class C fly ash (FA) often suffer from a flash geopolymerisation rate, making it difficult to work. Class F FA, however, may have low reactivity, so an admixture must be added to solve this problem. This research highlighted the use of mixed class C and class F FA as a precursor for geopolymer preparation to alleviate such problems. Optimisation of the precursor recipe and the solid-to-liquid ratio (S/L) was assessed. The highest compressive strength (30.54 MPa) and a suitable initial (136 min) and final (240 min) setting times were obtained with S/L = 2.00, and 50 : 50 proportions of class C to class F FA. The fire resistance test on a 10 mm thick geopolymer panel at 1000°C for 180 min showed a maximum temperature of ∼570–580°C on the reverse side without catastrophic failure, suggesting it can be applied for use in fire retardation.

Low-reactivity fly ash (LRFA), obtained from combustion in a low-temperature furnace, is a hazardous waste with low pozzolanic activity that is unsuitable for engineering applications. This study investigated the effects of mixing procedure (sequence mixing method (SMM) and conventional mixing method (CMM)) on the properties of a geopolymer made from LRFA blended with calcined kaolinite clay as a precursor. A mixture of sodium hydroxide and sodium metasilicate was used as the alkaline solution. It was found that the SMM was required to obtain LRFA geopolymers with sufficient strength for engineering applications. Using the CMM, the geopolymer had high porosity and low strength because gas bubbles were generated in the reaction between the alkaline solution and impurities in the LRFA during the hardening process. By contrast, in the SMM, the generated gas bubbles were released before the geopolymer hardened. As a result, the porosity was reduced and the strength was thus significantly increased. The 7-day strength of the geopolymer fabricated using the SMM was higher than the minimum requirement for Portland cement. In addition, the SMM reduced the setting time of the LRFA geopolymer, which is advantageous for rapid repair applications.