1

Introduction

Interestingly, Portland cement appeared in dentistry before 1878 . MTA (mineral trioxide aggregate), a Portland cement-based formulation was developed more than 20 years ago as a root-end filling material but its potential for new clinical applications later became evident thanks to its innovative hydraulic properties and sealing ability. We have now more information on its properties and many new MTA-derived materials have been developed.

This paper uses the term hydraulic calcium silicate cement (HCSC) to refer to the entire family of MTA-like cements. A long list of new materials based on the original formulation and/or with minor modifications has been introduced in routine clinical practice ( Table 1 ). A light-curable HCSC is advisable in many clinical cases and at least one material is now on the market. Other experimental materials have been proposed, so many potential new products could soon be available and more very innovative HCSCs can be expected in the future.

Despite of the dearth of information from the first in vitro experiments and the lack of clinical studies, HCSC gained the trust of many operators who proposed its clinical use for apicogenesis, pulp capping and other procedure. Tay and Pashley introduced the concept of “biomimetic remineralization” of partially demineralized dentin using MTA. This represented a major innovation and opened the way to potential new applications for HCSCs-based materials. At the moment the use of HCSCs-based technology for dentin remineralization is not far from the clinical application.

The history of calcium silicate Portland cements dates back to Roman times when a cement able to set in water made by grinding together lime and a volcanic product found at Puteoli (hence called pozzolana ) around Neapolis (the places described by Pliny the Elder). Pozzolana helped Roman concrete set quickly even when submerged in water, allowing the construction of bridges, harbors, aqueducts, monuments and buildings. During the middle ages the secret of cement was lost. In the 18th century John Smeaton, an English engineer, rediscovered the correct proportions of cement using clay and limestone. In 1824, Joseph Aspdin, an English bricklayer, patented a process for making what he called Portland cement. The first great bridge built in the USA in the late 19th century was made of Portland–Pozzolanic cements. Modern Portland cement is made by mixing substances containing lime, silica, alumina, and iron oxide and then heating the mixture until it almost fuses. Pozzolana is still the main component of many Portland cements.

Calcium silicate Portland cements set through a hydration reaction after mixing with water or water-containing liquids. Various hydration products form during the reaction, namely different phases of calcium silicate hydrate (CSH) as porous colloidal CSH gel and radial acicular CSH crystals, rombohedral crystals of portlandite (calcium hydroxide), needle-like crystals of ettringite (hexacalcium aluminate trisulphate hydrate) ( Fig. 1 ), and calcium monosulfoaluminate or calcium monocarboaluminate. Porous CSH hardens by the formation of a solid network within 1–6 h. The setting reaction requires several days to complete the hydration and hardening phases and includes dissolution and reprecipitation processes of the unhydrated di- and tri-calcium silicate phases (C ₂ S and C ₃ S) and formation of hydration products like calcium di- and tri-silicate hydrates and calcium hydroxide . In this phase, CSH has layer structure, with a layer growing radially from the calcium silicate particles and resulting in a fibrous needle-like complex structure, and cuboidal calcium hydroxide crystals form among the hydrating cement compounds ( Fig. 2 ). This phase is difficult to inspect under standard SEM. As demonstrated by Gandolfi et al. , environmental SEM (ESEM) analysis can provide more information on the morphology of the early stage of Portland and dental calcium silicate/MTA cements ( Fig. 3 a and b ). Original native Pozzolanic cements are very similar to HCSCs under ESEM ( Fig. 4 ). Moisture ( i.e. biological fluids) is essential to allow the setting reaction and to develop/activate the cement’s bioactivity, such as the formation of apatite . The following stages have been proposed by Gandolfi et al. for the nucleation of calcium phosphates on the HCSC surface:

• 1.

  A solid–liquid interface forms on the mineral particles, and ion dissolution occurs almost immediately. Ca ²⁺ ions rapidly migrate into the mixing solution and portlandite (Ca(OH) ₂ ) forms.

• 2.

  Silicates are attacked by OH ion groups in an alkaline environment and a CSH phase forms on mineral particles. CSH is a porous, fine-grained/fibrous and disorganized hydrated silicate gel layer containing Si-OH silanol groups and negative surface charges that may act as nucleation sites for apatite formation.

• 3.

  CSH contains an excess of calcium hydroxide/portlandite formed by OH ⁻ ions from dissociated water molecules and Ca ²⁺ ions from the cement particles, so a strong continuous outflux of calcium hydroxide from CSH occurs during the first hours after mixing causing a marked rise in the pH and an increase in calcium ion concentration in the surrounding environment. Portlandite crystals nucleate inside the hydrating wet paste and are also detectable on the surface by ESEM/EDX. The release of portlandite persists for a long time in immersion/storage conditions, as revealed by the high pH of the soaking medium which remains constant at 11.0–12.0, depending on the chemistry of the material.

• 4.

  When exposed to a phosphate-containing fluid – simulated body fluids (SBFs) or body fluids – a sequence of reactions take place on the surface of HCSCs between calcium from the cement and phosphate from the solution, namely the absorption of Ca and P ions on the silanol groups (Si-OH) of the silica-rich CSH surface and the precipitation of calcium phosphates and apatite (as a result of local supersaturation) which matures into a B-type HCA phase at increasing storage times. The calcium phosphate apatite deposits form a layer of spherulites filling the superficial porosities ( Fig. 3 b) . EDX, FTIR and micro Raman analyses have been used to demonstrate the formation of calcium phosphate and apatite deposits on cements after soaking in different SBFs ( Figs. 5 and 6A, B ) .

  

  Kinetics of apatite formation in SBF of a HCSC Portland-based cement monitored over time by SEM and EDX analyses. Fresh cement mainly displayed Ca, Si, S, Cl and Al peaks (all components of the cement). After 7-day soaking in SBF the surface became covered by aggregated spherulites. During the soaking time the coating increased in thickness and continuity and consequently the cement surface shifted from a calcium silicate to a calcium phosphate composition. With long (28 days) soaking times, Si completely disappeared, while P resulted in a growing peak.

  

  (A) Raman analyses of different freshly mixed calcium silicate cements . (B) Raman analyses of a conventional hydraulic calcium silicate cement after soaking in SBF. The apatite peaks appearing during soaking in SBF narrow and increase in intensity with aging .

Morphology of the early phase setting reaction of a calcium silicate cement (HCSC) visualized by environmental scanning electron microscopy (ESEM) 10 min after the mixing procedures. The setting reaction has been described as dissolution-precipitation process with gradual dissolution of the unhydrated calcium silicate particles and the subsequent formation of hydration products as radial elongated needle-like calcium silicate hydrate crystals and calcium hydroxide (portlandite) within a calcium silicate hydrate (CSH) amorphous gel. At this time, the hydrated and unhydrated phases are still present and allow rapid ion exchange.

Hydration of HCSCs: typical morphology of the radial elongated needle-like calcium silicate hydrate crystals and calcium hydroxide.

(a) ESEM/EDX image of an HCSC after 24-h aging in simulated body fluid (SBF). The hydration is at the initial stage and all the particles are immersed in a soggy jelly-like phase. (b) ESEM image of the previous sample. CaP-rich aggregated microspherulites coating the calcium silicate particles become visible after water removal by ESEM (high void) procedures. These spherulites constitute the substrate for the subsequent bionucleation of apatite.

ESEM image (in semi-wet conditions i.e. in low void ESEM conditions) of a Pozzolanic cement after soaking in SBF showing aggregates of apatite spherulites nucleated on the cement surface.

2

Properties

HCSCs are evolving rapidly as they may be used for a number of clinical applications where previous materials failed. They can set in wet environments such as in the presence of water, blood, dentinal fluid, saliva, etc. , and have satisfactory/good biological properties. This is the principal reason they have gained great attention for clinical many applications.

Historically, gray ProRoot MTA was the only HCSC on the market for a long time, subsequently flanked by white ProRoot MTA. Many of the early studies were done on these two materials and they still represent the gold standard to compare newer cements. New HCSCs have been developed and marketed while many experimental materials/prototypes have been proposed to investigate their effectiveness and to pass/obviate some of the drawbacks of the original cements such as: long setting time, difficult handling and manipulation, risk of wash-out in a too moist environment ( i.e. bleeding sites), low radiopacity, and high cost. Table 1 is schematic list of HCSCs present on the market (marketed in some countries under different brand names) and their clinical uses.